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LAB 06: Seed Plant Synapomorphies
Introduction to non-flowering seed plants (Gymnosperms)
A seed is a highly modified megasporangium, so seed plants are heterosporous. We will review
important differences between heterosporous non-seed plants and seed plants. There are five
lineages of extant seed plants, four of which are gymnosperms (seeds not enclosed in a fruit) and
one lineage of angiosperms (seeds in a fruit). We will also review some potentially confusing
differences in what the terms dioecious and monoecious refer to when applied to homosporous
versus heterosporous plants.
MICROSPORANGIA, MICROSPORES, MALE GAMETOPHYTES
Microsporangia. There is no fundamental difference in the function of microsporangia in
heterosporous non-seed and seed plants. The structure of the endosporic male gametophytes and
the way they function, however, is drastically different in the two groups of plants.
Microspores and Male Gametophytes: In heterosporous, non-seed plants an entire antheridium
develops within the microspore wall (review the structure of the Selaginella male gametophyte).
It has jacket cells surrounding a substantial number of spermatocytes. The microspore wall
eventually cracks open and many sperm are released and swim to the female gametophyte (in
dew, rainwater, pond water).
Because seed plants are heterosporous, the gametophytes are endosporic just as they are in
heterosporous non-seed plants. But the male gametophyte has undergone substantial reduction so
that there is no longer any trace of an antheridium. Male gametophytes of seed plants consist of
about 2-6 cells. The microspore, with its tiny internal gametophyte, is carried by wind or animals
to somatic tissues in the vicinity of the female gametophyte. It germinates there by producing a
tubular outgrowth. Two sperm are ultimately released from the male gametophyte and swim or
are conveyed to the female gametophyte. In seed plants the movement of sperm to the female
gametophyte is independent of water in the environment.
The tiny male gametophytes of seed plants are given a new name because they function in a new
way. We call them pollen. The transport and arrival of pollen in the vicinity of the female
gametophyte is called pollination. A male gametophyte (pollen grain) usually has one to several
vegetative cells. It has a single generative cell that produces two sperm. The male gametophyte
also has a single tube cell that directs the development of the pollen tube. In some pollen grains,
a distal weak spot on the exine (outer wall) of the pollen grain is the site of pollen tube
emergence. The proximal side of a pollen grain is the side that was in contact with other
microspores at the completion of meiosis (tetrad stage). The distal weak spot where the pollen
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tube emerges is usually seen as an aperture (opening) in the exine. In many angiosperms, there
are multiple apertures and a single pollen grain may develop more than one pollen tube.
There are two ways in which pollen tubes function. Haustorial pollen tubes have an exclusively
nutritive function. They digest the surrounding tissue for a period of weeks or months. The food
obtained is absorbed and used to support continued growth of the pollen tube and maintenance of
the male gametophyte. Eventually two flagellated, swimming sperm are produced by the
generative cell. They are released from the proximal end of the pollen grain - not from the pollen
tube itself. They swim to the egg and fertilization is accomplished. Haustorial pollen tubes are
found in cycads and Ginkgo. Haustorial pollen tubes represent the ancestral condition in seed
plants, indicating that the pollen tube may not have originally evolved as a sperm delivery
system. The haustorial process allows pollen grains to be light and easily transported because
food storage in the grains is unnecessary; sperm transport likely evolved later as a secondary
function. All other seed plants produce siphonogamous pollen tubes. Siphonogamous pollen
tubes grow at varying rates and do varying amounts of digestion of the surrounding tissue, but
ultimately also serve the function of conveying the sperm to the egg. The sperm are not
flagellated and are therefore non-motile.
A comparison of
male gametophyte
structure and
function in
heterosporous nonseed plants and seed
plants.
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MEGASPORANGIA, MEGASPORES, FEMALE GAMETOPHYTES
Megasporangia and megaspores: There are many important differences between heterosporous
non-seed and seed plants in the structure and function of megasporangia and megaspores. They
are listed below:
(1) As a megasporangium begins to develop, other tissues at its base begin to grow upward and
eventually entirely surround it except for an opening, the micropyle, at its distal end. There
may be a single such coat or integument around the megasporangium or two.
