Download This Document was created for a Botany class I taught at a different

This Document was created for a Botany class I taught at a different university
several years ago. It is a summary of information on the major groups of land
plants (Embryophyta) as well as some explanatory information about details of
plant structure and life cycles.
Important – I do not expect anyone to read this entire document. I am providing it
to you for background information on the various groups which you can use to help
you prepare your report. You can also use it to get a more complete explanation of
anything that is briefly mentioned in the lab.
Kevin Dixon
Sexual Life Cycles. Sexual reproduction is another major evolutionary innovation of the
eukaryotes. The selective advantage of sexual reproduction to an individual organism is
still unclear but it does allow for much greater genetic diversity within populations and
hence at least the potential for more rapid evolution. Although this is a fascinating topic
we will not consider it in detail until later in the course.
One consequence of sexual reproduction is that a species spends part of its life cycle as a
haploid and part of its cycle as diploid. In every sexual life cycle at some point a diploid
nucleus undergoes meiosis and produces four haploid nuclei (point A). At some other
point in the life cycle two haploid nuclei fuse to form one diploid nuclei (point B). What
happens between point A and point B (and between point B and the next point A) is
tremendously variable among eukaryotes. It is possible to divide the life cycles into three
groups which are outlined below. The categories are simplifications and not all
organisms fit neatly in one of them.
1. Gametic Meiosis. The only haploid cells are the gametes which do not reproduce or
grow into multicellular entities. The gametes’ only function is to fuse and form diploid
cells (zygotes). The diploid phase persists over a prolonged period of time, feeds, may
reproduce asexually and/or forms a multicellular organism which eventually produces
more gametes through meiosis. This life cycle is found in animals. It name is derived
from the fact that gametes are the direct product of meiosis.
2. Zygotic Meiosis. The only diploid cell in the life cycle is the zygote. This cell
undergoes meiosis to produce haploid cells which feed, reproduce asexually, and/or grow
into multicellular entities. Eventually the haploid stage will produce gametes (through
mitosis rather than meiosis) which unite to form a new zygote. This type of life cycle is
found in most green algae and in most fungi (usually in a highly modified form). It’s
name is derived from the fact that the zygote is the stage in the life cycle that undergoes
meiosis.
3. Sporic Meiosis. Both the diploid and haploid phases of the life cycle are long-lasting
and generally multicellular. The diploid phase (called the sporophyte) produces haploid
spores through meiosis. Each spore grows into a haploid organism called a gametophyte.
The gametophyte produces gametes through mitosis. Two gametes unite to produce a
diploid zygote which grows into a new sporophyte. This type of life cycle is typical of
land plants (Embryophytes). It’s name is derived from the fact that spores are the product
of meiosis. It is also known as Alternation of Generations referring to the sporophyte
(diploid) and gametophyte (haploid) generations.
Organisms exhibiting sporic meiosis can be further broken down based on the relative
sizes of the gametophyte and the sporophyte. If the gametophyte and sporophyte are
essentially identical in morphology then the life cycle is said to be isomorphic (e.g. some
green algae). If the gametophyte and the sporophyte are different in size then the life
cycle is said to be heteromorphic with either a dominant sporophyte (e.g. vascular plants)
or a dominant gametophyte (e.g. non-vascular plants).
It is also possible to have a life cycle of a unicellular organism that alternates between
gametes and zygotes. Generally one of the haploid or the diploid stage is longer-lasting
and/or more ecologically active (e.g. feeds) than the other and unicellular organisms are
classified as having either zygotic or gametic meiosis.
The selective advantages of different types of life cycles is a fascinating but sparingly
investigated subject.
*Embryophyta. The 'land plants' are the most diverse group of photosynthetic
organisms and the dominant group on land. As a group they are characterized by having
heteromorphic sporic meiosis, multicellular gametangia (gamete producing structures)
enclosed within sterile jacket cells, embryos (a juvenile multicellular stage enclosed
within the maternal gametophyte), and a large nonmotile female gamete and a small
motile female gamete (oogamous). The Embryophyta consists of of three groups of
nonvascular plants: the Anthocerophyta (hornworts), the Hepatophyta (liverworts), and
the Bryophyta (mosses) as well as the Tracheophyta (vascular plants). Relationships
among these four groups are uncertain and will be discussed at the end of the section on
the Bryophyta.
Invasion of the Land. Although the embryophytes are not the only photosynthetic
organisms to live on land and many of the embryophytes live in the water the title 'land
plants' is appropriate. Algae that live on land are only found in moist microhabitats or are
dormant except when it is wet. Life on land has a number of advantages over the aquatic
environment but also some additional challenges. Air contains a much greater
concentrations of oxygen and carbon dioxide than water. In water light is only available
at all relatively near the surface and the intensity of light drops off rapidly as it passes
through water. Light is much more available in the air. However, in water plants are
buoyant and and can float. Also water and nutrients are available through the aquatic
environment and specialized structures are not required for nutrient acquisition. On land
water and nutrients are generally unavailable in the atmosphere although they can be
abundant in the soil.
The nature of the terrestrial environment requires that land plants have specialized
structures for specific functions. Land plants are divided into two fundamental sections:
roots and shoots. Roots are specialized for acquiring water and nutrients from the soil.
Shoots are specialized for gas exhange and photosynthesis. Shoots are further sudivided
into stems and leaves. Leaves actually perform photosynthesis and gas exchange while
stems provide support to raise leaves above the substrate in order to maximize exposure
to sunlight.
Nonvascular plants lack true roots and the lack of a vascular system restricts the size of
shoots. As a result they typically grow very close to the substrate and live in moist
environments. Vascular plants are able to occupy a much greater range of terrestrial
habitats because they have a true root/shoot system.
Embryophyte Life Cycles. All embryophtyes have life cycles with heteromorphic
sporic meiosis. The sporophyte has sporangia (spore producing structures) that produce
spores through meiosis. Each spore will develop into a haploid gametophyte. The
gametophyte produces two kinds of gametangia (gamete producing structures), antheridia
(which produce male gametes) and archegonia (which produce female gametes). A male
gamete travels to an antheridium and unites with a female gamete to form a zygote. The
zygote is nourished by the gametophyte and develops into an embryo. The embryo
eventually grows into a sporophyte.
The preceding life cycle is homosporous, i.e. there is only one kind of spore. Therefore
there is only one kind of gametophyte with both antheridia and archegonia. This kind of
life cycle is common in the seedless plants. All seed plants and some seedless plants
have heterosporous life cycles. The sporophytes have two kinds of sporangia
(megasporangia and microsporangia) which produce two kinds of spores (megaspores
and microspores). the two kinds of spores produce megagametophytes and
microgametophytes respectively, The megagametophytes have archegonia and the
microgametophytes have antheridia. In other respects the life cycle is similar to
homosporous organisms.
Introduction to Nonvascular Embryophytes. Like the tracheophytes, the nonvascular
plants have antheridia and archegonia and embryos that are dependent on the female
gameotphyte. The three groups of nonvascular plants share one major trait that is not
found in the tracheophytes. They all have a dominant gametophyte (i.e. the largest phase
of the life cycle is haploid). For many other characteristics they are highly variable (both
within and among groups). Some have stomata (openings for gas exchange) while others
do not. Some have tissues for conducting water and nutrients while others do not. In
species without conducting tissue, water is absorbed over the entire surface of the
organism or carried externally through wick action. The leaves of nonvascular plants
(when present) are only a single layer of cells thick and the root-like structures (rhizoids)
serve only to anchor the plant to the substrate (no uptake of nutrients).
The nonvascular plants are much less diverse and conspicuous than vascular plants.
However they have one major advantage over the vascular plants; they can grow on
substrates from which no nutrients are available through roots (because nonvascular
plants do not obtain nutrients or water through roots). Mosses and liverworts are the
dominant groups of land plants growing on the surfaces of rocks and on the trunks,
branches, and leaves of vasuclar plants (epiphytes). Forests not only provide many
substrates for nonvascular plants but they also generally have higher levels of humidity
than other habitats. The extremely wet forests found in some coastal temperate regions
(e.g. northwestern North America) and montane tropical regions have the greatest
diversity and abundance of nonvascular plants (especially liverworts) of anywhere on
earth.
Nonvascular plants are also a dominant part of the flora in the tundra of high mountains
and the polar regions. Nonvascular plants also do well in moist habitats such as the edges
of streams and other bodies of water (these areas typically have high humditiy and the
plants are splashed or flooded frequently). For reasons that are not clear, they are not
abundant in completely aquatic habitats. Nonvascular plants are found in drier habitats
such as deserts and grasslands but are typically dormant except for periods of heavy rain.