Gymnosperms usually have one integument. In angiosperms the ancestral number of
integuments is two, but in derived angiosperm families it has been reduced to one. An
integumented megasporangium with its internal female gametophyte is known as an
ovule.
(2) In heterosporous, non-seed plants the number of meiocytes (megasporocytes) that develop
within a megasporangium is variable. Thus there may be as few as four megaspores
produced per sporangium or there may be many. In seed plants, a megasporangium rarely
develops more than one megasporocyte, thus the potential number of megaspores is four.
Meiosis usually occurs so that a linear tetrad is formed. The most common developmental
pattern is for the three distal meiotic products to degenerate. The surviving single
megaspore produces the female gametophyte. Later we will discuss some variations on this
theme where bisporic and tetrasporic female gametophytes occur in some taxa.
(3) The single megasporocyte is embedded in a solid tissue called the nucellus. The dominant
hypothesis with respect to the evolutionary history of the nucellus is that it represents sterile
tissue of the megasporangium.
(4) The megasporangium of seed plants is indehiscent. In other words, the single megaspore
with its internal female gametophyte is not released. This should be contrasted with the
behavior of heterosporous non-seed plants in which the megasporangia dehisce and the
megaspores, which are not fused to surrounding tissue, are released to the outside world
where fertilization of their internal female gametophytes takes place. Because the
megasporangia of seed plants are indehiscent, they evolved mechanisms that allow sperm
access to the female gametophyte.
(5) After fertilization, the integumented megasporangium (ovule) ripens into a seed. The
megasporangium stalk has an abscission zone that ultimately breaks down and frees the seed
to be dispersed. In non-seed plants, the only dispersal stage is the spore (in addition to
vegetative propagules).
Female gametophyte. The endosporic female gametophytes are very different in gymnosperms
and in angiosperms. In most gymnosperms, the single megaspore grows into a gametophyte that
consists of hundreds or thousands of cells and in most cases two or more archegonia differentiate
at the micropylar end. In angiosperms, the gametophyte is reduced to a few cells and lacks
archegonia.
FERTILIZATION, EMBRYO AND SEED DEVELOPMENT
In non-seed plants the basic unit of dispersal is the spore (megaspore in the case of
heterosporous, non-seed plants). Megaspores do not have a very great range of sizes, at least in
comparison to seeds, and all megaspores function more-or-less the same way. Embryos of
heterosporous non-seed plants must grow into a mature sporophyte wherever the megaspore
containing them happens to fall. Embryos of non-seed plants may grow continuously without
undergoing a period of dormancy or may remain quiescent during a period when environmental
conditions are unfavorable for growth (winter, dry season, etc.). But prolonged survival is
impossible. This should be contrasted with seeds that often contain considerable stored food and
may survive for years in a dormant condition. Maturation of a seed involves not only the
development of the embryo, but also numerous changes in the integuments as they ripen into a
tough, protective seed coat.
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MONOECY AND DIOECY.
Cruden & Lloyd (1995) have proposed a common terminology to describe sexual phenotypes
and breeding systems in all land plants. The terms “monoecy” and “dioecy” necessarily refer to
different things in heterosporous and homosporous species. A monoecious species bears both
male and female sex organs on the same individual plant (which can be described as bisexual or
hermaphroditic), whereas in a dioecious species there are separate male and female plants
(individual plants are unisexual). The potential for confusion lies in the fact that in heterosporous
plants the "individual" we are referring to is a sporophyte, whereas in homosporous plants the
"individual" is a gametophyte.
As an example of homosporous plants, consider moss. Mosses and other homosporous plants
produce only one kind of sporangium and therefore one kind of sporophyte. The terms
monoecious and dioecious in homosporous plants can only refer to gametophytes. A monoecious
moss species has archegonia and antheridia on the same gametophyte. A dioecious moss species
produces separate male gametophytes (antheridia-bearing) and female gametophytes
(archegonia-bearing). In many bryophyte species it has been shown that half of the spores from a
particular sporangium produce female gametophytes and half produce male gametophytes. This
fact implies that sex determination is a consequence of the segregation of sex chromosomes, as
in humans. But not all cases of dioecy result from segregating sex chromosomes. Other genetic
sex-determining mechanisms than XY chromosomes are known. It is also well known in other
organisms that sex is not always genetically determined; it can be environmentally determined
(e.g. in reptiles and fish), although in such a case you would not expect a 1:1 ratio of males to
females. For our purposes, the important point is that, in homosporous plants, the terms monoecy
and dioecy refer to gametophytes and not sporophytes.
Heterosporous plants produce two kinds of sporangia and two kinds of spores. Megaspores
develop internal female gametophytes and microspores develop internal male gametophytes.
Obviously, it is not possible for eggs and sperm to be produced by the same gametophyte.
However, both megasporangia and microsporangia may occur on the same sporophyte or
different sporophytes. So the term monoecious refers to species in which a single sporophyte
bears both mega and microsporangia. The term dioecious refers to the presence of
megasporangia and microsporangia on different individual sporophytes. All extant
heterosporous non-seed plant species are monoecious. For example, in Selaginella the
strobilus always contains both megasporangia and microsporangia. Seed plants may be either
monoecious or dioecious. Monoecy is the most common condition, but dioecy is not
uncommon. Pines, for example, are always monoecious, with microsporangia (in pollen cones)
and megasporangia (in seed cones) developing on the same tree. Ginkgos, on the other hand are
always dioecious, with separate "male" and "female" trees. The terms male and female are in
quotation marks to remind you that, technically speaking, the sporophyte is not the sexual stage
of the life cycle and cannot be male or female. But biologists routinely ignore that fact and slip
into using the terms male and female to describe the sporophyte where dioecy is involved. In
flowering plants, flowers are usually bisexual, containing both anthers (which bear
microsporangia) and carpels (which bear megasporangia). The term monoecy is usually reserved
for the situation in which male organs and female organs are in separate flowers on the same
plant. Dioecy is obviously used to describe the situation where male and female flowers occur on
separate plants. An example of a dioecious flowering plant is the Holly tree. A female Holly has
flowers that bear only female organs and is easily spotted because of the bright red fruits it
develops. Male hollies are generally recognizable as mature trees that lack fruit.
Cruden, R., and Lloyd, R. (1995). Embryophytes Have Equivalent Sexual Phenotypes and Breeding Systems -­‐ Why Not a Common Terminology to Describe Them. Am. J. Bot. 82, 816–825. 69
Bryophytes
Homosporous
Non-seed Tracheophytes
Homosporous Heterosporous
Seed plants
Heterosporous
Monoecy and dioecy in land plants.
REPRODUCTIVE CHARACTERISTICS OF GYMNOSPERMS
(1) All gymnosperms have "naked" seeds. In other words, ovules are exposed to the outside
world at the time pollination takes place - thus pollen can land directly on or near micropyles.
When you see the tightly shut cones of pines or other conifers, you may doubt the truth of the
above statement, but at the time of pollination the young cone has cone scales that are
separated from each other- leaving a space into which windborne pollen can blow. In
gymnosperms that do not have cones, the "naked" aspect of the ovules is much more obvious
(e.g. Ginkgo biloba).
(2) Most gymnosperm ovules have a single integument (Gnetophytes are an exception).
(3) Gymnosperms are wind-pollinated (Cycads and Gnetophytes are a partial exception). In
most species wind-blown pollen is trapped by a sticky pollination droplet exuded through
the micropyle. Appearance of the droplet often correlates with the breakdown of the nucellar
tip, creating a space, which in conjunction with the space between the tip of the nucellus and
the integumentary lobes is called the pollen chamber. The way in which pollen is ultimately
brought into the pollen chamber varies with species. In some conifer species, pollen is
“wettable” and sinks in the droplet, accumulating at the mouth of the micropyle. As the
droplet dries or is metabolically retracted into the micropyle, the pollen is pulled into the
pollen chamber. In other species ovules are inverted at the time of pollination, so that the
micropyles face downward. Species with inverted ovules invariably have saccate (winged)
pollen. Saccate pollen is buoyant and non-wettable and stays on the droplet meniscus while
moving upward to the vicinity of the micropyle. As in the previous case, as the droplet dries
out and the meniscus recedes into the micropyle, pollen is pulled into the pollen chamber.
Some gymnosperms lack pollination droplets, in which case windblown pollen lands directly
on the protruding integumentary lobes and germinates there.
(4) Most gymnosperm female gametophytes consist of hundreds or thousands of cells and
have archegonia (some exceptions).