*Hepatophyta. These organisms are known as liverworts due to supposed resemblence
of some taxa to the human liver. In the middle ages these plants were thought to be a
treatment for liver ailments. Like all nonvascular embryophytes the gametophyte
(haploid stage) is dominant and the sporophyte is small and permanently attached to the
gametophyte. The gametophyte is a fairly small plant that grows low to the ground, on
rocks, or on other plants. There are five to six thousand species of liverworts. As a
group they are restricted to moist habitats and are the most abundant and diverse in the
tropics.
Liverwort gametophytes are either leafless, flattened and irregular in shape or consist of
leaves attached to stems (resembling mosses). The former are known as thallose
liverworts and the latter as leafy liverworts. Leafy liverworts differ from mosses in
lacking any tissue differentiation in their stems, and a midrib on their leaves. They also
have leaves arranged in sets of three, two equal in size and a third, smaller leaf.
Liverworts appear to be the only embryophytes to lack stomata (opening in the surfaces
of leaves or stems to allow for gas exchange for photosynthesis) and any kind of
conductive tissue. In thallose hepatophyta the entire upper surface of the gametophyte is
used in photosyntheses. Thallose liverworts often have airspecies in the middle of the
thallus that is accessible through a single pore.
Most liverworts (90%) have unisexual gametophytes (either male or female) even though
they do not have two kinds of sporangia. In the most ancestral taxa the gametangia are
exposed. In leafy forms they occur within a protective covering at the end of a stem or
along the side. In thallose species the gametangia are either embedded in the thallus or
(in the Marchantiales) on stalks. The antheridia produce sperm that require rain drops to
swim into an archegonia. After fertilization the archegonium becomes enlarged and
forms a structure called a calyptra in which the sporophyte develops. The sporophyte has
a foot which attaches it to the calyptra, through which it recieves nutrients. Otherwise,
the calyptra is basically a mass of tissue that produces spores through meiosis. The
hepatophytes are unique among the embryophytes in having sporophytes that remain
permanently enclosed within the gametophye.
*Anthocerophyta. These plants are called hornworts, a reference to the shape of their
sporophytes. There are only around 100 species in this taxon. The hornworts all have
gametophytes that resemble those of thallose liverworts. The antheridia and archegonia
are embedded in the surface of the thallus similar to many taxa of hepatophytes. The
anthocerophyta also share the trait of unicellular rhizoids with the liverworts.
The hornworts differ from the liverworts and all other plants in several important
respects. The sporophytes is an erect spike growing from the surface of the gametophye.
Unlike the sporophytes of mosses and liverworts the sporophyte of hornworts contains
photosynthetic tissue and merisematic tissue (at the base). Therefore the sporophyte is
capable of providing its own nutrition and of indefinite growth. However it remains
permanently attached to the gametophyte.
The gametophytes of Anthocerophyta also have a number of unique traits. They have a
cavity inside the thallus but it is filled with mucilage instead of air (as in Marchantia).
Anthoceropyta have a single large chloroplast per cell, a trait typical of green algae not
found in any other taxon of embryophyte. They also have pyrenoids, an organelle used to
synthesize starch, another algal trait. The archegonia are not discrete structures but
masses of undifferentiated tissue below the neck cells. Most anthocerophyta are
monoecious (having both antheridia and archegonia on the same gametophyte) although
typically both kinds of gametangia are produced at different times.
*Bryophyta. These are the mosses, the most diverse and successful group of nonvascular plants with about 10,000 species. They are found in most terrestrial habitats,
especially those in which water is abundant. In arid environments such as the desert
around Phoenix mosses are able to persist by remaining dormant during periods of
drought. Mosses commonly grow on rocks or on other plants.
Moss gametophytes are generally similar to leafy liverworts except that they have
stomata, almost all species have water conducting cells (hydroids), some species have
nutrient conducting cells (leptoids), they have multicellular rhizoids, and the leaves
typically have a thickened midrib and are all equal in size. The gametophyte consists
horizontal stems attached to the substrate with rhizoids and aerial stems that grow
upwards from the horizontal stem. The antheridia and archegonia are located on the ends
of individual aerial stems or the ends of side branches surrounded by leaves. Depending
on the species the antheridia and archegonia may be on the same stem, on separate stems
on the same plant, or on different individual plants. After fertilization the sporophyte
grows up out of the archegonia, unlike hepatophytes where the sporophyte reamains
enclosed. The sporophyte consists of a stalk and a capsule that contains the results of
meiosis; the haploid spores. The top of the capsule is a lid-like structure called the
operculum. When the spores are mature the operculum opens and releases the spores.
The spores develop into filamentous structures called protonemata. The protonemata
mature into gametophytes. Variation in sporophyte structure divides the mosses into
three major groups.
The Bryidae are the true mosses and include the vast majority of the Bryophyta. In this
taxa the stalk is a part of the sporophyte called the seta. The operculum is attached to the
rest of the capsule by a toothed structure known as the peristome. As capsule matures the
peristome dries and the operculum slowly disengages from the capsule releasing the
spores. One taxon of bryids, the Polytrichidae has gametophytes with leptoids, other
groups have sporophytes with leptoids. Most bryids have dioecious gametophytes and
the ones that are monoecious typically have antheridia and archegonia that mature at
different times.
The other two groups of mosses are the Sphagnidae and Andreaeidae. The Sphagnidae
are the sphagnum mosses which are typically found in bogs. They have leaves with no
midribs and they lack rhizoids, hydroids, and leptoids. The stalk of the sporophyte is
actually gametophyte tissue and the capsule lacks an operculum. The capsules open
explosively dispersing the spores over a large area. The Andreaeidae are the granite
mosses. They have sporophytes which usually lack stalks, leptoids, and hydroids. They
have lobed protonemata and typically grow on granite substrates usually in the arctic and
high in mountains.
Phylogeny of nonvascular and vascular embryophytes. The relationships among the
three groups of nonvascular plants and the Tracheophytes are not well understood. It is
generally agreed that the nonvascular plants are not a monophyletic group but rather that
some of them are more closely related to the tracheophytes than others. The mosses are
the group that is the most similar to the Tracheophytes in structure. It is unclear if the
similarities between these two groups indicate a close relationship or convergence. The
hepatophytes are the group with the simplest morphology. It is unclear if this represents
an ancestral state or an evolutionary reduction in complexity. The Anthocerophyta have
the most complex and independent sporophytes of the nonvascular plants but they also
retain several algal characteristics not found in any other embryophytes.
A large number of possible trees have been suggested. We will discuss several different
options (a subset of those that have been proposed). Option 1: The Anthocerophyta are
the sister taxon to all the other embryophytes. The hornworts are the most distinctive
group of land plants, retaining a number of algal characteristics and having a unique
sporophyte. This option requires that the meristematic tissue and photosynthetic abilities
of both hornwort and tracheophyte sporophytes either evolved twice or were secondarily
lost in liverworts and mosses. If we assume that option 1 is true then either the liverworts
and the mosses jointly form a monophyletic sister taxon to the vasuclar plants (option
1A) or the liverworts form a sister taxons to a monophyletic group consisting of the
mosses and the vascular plants (option 1B). Option 1A requires either than the
conductive structures of mosses and vascular plants evolved independently or that the
simple morphology of the liverworts is a case of evolutionary loss of traits rather than
retention of ancestral traits. Option 1B does not require either of these conditions (except
for traits such as stomata which are present in the hornworts but absent in the liverworts).
Option 2: The Hepatophyta are the sister taxon to the rest of the embryophytes. This
allows all of the simple morphological traits of the liverworts to be ancestral in nature. It
also requires that the liverworts lost the algal characteristics found in the hornworts in a
separate evolutionary event from other embryophytes. Option 2A: The hornworts are the
sistergroup to a monophyletic moss/vascular plant clade. This requires 2 losses of the
ancestral algal traits of hornworts (once in the liverwort ancestor and once in the
moss/vascular plant ancestor) and two separate evolutions of photosynthetic,
meristematic sporophytes (once in hornworts and once in vascular plants). Option 2B:
The mosses are the sister taxon to a monophyletic hornwort/vascular plant clade. This
requires three separate losses of the algal traits found in hornworts (in liverworts, mosses,
and in vascular plants) but only one evolution of photosynthetic, meristematic
sporophytes.
*Tracheophyta. The majority of embryophytes have a vascular system. Not only are
there 250,000 plus species of vascular plants compared to less than 20,000 species of
nonvascular plants but the vascular plants attain a much greater range of sizes and live in
a much wider range of habitats. Vascular systems allow plants to attain much greater
heights than they could otherwise. There are two reasons for this. The vascular system
provides support for the stem allowing plants to grow tall without falling over. More
importantly, the vascular system allows for transportation of nutrients and water between
the roots and the shoots. Therefore photosynthesis can occur at a location remote from
the substrate.
The stem and roots of a tracheophytes contains a stele, a bundle (or bundles) of vascular
tissue that runs the length of the plant. In roots and in the stems of a few vascular plants
the stele is a single solid bundle known as a protostele. In other stems the stele consists
of a tube of vascular tissue with some other cell type (or acellular material) in the middlle
(pith). This is known as a siphonostele. In most vascular plants the stele consists of a
number of separate bundles each running the length of the stem. This is a eustele.
All vascular plants have a dominant sporophyte generation that is capable of
photosynthesis and existence independent of the gametophyte. Seedless vascular plants
have freeliving gametophytes with motile male gametes. Although these plants can
survive as sporophytes in a wide range of environments they require free water for sexual
reproduction (sperm needs water in order to swim to the archegonium). Seed plants have
a female gametophyte that is enclosed and nourished by the parental sporophyte. Free
water is no longer required for reproduction.
The Tracheophyta is generally thought to be a monophyletic group (i.e. the vascular
system only evolved once). There are two distinct sister groups within the vascular
plants. The Lycophyta (club mosses and their relatives) are one group and all the other
seedless vascular and seed plants make up the other. A few studies have indicated that
the Lycophyta may have a separate evolutionary origin from the rest of the Tracheophyta
(i.e. the sister taxon to the Lycophyta is some group of nonvascular plants) but this is not
a widely held position.
Fossil Seedless Vascular Plants. The fossil record of the earliest plants is very
incomplete. Fossil land plants are known from the upper Silurian (400-410 million years
ago) but they are fragmentary and the nature of the plants is unknown. There are several
well known fossil vascular plants known from the Devonian. Because they are small and
soft-bodied nonvascular plants are poorly represented in the fossil record.
The early (lower) Devonian flora (390-400 million years ago) consists of vascular plants
that are divided into three groups: the Rhyniophyta, the Zosterophyllophyta, and the
Trimerophyta. The few species in these early groups with known reproductive cycles had
identical gametophytes and sporophytes. The Rhyniophyta consisted of fairly small
plants (the largest attained heights of 0.5 m and most were considerably smaller) with
horizontally growing branches (rhizomes) that produced aerial stems. The aerial stems
were leafless, branched dichotomously (i.e. the two branches were equal, there was no
side branch or main branch), and had sporangia at the ends of the branches. The
Trimerophyta were similar to the Rhyniophyta except that they were larger (up to 1 m
tall) and had unequal branching giving rise to main stems with side branches. The third
group, the Zosterophyllophyta were superficially similar to the other two but they
differed in two important respects: their sporangia were born laterally (along the sides of
the aerial stems) instead of terminally (at the end of stems) and their xylem (vascular
cells) matured in the exterior of the stele first and the interior of the stele later (opposite
to the other groups). It is possible that the Zosterophyllophyta represent a completely
independent evolution of the vascular system in plants from the other two groups but
there is insufficient evidence to support or refute this hypothesis.
By the upper Devonian and lower Carboniferous (340-360 million years ago) the vascular
plants had greatly diversified. The three main groups of extant seedless vascular plants
(fern, horsetails, and lycophytes) were in existence as well as a number of extinct forms.
The climate of the earth at this time was mostly moist and warm and these seedless plants
evolved into very large treelike forms that formed great forests.
Extant Seedless Vascular Plants. There are 3 or 4 different clades of seedless vascular
plants. The Lycophyta, as mentioned above, are the most distinct group representing a
lineage that diverged from the main line very early in the evolution of vascular plants.
The Pterophyta and the Sphenophyta are both related to the seed plants (Spermophyta).
It is not certain which of these is most closely related to the seed plants or if they form a
monophyletic group. The final group is the enigmatic Psilophyta which was once
thought to be related to the earliest vascular plants but which many now consider to be a
group of unusual ferns.
Lycophyta. This group of about 1,000 species contains the club mosses and the
quillworts. As in the earliest vascular plants, the sporophyte of the club mosses
(Lycopodium and Selaginella) has an underground stem called a rhizome. Individual
adventitious roots obtain nutrients from the soil and tranfer them to the rhizome. Aerial
stems bearing leaves and sporangia grow vertically off the rhizome. In the Lycophyta the
leaves are microphyllous. A club moss superficially resembles a large bryophyte
although closely related taxa attained the size of large trees in the Carboniferous. Each
leaf contains a single vascular strand so there is only one vein. As in the
Zosterophyllophyta the sporangia are born laterally and the xylem matures from the
outside of the stele to the inside. It seems very likely that the Lycophyta and the
Zosterophyllophyta are very closely related and that the divergence of the Lycophyta
from the other extant tracheophytes dates back to the earliest days of the vascular plants
In Lycopodium the sporophytes are homosporous, that is there is one kind of spore. The
spores are released from stobili, conelike structures composed of sporophylls. A
sporophyll is a leaf with a sporangium attached to it. Each spore has the potential to
develop into a gametophyte. As there is only one kind of spore, there is only one kind of
gametophyte bearing both antheridia and archegonia. The gametophyte can either be
photosynthetic and occur on the soil surface or live underground and feed with the
assistance of mutualistic fungi. Sperm swim though rain water, dew, or some other
external water source from the antheridia to the archegonia and fertilize the eggs. The
embryo develops within the archegonium and then the young sporophyte grows directly
out of the gametophyte. The 200 or more species of Lycopodium grow mostly in moist
habitats, primarily in the tropics. Many species are epiphytes (growing on other plants)
or grow in bogs. The various species of Lycopodium all have protosteles with
considerable variation in the arrangement of xylem and phloem.
Selaginella (700 species) is superficially very similar to Lycopodium except that it has a
heterosporous life cycle. The strobili contain two distinct kinds of sporangia
(megasporangia and microsporangia) which produce megaspores and microspores
respectively. The megaspores develop into megagametophytes which contain antheridia.
The microspores develop into microgametophytes which are generally hollow, spermproducing capsules. In other respects, the life cycle is similar to that found in
Lycopodium. Selaginella is ecologically similar to Lycopodium except that few species
grow as epiphytes and quite a few species grow in arid environments. Selaginella is the
most anatomically diverse taxon within the Lycophyta. Most species have protosteles but
some have siphonosteles or eusteles.
The third major taxon of lycophytes is the quillworts, members of the genus Isoetes
which contains over 70 species. These plants superficially resemble a grass or sedge and
are usually found in aquatic or semi-aquatic habitats. The anatomy of this plant is unlike
that of any other seedless plant. The only leaves are the sporophylls which do not form
strobili. The sporangia occur in the thickened bases of the sporophylls. All the
sporophylls on a plant are attached to a structure known as a corm. This corm contains a
small central core of vascular tissue (a protostele). Surrounding the stele is a large area
of cortex tissue that is lost and regrown every year. Vertical growth is extremely slow
and the corm is typically wider than it is tall. Isoetes is heterosporous and molecular
analyses strongly support the hypothesis that Isoetes and Selaginella are sister taxa with
Lycopodium as the basal lycophyte group. All three of these taxa are found throughout
much of the world. There are two other monotypic (single-species) taxa: Phylloglossum
(found in Australia and New Zealand) and Stylites (found in the Andes) which are closely
related to Lycopodium and Isoetes respectively.
Psilophyta. This taxon has a very uncertain phylogenetic position. The two genera of
plants within the Psilophyta are extremely distinctive and they have often been assumed
to be related to the Rhyniophyta, the earliest known vascular plants. Some morphologists
have asserted that the simple nature of the Psilophyta is derived rather than ancestral and
that they are a group of unusual ferns, a hypothesis supported by molecular data.
The two genera are Psilotum which is found throughout the tropics and subtropics of the
world and Tmesipteris which is found in Australia and elsewhere in the southern Pacific.
The total number of species is uncertain but is probably no more than ten. These plants
are unique among vascular plants in lacking both leaves and roots. Psilotum sporophytes
consist of colorless rhizomes that bear photosynthetic aerial stems. The rhizomes are
attached to the substrate by rhizoids similar to those in nonvascular plants. The aerial
stems bear small scale-like structures. Unlike leaves these structures have no vascular
connection to the stem. Paired sporangia are found at the end of some branches. The
gametophytes are subterranean and strongly resemble portions of the sporophyte
rhizome. The antheridia and archegonia are found on the same individual gametophytes.
The life cycle is essentially the same as in Lycopodium.Tmesipteris is very similar to
Psilotum except that its leaf-like structures are considerably larger (up to 2 cm long) and
they have vascular connections to the stem.
Because of their similarity to the Rhyniophytes many evolutionary botanists have looked
with great interest at the Psilophyta for clues as to the nature of the earliest land plants.
This is premature, given the uncertainty of the true evolutionary position of this group.