(5) Gymnosperm female gametophytes undergo extensive free-nuclear development before
becoming cellular. In other words, the division of the megaspore nucleus is not accompanied
by wall formation, likewise the many divisions that follow - so that the megaspore cytoplasm
is filled with numerous free nuclei. At some point, wall formation takes place and archegonia
differentiate.
(6) The early development of the embryo is free nuclear, with some exceptions. The extent
of the phenomenon varies widely. For example, in the Cycads there may be over a thousand
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free nuclei before walls develop, whereas in conifers there may be as few as four free nuclei.
Angiosperms undergo cellular embryo development from the outset.
(7) Cleavage polyembryony (splitting of one embryo into several) is common.
Pollination mechanisms in conifers.
A, erect ovule, non-saccate pollen sinks into pollination droplet. B-E inverted ovules.
B, saccate pollen penetrates pollination droplet.
C, pollination droplet not extruded or absent, pollen floats into ovule in rainwater.
D, pollen adheres to papillate cells at tip of micropylar lobes.
E, pollen germinates on scale, cone axis or bract-scale, long pollen tube penetrates micropyle.
Intro to Cycads and Ginkgo
Cycads were much more numerous and widespread 150 million years ago when the climate was
wet and warm over the entire earth's surface. Cycads are dioecious, with individual plants
forming either pollen cones or seed cones, but never both. Seed cones can be very large, often
exceeding 20 kg! In the genus Cycas, ovules are attached along the sides of the petiole-like base
of the megasporophylls. In the other genera, the spirally arranged megasporophylls are not leaf
like. The dominant hypothesis is that the leaf-like megasporophylls of Cycas represent the
ancestral form.
Cycad megasporophylls. A, Cycas revoluta; B, Cycas circinnalis; C, Dioon edule;
D, Ceratozamia mexicana; E, Zamia floridana.
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Male cones bear spirally arranged microsporophylls that are not at all leaf like. They bear
numerous abaxial microsporangia that may be arranged in distinct groups like the sori seen on
fern leaves.
Cycad microsporophylls. A, Cycas; B, Zamia.
Pollination, fertilization and embryo growth. For many years it was assumed that Cycads
were wind-pollinated. Interestingly, both male and female cones of virtually all genera have been
shown to be strongly thermogenic. Thermogenicity in flowering plants is a process that
volatilizes insect-attracting odors. Detailed studies of several Zamia species show that pollination
is carried out by beetles that feed on starch-rich tissues of the male cone. Although this food
source is absent from the seed cones, the pollen-covered beetles are still attracted to the females
by the strong odor emitted by their thermogenic cones. It is now thought that, depending on the
species, cycads are either exclusively insect-pollinated or pollinated by a combination of factors.
About four to five months following pollination, the proximal end of the pollen grain bursts,
releasing the two sperm. They swim through the liquid of the fertilization chamber and penetrate
the archegonial necks. A single sperm nucleus fuses with each egg nucleus.
Diagram of longitudinal sections of a cycad ovule showing male and female gametophytes at the micropilar end.
A, pre-pollination ovule. B, ovule immediately before fertilization, with haustorial pollen tubes.
Ginkgo
Ginkgo is a dioecious species, producing separate male and female trees. Microsporangiate
strobili arise in the axils of short shoot bud scales and leaves. A strobilus consists of a stalk
bearing numerous spirally arranged appendages, each of which usually has 2 microsporangia at
its tip. Ovules are not found in strobili, but develop in pairs at the tip of stalks which arise in a
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position equivalent to that of the microsporangiate strobili - in the axils of short shoot bud scales
and leaves.
Short shoots of Ginkgo
biloba bearing
(A) male strobili or
(B) ovules.
CONIFERS
Conifers and Gnetales
Conifer reproductive Morphology
(1) Cones are unisexual. A cone may bear microsporangia (pollen sacs) or megasporangia
(ovules), but not both. This should be contrasted with heterosporous non-seed plants
(e.g. Selaginella) where a strobilus usually contains both micro and megasporangia. Some
extinct gymnosperms had bisexual cones. Some extant gymnosperms (Gnetales) have cones
that bear both pollen sacs and ovules, but they are not functionally bisexual because the
ovules in bisexual cones are sterile.