Sphenophyta. These are the horsetails (Equisetum). Similar to the club mosses, these
are only relatively small plants today but formed large trees in the Paleozoic. There are
only about 30 species of Equisetum extant today but they are found throughout the world
except for the Australian region. The horsetails are found in moist habitats such as
marshes. The Sphenophyta clearly belongs to the same lineage as the ferns and the seed
plants but the relationship between these three groups in unclear.
All horsetails are quite similar to one another. The sporophyte has a horizontal rhizome
with adventitious roots, similar to those found in Selaginella and Lycopodium. The aerial
stems are highly distinctive, being unlike those of any other plants. The stems are
segmented hollow tubes, with a whorl of very small leaves at each node (joint) on the
stem. The leaves are not capable of photosynthesis which is performed by the stems.
Many fossil sphenophytes have large complex leaves indicating the simple nature of
Equisetum leaves is probably a derived trait. The aerial stems may be branched or
unbranched. The stele is a siphonostele surrounding the hollow core. The sporangia are
organized into cone-like strobili at the ends of the branches.
The life cycle is homosporous in that there is only one kind of sporangium and one kind
of spore. However some of the spores develop into male gametophytes containing only
antheridia and the rest develop into female gametophytes containing only archegonia.
The female gametophytes eventually develop antheridia and lose their archegonia
becoming male. The proportion of each sex of gametophyte is influenced by factors such
as light and crowding. The gametophytes are flattened structures that are capable of
photosynthesis.
Pteridophyta. These are the ferns. The pteridophytes are by far the most diverse group
of seedless vascular plants, in terms of both number of species (12,000) and ecology.
The basic structure of the fern sporophyte is similar to that of the other seedless
tracheophytes. A horizontal rhizome is attached to the substrate with adventitious roots
and gives off aerial stems with leaves and sporangia. The leaves of ferns are
megaphyllous, they have multiple vascular strands, giving rise to many veins per leaf.
Most ferns have large compound leaves called fronds. Typically there is only one frond
per aerial stem. The vascular system is highly variable.
Ferns differ strikingly from other vascular plants in the organization of their reproductive
structures. All other vascular sporophytes have strobili, groups of specialized leaves (or
branches) that bear sporangia (flowers are strobili). In most ferns the sporangia are
scattered in small bundles called sori on the surfaces of the leaves. There is no
distinction between sterile leaves and fertile leaves (sporophylls) and no organization of
leaves into larger reproductive structures. The sporangia within a sorus may be gradate
(all developing at the same time) or mixtate (developing at different times).
The gametophytes of ferns are known as prothalli. They are usually flat, heart-shaped,
photosynthetic structures. Most ferns are homosporous but the prothalli often have only
antheridia or archegonia. The young sporophyte in some taxa often develop a true root
briefly but this is alway superseded by the rhizome, adventitious root system typical of
seedless tracheophytes.
Most of the ferns belong to single large taxon, the leptosporangiate ferns. These ferns
have small thin-walled sporangia, each derived from a single cell that produce a set
number of spores (no more than 128). There are two clades of ferns (possibly three if the
Psilophyta are actually ferns) that are basal to the leptosporangiate ferns. These are
known as eusporangiate ferns. They have massive, thick-walled sporangia derived from
multiple cells. Each sporangium can produce thousands of cells. The two taxa of
eusporangiate ferns are the Ophioglossales and the Marattiales.
Ophioglossales. Most of the eighty species in this clade are in two widespread genera:
Ophioglossum (Adder's Tongue) and Botrychium (grape ferns). These ferns are almost
unique in having leaves divided into lower sterile portions and fertile upper portions with
spikes of sporangia. The prothalli are subterranean and associated with fungi and can be
as large as 6 cm long.
Marattiales. This group consists of about 200 species of large tropical ferns. The large
sporangia are organized into sori which are spread over the surface of the leaves. The
gametophytes are large, long-lived, and photosynthetic, resembling thallose liverworts.
Leptosporangiate Ferns. As described above these are the great majority of the ferns.
The most basal group of leptosporangiate ferns is Osmundales. This group is
intermediate in several respects between the eusporangiate and the other leptosporangiate
ferns. The leaves are divided into sterile and fertile portions. The sporangia are initiated
by more than one cell and they are relatively large. The number of spores is large for a
leptosporangiate sporangium (128 or 256) but much smaller than the thousands produced
by a eusporangium. The prothallus is similar to that of the Maratialles.
The rest of of the leptosporangiate ferns are divided into a large number of taxa, based in
many cases on the structure of the sori, particular the indusia (the structures that cover
and protect the sporangia). These ferns live in a wide variety of habitats, including
deserts, grasslands, and as epiphytes as well as the more typical fern habitats of forests
and swamps.
Other taxa that are thought to be relatively basal within the leptosporangiate clade include
the Schizaceae which lack sori (the sporangia occur singly) and (in some species) have
filamentous or underground prothalli, the Gleicheniaceae which have gradate sporangia
organized into sori with no indusia and large, slow-growing gametophytes, and the
Hymenophyllaceae which have gradate sporangia and a thin strap-shaped or filamentous
thallus.
Two other important groups of basal leptosporangiate ferns are the tree ferns and the
aquatic heterosporous ferns. The tree ferns are found primarily in very moist tropical and
mild temperate habitats. These are the only extant seedless plants to attain tree size. The
trunks of tree ferns are not the product of secondary growth (i.e. they are not composed of
wood) but are largely composed of masses of adventitious roots. The true stem is a
slender structure in the center of the trunk. The water ferns include the Marsileales and
the Salvintales. Both groups are heterosporous with microsporangia and megasporangia.
In both groups the sporangia are encased within hard, bean-like structures called
sorocarps near the base of the plant. The sorocarp of the Marsileales (which are typical
rooted plants) is derived from many sori while the sorocarp of the Salvintales (which are
floating plants) is derived from a single sorus.
Most of the leptosporangiate ferns belong to a large clade known as the ‘polypodiaceous
ferns’. These ferns have mixtate sori and share a number of sporangial traits. The
indusia have been lost in some taxa and in a few cases the sporangia are no longer
organized into sori but cover the undersides of the leaves in a felt. Most of the ferns in
North America belong to this clade. One family of ‘polpodiaceous ferns’, the Pteridaceae
contains the arid-adapted ferns that are the most diverse group of seedless vascular plants
in Arizona.
Spermophyta (Seed Plants) The evolution of seeds removed the need for free water in
sexual reproduction in plants. Seeds also provide nutrition for the female gametophyte
and the young sporophyte.
All seed plants are heterosporous. The microgametophytes are pollen grains and the
megagametophytes are permanently encased within the megasporangium. The
megasporangium, megaspores and megagametophytes are all contained within an
integument and are collectively known as an ovule. After fertilization of an egg the ovule
becomes a seed.
The mega- and microsporangia are organized into strobili. The strobili can have the
typical pine cone-like shape previously seen in the Lycophyta and the Sphenophyta or
they can be highly variable in appearance (e.g. flowers). If an individual sporophyte has
strobili with only one kind of sporangia than the plants are said to be dioecious (separate
sexes). If a sporophyte has both microsporangia and megasporangia but they are found in
separate strobili then the plant is said to be monoecious. If a sporophyte has both
megasporangia and microsporangia in the same strobili then it is said to be synoecious
(hermaphroditic). All three types of plants are known in the Spermophyta and in some
cases more than one type is found within a species.
The microsporangia have microsporocytes (microspore mother cells) which produce
microspores through meiosis. Each microspore then develops into a microgametophyte
(a pollen grain). The ‘typical’ pattern of non-flowering seed plants is for the pollen grain
to consist of four cells: two prothallial cells, a tube cell, and a generative cell. Variations
will be noted under specific taxa.
The megasporangia produce megaspore mother cells (megasporocytes) which produce
megaspores through meiosis. Typically only one megaspore is retained after meiosis.
The megaspore develops into a megagametophyte which typically has several archegonia
(e.g. 2 or 3 in pines).
A pollen grain lands on the micropyle, an opening in the integument at the top of the
ovule. The micropyle has a drop of fluid in it which causes the pollen to adhere to the
ovule. The tube cell then grows down into the ovule and into the archegonia. The
generative cell then develops into two cells one or both of which may function as sperm.
These cells travel down the tube and fertilize the egg.
After fertilization the ovule becomes the seed. The integument and (in some cases) other
portions of the ovule form the seed coat. The interior of the cell contains the remains of
the megasporangium (the nucellus) and the megagametophyte as well as the zygote. The
zygote divides several times and one layer of cells forms the embryo and another forms
the suspensor. The embryo develops into an elongate structure with an epicotyl end
containing one or more cotyledons. The cotyledons are embyonic leaves which serve as
nutritional sources while in the seed and are the first photosynthetic organs of the
seedling plant. The other end of the embryo is the hypocotyl, the embryonic root.
There are five groups of extant seed plants: the Cycadophyta, the Gingkophyta, the
Coniferophyta, the Gnetophyta, and the Anthophyta (Angiosperms). The first two are
ancient groups with extensive fossil records and very small numbers of extant species
with restricted distributions. The Coniferophyta are also an ancient group with relatively
few extant species but they are an ecologically successful group today with a very wide
geographic distribution. The Gnetophyta are an enigmatic group with a poor fossil record
and a small number of very divergent taxa. The angiosperms represent almost 90% of
extant embryophyte species and are by far the most abundant plants in most terrestrial
habitats.
Fossil Seed Plants The seed plants first appeared in the Carboniferous during the era of
great forests of seedless plants. The probable ancestors of modern seed plants were the
progymnosperms which first appeared in the late Devonian. These plants had vegetative
structures similar to seed plants (secondary growth) but reproduced using spores. The
Pteridospermophyta (seed ferns, seed plants which probably are not related to any extant
groups) and the Cordaitales (basal members of the conifer lineage) are two groups of seed
plants that appeared in the Carboniferous. In the cooler and drier Permian era the
seedless plants declined and the seed plants became dominant. Two extant groups the
cycads and the Gingkoales first appeared at this time. The seed ferns persisted into this
era as did the Cordaitales. Several now extinct groups also appeared in the Permian such
as the Voltziales (probable ancestors of modern conifers) and the Cycadeodiales (a group
similar in appearance to the cycads but which may not have been closely related to them).
The Triassic and Jurassic eras were dominated by the cycads and the Cycadeodiales. The
modern groups of conifers appeared in the Jurassic and became the dominant plants by
the beginning of the Cretaceous. The Gingkoales also increased in abundance in the
Jurassic and early Cretaceous. The angiosperms first appeared in the early Cretaceous
and by the end of that era they were the dominant group of land plants and all other seed
plants had declined dramatically.
Cycadophyta The cycads are palm-like plants with a typically unbranched trunk
crowned by an apical cluster of large compound leaves. There are almost 200 species of
cycads found in tropical and subtropical regions throughout the world. Most species have
restricted geographic distributions although they may be locally abundant where they are
found. Areas of particularly high cycad diversity include southern Mexico, southern
Africa, and Australia.
The largest cycads can attain heights of close to 20 m but most are considerably shorter.
The stems of cycads are columnar (same diameter from top to bottom) and most of the
stem thickening occurs due to a primary thickening meristem similar to that found in
palms (see sections on angiosperm anatomy). Some true secondary growth also occurs
but it is of minor importance. Cycads are slow growing, very long-lived plants. The
wood is manoxylic - it has relatively large xylem cells and a lot of non vascular tissue
(pith). Cycad wood is therefore relatively soft. Most woody plants have pycnoxylic
wood, with smaller xylem cells and less wood. Many basal angiosperms have manoxylic
wood as well.
Cycads are dioecious with separate male (microsporangiate) and female
(megasporangiate) sporophyte plants. The strobili (pollen cones and seed cones) are
large conelike structures that are similar to the strobili of seedless plants such as
Equisetum and Selaginella except for their size and the presence of ovules. The
megastrobili (seed cones) of some species can reach a meter in length and are among the
largest reproductive structures of any plant. In most cases the strobili are produced at the
apex of the plant but in some cases they are produced laterally. Each megasporophyll is a
simple leaf and usually bears two (up to ten in the genus Cycas) ovules. The
megagametophyte which forms within the ovule is a large structure up to 5 cm long
containing many starch filled cells. A megagametophyte contains between two and eight
archegonia depending on the genus. The pollen grain (microgametophyte) is four celled
as described in the introductory section on seed plants. Insects are thought to be the most
important pollinating agents (mostly beetles) for cycads although wind pollination may
also pay a role. The most novel aspect of cycad reproduction is that the sperm are motile,
multiflagellated cells unlike most seed plants. Most cycads have two cotyledons (are
dicotyledonous) but some have one or three.
Cycads first appeared in the Permian and were one of the dominant group of plants in the
Triassic and Jurassic along with the similar appearing Cycadeoids. There are currently
only eleven genera of living cycads comprising slighlty less than 200 species. Five of the
genera and slightly over one third of the species are restricted to the tropical subtropical
regions of the new world with the genus Zamia making up over two thirds of the total
number of species. Three genera are restricted to Australia with most of the species in
Macrozamia. Two genera are restricted to Africa with almost of all those species in
Encephalartos. The remaining genus is Cycas which contains over forty species and is
found in eastern Asia and northern Australia with one species in Madagascar. It contains
the commonest species in cultivation, Cycas revoluta, which is native to southern Japan.
Gingkophyta The Gingkophyta are an ancient group first known from the Permian and
with an extensive fossil history in the Mesozoic. The only known extant species, Gingko
biloba, was discovered by western scientists growing in temple gardens in China and
Japan in the nineteenth century. No wild populations of G. biloba have ever been found
but it has been widely established around the world (particularly in urban areas in
temperate regions).
Gingko biloba is tree that superficially resembles a woody deciduous angiosperm,
attaining a height of 30 m. It has vigorous secondary growth producing hard pycnoxylic
wood similar to that found in conifers and in many angiosperms. The leaves are fan
shaped with parallel veins that radiate out from the petiole (the 'stem' of the leaf) and are
shed during the winter. The plants are dioecious. Microsporangiate (i.e. male) plants
have small microstrobili that hang down from branches and resemble catkins (reduced
flowers of wind-pollenated trees). The ovules occur in pairs and each ovule is
surrounded by a fleshy collar of tissue. The collar and ovule resemble a fruit but the the
tissue does not form an ovary (see angiosperm reproduction section) and it does not
develop further after pollination.
The reproductive cycle of Gingko is similar to that of the cycads with a large
megagametophyte and flagellated sperm. The pollen tube is a highly branched structure
unlike most other seed plants. The embryo is dicotyledonous. The megagametophyte
has either one or two archegonia. Gingko is wind pollinated.
Coniferophyta The conifers are by far the most diverse and speciose group of seed
plants outside of the angiosperms. There are around 600 species found in the temperate
regions of the northern and southern hemispheres. All confers are trees or shrubs with
pycnoxcylic wood and vigorous secondary growth similar to Gingko. Conifers include
the largest and oldest individual plants known with redwoods attaining heights of around
100 m and bristlecone pines living almost 5,000 years. In northern boreal and montane
forests conifers such as spruces, firs, and pines are the dominant plants. They are
economically important as sources of timber. While many northern taxa are widespread
plants withy enormous populations there are many other species with restricted
distributions similar to cycads or Gingko. Examples include the Norfolk Island ‘Pine’
and a number of species of pines and cypresses restricted to limited areas in coastal
California.
Most conifers have leaves modified into needles (as in pines and spruces) or into
flattened sprays (as junipers and cypresses) but some have broad leaves similar to those
of angiosperms (e.g. the southern hemisphere genus Podocarpus). The leaves are
typically evergreen and long lasting but a few taxa such as the bald cypress of the
southeastern U.S.) are deciduous. The reduced evergreen leaves are thought to be an
adaptation to reduce water loss in water limited environments (e.g. during the winter in
cold habitats when all the water in the soil is frozen).
The ancestors of the conifers appeared in the Permian and most of the modern families
appeared in the Jurassic or the Cretaceous. The conifers were the dominant group of
plants in the early Cretaceous before being supplanted by the angiosperms.
In most taxa the strobili are conelike structures although in some taxa such as the junipers
the ovules are surrounded by fleshy tissue as in Gingko. Male cones are composed of
microsporophylls as in other groups but the scales of female cones are derived from
entire branches instead of single leaves. Unlike most other 'gymnopserms' the conifers
are mostly monoecious (with Podocarpus as a notable exception), with both male and
female cones on the same plant. The number of archegonia within a gametophyte and the
number of cotyledons per embryo (e.g. seven or eight in pines) is highly variable among
the different families. As far as is known all conifers are wind pollinated.
Gnetophyta. The Gnetophyta consists of three very different taxa of extant plants.
There is a sparse fossil record of these plants from the Mesozoic but the evolutionary
history of these plants is not well understood. Molecular analyses indicate that the
Gnetophyta are a monophyletic group that is the sister taxon to the angiosperms. These
plants do share some characteristics with angiosperms but it is uncertain if these
similarities are the result of common ancestry or convergence.
The three genera include Gnetum a group of about 30 species of vines and shrubs found
in moist tropical habitats in South America, Africa and southeast Asia. These plants
resemble typical tropical dicotyledonous angiosperms except for their reproductive
structures. The second genus is Ephedra which consists of woody shrubs with jointed
photosynthetic stems and highly reduced leaves that occur at the nodes (joints). Ephedra
(mormon tea) occurs worldwide (about 40 species) in arid regions or areas with saline
soil. Superficially, Ephedra, resembles the horsetails, Equisetum, but can be easily
distinguished by the presence of woody, highly branching stems, ovules, seeds, and
pollen in Ephedra which also grows in much more arid conditions than Equisetum. The
final genus is Welwitschia which consists of a single species found in the Namib desert of
southwestern Africa. The sporophyte consists a short wide stem that is mostly
subterranean with a broad concave apex. The stem is connected to a very long taproot.
The apex of the stem bears two enormous straplike leaves. These leaves have active
meristems at their bases and grow thoughout the life of the plant often reaching lengths of
3 m. The ends of the leaves fray and split giving the appearance of multiple leaves.
Plants may live up to 2,000 years.
The Gnetophyta are normally dioecious although some individuals of Ephedra and
Gnetum may have sporangia of both types. All gnetophytes have two cotyledons. It is
not known if insect pollination is important for any of the Gnetophyta. The wood of
gnetophytes is intermediate between the manoxylic and pycnoxylic states and contains a
type of xylem known as vessel members. Vessel members are found in most
angiosperms (missing in some basal taxa) and also in such distantly related taxa as
Selaginella. Ephedra exhibits double fertilization similar to that found in angiosperms
(see section on Angiosperm reproduction) while both Gnetum and Welwitschia have
highly reduced archegonia.
Relationships among Extant Spermophyta (Seed Plants). Traditional systematists
relying on morphological traits and molecular systematists often disagree strongly about
the relationships among the five extant taxa of seed plants. Morphologists have tended to
interpret the similarities of the gnetophytes and the angiosperms in reproductive and
vascular traits as convergence. They point out that the two groups appear to have
diverged early in the Mesozoic long before these traits would have likely appeared in
either group. Many morphologists doubt that the three groups of gnetophyta are even
closely related to one another, which is not surprising given their striking differences.
Some morphologists have postulated a close relationships between the cycads and the
angiosperms based on the similarity of the soft manoxcylic wood of cycads and basal
angiosperms. Molecular analyses strongly and consistently support both the monophyly
of the Gnetophyta and the sister taxa status of the gnetophytes and the angiosperms.
Relationships among the other three groups of seed plants and the gnetophyte/angiosperm
clade are less consistently supported by multiple analyses. The most common hypothesis
is that the cycads are the sister taxon to all the others and Gingko is the sister taxon to the
conifers, gnetophytes and angiosperms. The conifers would then be the sister taxon to
the Gnetophyta/angiosperm clade. Suggested alternatives include having Gingko and the
conifers forming a monophyletic sister taxon to Gnetophyta/angiopserms or that the
conifers may be in paraphyletic taxon. The common factors in all recent phylogenetic
hypotheses are the sister relationship between the gnetophytes and the flowering plants
and the distant relationship of the cycads to all other extant seed plants.
Origin of Flowering Plants. Charles Darwin refered to the origin of angiosperms as a
'damnable mystery' and even today, over a hundred years later, the nature of the earliest
angiosperms and their closest relatives among the other seed plants are not known with
any certainty. Fossils of flowering plants are known from the early Cretaceous and
become abundant by the late Cretaceous. It is likely that flowers first evolved prior to
that, possibly as early as the Triassic, the era in which the angiosprems and the
Gnetophyta diverged.
Several different hypotheses have been presented for the success of the angiosperms and
their rapid (in evolutionary terms) rise to become the dominant group of land plants. One
hypothesis is that the evolution of flowers and fruit allowed for more efficient pollination
and seed dispersal through coevolution with animals. However animal pollination and
dispersal is known from other extant seed plant groups (e.g. cycads). A second
hypothesis is that the vascular system of angiosperms with vessel members and seive
tubes is inherently more efficient allowing them to exploit more arid environments.
Unfortunately the most basal angiosperms known generally live in moist environments
and may lack vessel members. A third hypothesis is that angiosperms are capable of
much more rapid reproduction than other seed plants. This rapid reproductive rate
allowed them to colonize disturbed areas and evolve small herbaceous bodies. Among
seed plants other than angiosperms all plants are woody shrubs or trees that do reproduce
fairly slowly. Pines for example take about two years from the iniation of ovule and
pollen production until a seedling appears.
Three scenarios have been presented for the origin of the angiosperms. One is that they
are not a monophyletic group and that the diversity of the flowering plants represents
multiple origins. The hypothesis was proposed by paleobotanists and it is not supported
by the overwhelming evidence that the angiosperms are monophyletic. The second
hypothesis is the ‘ancient upland’ hypothesis that proposes that angiosperms evolved as
shrubs and trees in drier upland habitats in which their superior vascular systems gave
them an intial advantage and then secondarily invaded other habitats. Again almost all
basal angiosperms live in moist habitats which does not support this hypothesis. The
third and newest hypothesis is that angiosperms arose as shrubs and herbaceous plants in
streamside habitats. Theses habitats are moist and subject to high levels of disturbance
(flooding) which would favor small, fast-growing, and fast-reproducing plants. These
ancestral plants would be similar to the ‘Paleoherbs’ (see next section). Plant-animal
coevolution may have then helped the spread of angiosperms into other habitats.
Diversity of Living Angiosperms. The systematics of angiosperms has been a subject of
controversy for many years. In the last decade there have been several phylogenetic
analyses of the angiosperms using molecular techniques. There are several robust results
from these analyses but quite a bit of uncertainty remains.
There are two large monophyletic clades that all analyses support: the monocotyledons
(monocots) and the eudicotyledons (eudicots). These two clades comprise the vast
majority (>90%) of all angiosperms. Monocots have a single cotyledon, lack secondary
growth, and generally have parallel venation in their leaves, a fibrous root system, and
floral parts in multiples of three. Examples include grasses, orchids, palms, lilies, and
aloes. Eudicots have two cotyledons, may or may not exhibit secondary growth, and
generally exhibit branching venation in their leaves, a tap root system, and floral parts in
multiples of four or five. Examples include the vast majority of woody trees and shrubs,
plants with composite flowers (e.g. daisies and sunflowers), cacti, roses, geraniums, and
tomatoes.
All other angiosperms are dicotyledonous which appears to have been the ancestral
condition for the entire group (a hypothesis that is supported by the fact that the
Gnetophyta, Gingko, and the majority of cycads are also dicotyledonous; the
polycotyledonous state of many conifers appears to be a derived state). These basal
angiosperm groups fall into two basic grades (groups based on similarity of structure
rather than phylogeny): the paleoherbs and large-flowered tropical and subtropical trees
and shrubs.
Paleoherbs are either shrubs or herbaceous plants with small flowers, often lacking sepals
and petals with small numbers of floral parts. Many of the paleoherbs also exhibit traits
that are similar to monocots (floral parts in threes, stem vascular anatomy (Piperaceae)).
Examples of paleoherbs include the Piperaceae (an abundant tropical family including
black (not bell) pepper), Nymphaceae (water lilies - unusual paleoherbs with large
flowers), Ceratophyllum (a widespread submerged aquatic plant), and Sauraceae (lizard’s
tails - includes a species of flower found in wet areas of Arizona).
Other basal angiosperms include a large number of tropical trees and shrubs (mostly in
the Lauraceae - avocados, cinnamon, sassafras and the Annonaceae - soursop) with a few
temperate representatives such as Magnolia. These plants typically have large and
indeterminate numbers of floral parts arranged in spirals. Petals and sepals are usually
not well differentiated from one another. There are a number of evolutionary trends in
flower morphology in plants. Fusion of floral parts results in plants unusual shapes and
bilateral symmetry (as in orchids). Fusion of multiple flowers into one functional unit
results in ‘flowers’ that are really entire infloresences (as in daisies and sunflowers).
Wind pollination results in small flowers with reduced or absent petals and sepals (as in
grasses).
All the basal angiosperms are monosulcate - their pollen grains have a single groove.
They share this trait with the monocots. The eudicots have trisulcate pollen. Eudicots
(and most of the basal angiosperms) all share the following traits: two cotyledons, floral
parts usually in multiples of four or five, branching veins in the leaves, tap roots,
secondary growth in some taxa, stem vascular bundles organized in a ring. Eudicots were
traditionally classified into six major taxa. The two basal taxa were the Ranunculids,
containing mostly herbaceous plants such as columbine, mustards, and poppies and the
Hamamelids which contain mostly trees such sycamores, birches, oaks, beeches, etc.
Molecular analyses have confirmed the monophyly of the Ranunculids which are either
the basal eudicot taxon or close to the base. The Hamamelids are paraphyletic or
polyphyletic and exist as at least two distinct clades and possibly more. The other four
groups were the Dilleniidae, Caryophyllidae, Rodiae, and Asteridae. All these groups
have plants exhibiting the entire range of plant bodies. The Dilleniidae was found to be
grossly paraphyletic (retaining ancestral traits) and its members are now incorporated
into all the other groups. The Caryophyllidae includes the cacti, iceplants, spinach/beet
family, carnations, and bougainvilleas. It may be nested within the Asteridae but it is
definitely a monophyletic unit. The Rosidae contains such diverse plants as roses, apples,
pears, plums, pomegranates, citrus, walnuts, hickories, maples, geraniums, carrots, and
grapes. The Asteridae includes milkweeds, olives, potatoes, tomatoes, mint, morning
glories, and the huge sunflower family.
The monocotyledons all share the following traits: one cotyledon, floral parts in multiples
of three, unbranched veins in the leaves, a fibrous root system, the absence of secondar
growth, and stem vascular bundles forming no apparent pattern. Monocots are mostly
herbaceous plants but some large tree-like forms have evolved in the absence of
secondary growth. Growth form appears to be more constrained in the monocots than in
the eudicots and major taxonomic units conform more closely to familiar groups of
plants. The two basal taxa of monocots are the Alismatales and the Arales. The
Alismatales are aquatic plants (a variety of aquatic ‘grasses’) while the Arales are
tropical herbs, shrubs, and vines that include many popular house plants (Philodendron,
Monstera, Spathiphyllum, Pothos, etc.). There is a large assemblage of monocots
(Lillidae) that has been variously classified within the Lilliales, Asparagales, and
Orchidales. These plants may make-up a monophyletic group or they may be
paraphyletic. Many sub-taxa have been transferred between these large groups as
classifications have changed. We will not attempt to distinguish among sub-groups of the
Lillidae but merely list some members. Included are familiar herbaceous flowers such as
lilies, irises, daffodils, and orchids onions, succulent and semisucculent plants such as
Aloes, Agaves (century plants), Yuccas, and Dracaenas. Plants in this latter category
often exhibit a ‘woody’ growth form and a few species become fairly large trees. The
rest of the monocots form a single clade with three major taxa. The Arecales are the
palms, all tree-like ‘woody’ tropical plants which form the sister-taxon to the other two.
The Zingiberales includes ginger, bananas, and the tropical bird of paradise flowers. The
latter two groups are similar to palms in growth form. The remaining group is the
Commelinales which includes the bromeliads (pineapples and many tropical epiphytic
plants), the grasses and many similar families (sedges, reeds, rushes).
Angiosperm Reproduction The flowering plants are distinguished from all other seed
plants by several traits. Sporangia are located in specialized strobili called flowers. The
details of flower construction vary greatly across the angiosperms. A key trait of flowers
is that the ovules are encased within a larger structure known as a carpel (a modified
megasporophyll). After fertilization of the egg cell within the megagametophyte, part of
the carpel (the ovary) becomes the fruit containing the seed(s).
A fruit consists of the following structures. A flower is attached to the stem of a plant by
a peduncle (the stem of the flower). The receptacle is the top of the peduncle, where the
floral parts are attached. The sterile parts of the flower are collectively known as the
perianth. The perianth usually consists of sepals and petals which are modified leaves.
Sepals are typically leaflike although in some flowers they may be brightly colored.
They are attached to the receptacle below the petals and are collectively known as the
calyx. The petals are often brightly colored although they are reduced or absent in many
species. Many basal angiosperms do not have a strong differentiation between sepals and
petals (in which case they are called tepals).
The fertile parts of the flower are located within the perianth. The microsporophylls are
known as stamens and the megasporophylls are known as carpels (pistils). The stamens
are composed of a stalk, called a filament, and the anther which contains microsporangia.
Pollen is produced within the anthers much as it is in other seed plants. Each pollen grain
consists of only two cells a generative cell and a tube cell. The generative cell may
divide in some species giving rise to a three-celled pollen grain.
The carpel consists of an ovary which contains one or more ovules, a stigma which is the
area of pollination and a style which is a stalk connecting the ovary to the stigma. Each
ovule contains a single megasporocyte (mega spore mother cell) which produces four
megaspores of which only one survives (the one furthest from the micropyle). The
remaining megaspore undergoes three mitotic divisions, resulting in an eight nuclei
megagametophyte called the embryosac. Three of the nuclei migrate to the end of the
megagametophyte closest to the micropyle; the egg cell and two synergid cells. The
three nuclei at the other end become antipodal cells. The remaining two nuclei remain in
the middle but do not complete cell division; they are known as the polar nuclei.
Pollen grains are transfered onto the stigma by wind, water, or an animal. The tube cell
forms a pollen tube and two sperm nuclei (from the generative cell) travel down it. One
of the sperm nuclei fuses with the egg cell to form a zygote while the other fuses with the
two polar nuclei to form a triploid cell (double fertilization). The zygote develops into
the embryo as in other seed plants while the triploid cell forms a tissue known as the
endosperm. An angiosperm seed is a complex structure consisting of an integument and
nucellus derived from parental sporophyte tissue (diploid), an embryo (also diploid but
derived from the zygote), a triploid endosperm. Nutrients can be stored in the nucellus,
the endosperm, or the cotyledons of the embryo. All angiosperms have either one
cotyledon (monocots) or two cotyledons (dicots).
Seeds are encased within the ovary which becomes the pericarp - the fruit. The
thickness of the pericarp varies: it can be thick and fleshy as in a peach or a thin layer as
in a wheat grain. The pericarp is divided into three layers: the exocarp, the mesocarp,
and the endocarp. In a fleshy fruit like a peach these layers can be quite distinct: the
exocarp is the skin, the mesocarp is the flesh, and the endocarp is the wall of the pit. It
seeds with only a thin pericarp the layers are generally not obviously distinct. A simple
fruit is derived from a single carpel or a few carpel (e.g. a peach or a plum). An
aggregate fruit is comprised of many carpels from the same flower (e.g. a strawberry or a
rasberry). A multiple fruit is comprised of carpels from many flowers in the same
infloresence (e.g. a pineapple).
There are three types of fleshy simple fruits: berries, drupes, and pomes. Berries (e.g.
grapes and tomatoes) and drupes (peaches and cherries) both may be derived from a
single carpel or several united carpels. In berries the endocarp is fleshy and in drupes the
endocarp is stony or fibrous (as in a peach pit or the 'shell' of a coconut). Pomes exist
within only one phylogenetic group containing the apples and pears. The fruit is always
derived from several fused carpels and the endocarp forms a membraneous layer. The
flesh of the fruit is derived from the base of the perianth rather than the pericarp.
Floral and fruit structure vary considerably among the angiosperms. These differences,
in part, reflect the ecological strategies of the plants and the roles of the floral parts and
the fruit. Wind-pollinated plants typically have small flowers with a reduced or absent
perianth (petals and sepals). Animal pollinated flowers typically have a ‘showy’ perianth
and may also have a strong odor. Animal pollinated flowers may also produce nectar.
Reproductive strategies of plants will be discussed in detail later.
Wind dispersed fruits are usually very small and often have fuzz or a wing-like structure
attached (dandelions, maples). Water dispersed fruits(coconuts) can float. Some plants
have fruits with internal structures under tension that flings the seeds as the fruit ripens.
Animal dispersed plants can be actively or passively dispersed. Passive animal dispersal
occurs with fruits such as burs that stick onto the exterior of an animal. Active animal
dispersal occurs when the pericarp of a fruit is attractive to animals because of scent,
appearance, and/or nutritional value. As they eat the fruit the animals disperse the seeds.
Angiosperm Anatomy Plant anatomy at the level of tissues and organs is relatively
simple in comparison to animals. Individual angiosperms (and all other seed plants) are
divided into two major sections: a shoot and a root. Shoots can further be divided into
stems and leaves. Leaves consist of petioles (the ‘stems’ of leaves) and blades. Blades
contain veins which are bundles of vascular tissue. Many plants have leaves with one
major bundle of vascular tissue running down the center of the blade with many other
veins branching off from it.. This large bundle is known as the mid-rib.
The point of attachment of a leaf to a stem is known as a node. The area of stem between
two nodes is known as an internode. The tip of a stem is known as an apex. Each apex
contains a group of rapidly dividing cells known as an apical meristem. An apical
meristem is the region of stem elongation and growth. Each node contains an axillary
bud which is a group of cells that has the potential to become a new apical meristem and
form a branch off the main stem. Stems typically exhibit apical dominance such that
nodes nearer to the apex are less likely to form branches than nodes further from the
apex.
Roots can either have a major central structure known as a taproot or the many smaller
root branches can form a fibrous mass. Roots are covered with many small root hairs
which serve to absorb material out of the soil.
Plant Tissues and Cell Types. Plant tissues are composed of three basic cell types:
ground tissue, dermal tissue and wascular tissue. Ground tissue consists of tissue that
functions in photosynthesis, storage, regeneration, and support. There are three types of
ground tissue: parenchyma, collenchyma, and sclerenchyma. Parenchyma cells are living
at maturity and function in photosynthesis, storage and regeneration. Collenchyma cells
are also living at maturity but are thicker walled than parenchyma. They are used for
support of areas of a stem that are still actively elongating. Sclerenchyma cells are very
thick walled and may die after reaching maturity. Sclerenchyma functions in supporting
stems with secondary growth and in protection. There are two types of sclerenchyma
cells: fibers which are long and slender and sclereids which are shorter and thicker.
Dermal tissue consists not only of the cells covering the surface of roots, stems, and
leaves that collectively make up the epiderm. The epiderm consists large of relatively
unspecialized cells but also contains such specialized cells as guard cells which control
the opening of stomata and trichomes (plant hairs). In plants with secondary growth the
epiderm is lost and is replaced with the periderm consisting of the cork cambium and
cork tissue (see below).
Vascular tissue serves to transport water, carbohydrates, and nutrients within the plant.
There are two major tissue types: xylem and phloem. Xylem transports water and
nutrients from the roots up into the stems and leaves and is dead at maturity. Phloem
transports carbohydrates within the plant and is capable of transport in any direction and
is alive at maturity. There are two types of xylem cells: tracheids and vessel members.
Tracheids are long slender cells with overlapping ends. The entire cell wall of each
tracheid is covered with pores that allow water to move between cells. Vessel members
are cylindrical cells with porous ends. Long chains of vessel members are lined up end to
end to make long continuous tubes known as vessels which are thought to be more
efficient conductors of water than tracheids. Tracheids are found in all vascular plants
while vessel members are found in almost all Gnetophyta and Angiosperms as well as a
few seedless plants such as some Selaginella and Equisetum.
Phloem consists of two types of cells: sieve elements (non angiosperms) and sieve tube
members (angiosperms). Similarly to the two types of xylem the sieve elements have
pores all over the cell surface while the sieve tube members have pores concentrated on
the ends. Both types of cells have adjoining cells that provide them with nutrients. For
sieve elements these are called albuminous cells and for sieve tube members these are
called companion cells.
Root Anatomy The tip of a root consists of a root cap, a group of cells that function to
protect the apical meristem from damage as it is pushed through the soil. The cells of the
root cap produce a substance called mucigel which forms a slimy sheath that reduces
friction on the root as it grows. Cells are constantly sloughed off of the root cap and they
are replaced by the apical meristem. The apical meristem sits directly behind the root
cap. There are two types of apical meristems in roots. In one type the cells all contribute
equally to the different tissues of the root. In the other the meristem is divided into
sections that produce different portions of the root. One section produces the root cap,
another the protoderm and ground meristem and a third produces the procambium. The
last three mentioned tissues mature to become ground tissue, dermal tissue and vascular
tissue respectively. The meristem consists of quiescent center with relatively inactive
cells with a region of cell division behind. Behind the region of cell division is a region
of cell elongation where most of the actual growth occurs. Behind that is the region of
cell maturation where root cells develop into their final form.
The interior of a root has a solid ridged core of xylem in dicots with the phloem located
in the grooves between the ridges (root has a protostele). In monocots the xylem forms a
hollow ring (a siphonostele) with pith on the inside. There is a tissue layer called the
pericycle immediately outside the xylem and the phloem. The pericycle contributes to
secondary growth and produces lateral root branches. Ouside of the stele is the cortex.
This tissue functions primarily for transport of materials into the stele and for storage.
The inner layer of the cortex is endodermis a tissue that controls the uptake of materials
into the stele. The endodermis contain Casparian Strips. These are composed of a fatty
material called suberin that blocks movement of sbstances between the walls of the
endodermis. Everythng moving in or out of the stele must therefore pass through the
cells of the endodermis.
The cortex is surrounded by the epidermis which produces the root hairs that perform
uptake of material from the soil. The cortex and the epidermis are lost in roots with
secondary growth. In these plants the part of the procambium remains meristematic and
becomes the vascular cambium. This tissue forms a continuous band running between
the xylem and the phloem around the cell. The vascular cambium produces secondary
xylem on its inner surface and secondary phloem on its outer surface. As the stele
expands outwards the cortex and epidermis are sloughed off. The pericycle then forms
the cork cambium, a second meristematic ring of tissue. The cork cambium produces
cork on the outside and phelloderm on inner surface to protect the root. This is known as
bark.
Stem Anatomy The apical meristems of stems are different from those in roots. They
have no protective cap and there are no clear cut zones of cell division, elongation, and
maturation.. In addition to producing stem tissue, the meristems of stems are responsible
for producing leaf primordia and axillary meristems. The leaf primordia are produced so
rapidly relative to cell division in the meristem that initially there is no distinction among
them. (no internodes). The spaces between the nodes develop through elongation of the
cells which is also the primary mechanism for stem elongation.
The meristem consists of two parts. The tunica is comprised of the outermost layer(s) of
cells which divide anticlinally (perpendicular to surface plane of meristem). The tunica
primarily contributes to surface growth. The corpus is below the tunica and divides in
many planes. It produces the bulk of primary tissue of the stem and is divided into central
mother cells and peripheral meristem (derived fromboth tunica and corpus). The
peripheral meristem is very mitotically active. The tunica gives rise to the protoderm
while the corpus gives rise to the ground meristem (cortex) and the procambium. The
peripheral meristem gives rise to ground meristem which forms the pith.
In dicots the vascular tissue either forms a continuous cylinder (xylem on the inside,
phloem on the outside) or a cylinder of individual bundles (still with xylem on the inside,
phloem on the inside). In either case there is pith inside the cylinder and cortex outside.
In species with secondary growth a vascular cambium forms between the xylem and the
phloem (in species with discrete vascular bundles the vascular cambium is derived from a
fasicular cambium within the bundles and an interfasicular cambium outside the bundles).
Secondary growth and the formation of bark occurs much as in roots. Monocots have
vascular bundles scattered throughout the stem in an apparently random fashion.
Secondary growth is unknown.
Leaf Anatomy Leaves begin as leaf primordia with strands of procambial tissue linking
them to the stem. These strands become leaf traces, vascular bundles that attach the leaf
to the main vascular system of the stem. The structure of leaves is highly tied to the
ecological circumstances of the plant. Variation in leaf structure will be discussed later.
A typical leaf has an epidermal layer enclosing parenchyma tissue with vascular bundles
(veins) through the parenchyma. The parenchyma consists of one or more layers of
tightly packed columnar cells immediately below the upper epidermal layer making up
the palisade parenchyma. This tissue performs most of the photosynthesis. Below the
palisade parenchyma is an area of loosely packed, irregularly shaped cells called the
spongy parenchyma (or spongy mesophyll). This area has many air spaces and vascular
bundles running through it. This is the major area of gas exchange between the cells and
the air. The lower layer of epidermal cells has many openings in it. These openings are
called stomata and they can be opened and closed by guard cells. The stomata allow for
gas exchange in and out of the leaf. While the stomata are open (to allow carbon dioxide
into the leaf and oxygen to escape) water is continuously lost. This water loss is known
as evapotranspiration.
Plant Transport Plants most move water and nutrients up from the soil and transport
carbohydrate within the plant. The movement of water through xylem is almost always
against the force of gravity and thus requires considerable energy. Transport through
xylem can be performed using active transport but this is not feasible in larger plants as
the maximum height that water can be raised this way is only a few feet.
Evapotranspiration causes plants to lose an immense amount of water through their
leaves. A sunflower can use up to 17 times the water of a human being on a hot day.
The water loss of evapotranspiration is necessary for plants as they must expose moist
cellular surfaces to the atomosphere in order to take up carbon dioxide. They make use
of this water loss to drive water and nutrient transport through xylem. This is the
cohesion-tension model of transport. Water molecules are polar and are thus cohesive
(they tend to form weak bonds to one another) and adhesive (they form weak bonds to
other substances). Both of these properties are easy to observe: water forms droplets and
these droplets tend to adhere to surfaces. As water molecules evaporate off the surfaces
of cells they pull on the molecules behind them which then pull on the molecules behind
them and so on. This creates tension which pulls all of the water molecules upwards, all
the way down to the roots. The adhesive properties of water prevents the column of
water in a vessel from sliding back down when the stomata close and evapotranspiration
ceases.
Nutrients are pumped into xylem using active transport in the roots and then carried to
the rest of the plant by the flow of water through xylem. The movement of carbohydrates
in phloem is very different. It is bidirectional moving carbohydrates from a source point
anywhere on the plant (leaf or storage organ) to a sink point (area of rapid growth). The
mechanism is called the pressure flow hypothesis. Sugars are pumped into the phloem by
active transport at the source point. Water follows (from xylem) by osmosis which builds
up pressure lead to flow in the phloem tube. At the sink point the sugar is transported out
and the water returns to the xylem by osmosis.