(2) Most species are monoecious, but dioecy is not uncommon. In a monoecious species, pollen
and seed cones occur on the same tree. A particular branch may produce only pollen cones
one or more years and then switch to seed cones for a period of time and vice versa.
(3) Simple vs. compound cones. A cone may be thought of as a reproductive short shoot. In all
conifer families, pollen cones are simple, meaning it has a cone axis and one set of
appendages (scales or microsporophylls). In the Pinaceae, seed cones are compound. The
seed cone axis bears first order appendages (bracts) and second order appendages
(ovuliferous scales) in the bract axils. In some families, cones appear to be simple because of
bract-scale fusions during ontogeny.
A, pollen cones are simple, one set of
appendages (microsporophylls) bear
abaxial pollen sacs (not shown). B, seed
cones are compound, first order bracts
and second order ovuliferous scales with
ovules (not shown).
(4) Ovules may be inverted or erect. In Pinaceae, ovules are attached to the scale surface and
are inverted (micropyles face inward toward the cone axis). In other conifer families, ovules
may be erect (micropyles face outward away from the cone axis).
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(5) Some conifer families have highly reduced seed cones. In the Cephalotaxaceae ("plum
yews"), the highly reduced cones bear only one or two ovules, but additional aborted ovules
are present and the reduced cone structure is obvious. In Podocarpus the cone is reduced to
one to several bracts, often fleshy, with each bract subtending a single ovule that is partially
or entirely surrounded by an enlarged ovuliferous scale (the epimatium).
(6) The Taxaceae (yews) lack seed cones. In this family, one or two ovules develop terminally
on short branches. The base of the ovule develops a fleshy aril. Developmental studies reveal
no signs of reduction from a more complex structure and there is no palaeobotanical evidence
for a cone-bearing ancestor. However, the wood anatomy and details of reproductive biology
are typical of conifers.
GNETALES: General features of reproductive morphology
Both pollen and seed cones of all three genera are compound. The basic architecture of the cones
seems to be fundamentally the same in the three genera. Although the cones of Gnetophytes are
unisexual, there is a definite tendency toward bisexuality. In all three genera, sterile ovules may
be present in the male cones.
Figure 13.2:
Diagrammatic representation
of gnetophyte cones.
The flower-like nature of secondary axes that contain both pollen-bearing structures and sterile
ovules can be very striking. However, DNA sequence data from extant gymnosperms indicates
that Gnetophytes and angiosperms are not closely related and that their shared morphological
characters are the result of convergence, not homology (Hansen et. al. 1999). The Gnetophytes
are most likely sister group to the Pinaceae (Burleigh and Mathews, 2004).
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Reproductive morphology of Ephedra.
Both wind and insect pollination seem to play a
role in Ephedra. In some species male plants
produce nonfunctional ovules that produce a sweet
pollination droplet that is presumed to play a role
in attracting insects to the male plants.
Ephedra cones. A-B, pollen cones; C-E, seed; d ovule detail
Reproductive morphology of Gnetum. All species of Gnetum are dioecious and most species
occur in humid rain forests, a habitat that is unsuitable for wind pollination. Some species are
moth-pollinated.
(Fused bracts)
B
C
D
Male and female cones of Gnetum gnemon.
A, microsporangiate strobilus;
B, long section through a node showing 4 developing microsporangiate fertile
shoots and one abortive ovule;
C, megasporangiate strobilus showing ovules and partially developed seeds;
D, long section of young ovule. (from Gifford and Foster, 1988)
A
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Reproductive morphology of Welwitschia. Welwitschia is dioecious. Cones form at the tips of
branched axes that develop in the leaf axils. Welwitschia ovules secrete a droplet with a high
sugar content that might be associated with insect pollination, but it might also simply function
as a pollen-trapping and germination medium. At present, the limited evidence available suggests
that Welwitschia is predominantly wind-pollinated, but that insect pollination also occurs to
some extent.
Significance of double fertilization in Ephedra and Gnetum.
Double fertilization has always been considered a unique event in angiosperms. It is now known
with certainty to also occur in Ephedra and Gnetum. In angiosperms, the second fertilization
event involves three nuclei in most cases, a sperm nucleus and two nuclei of the female
gametophyte. This triploid fusion product develops into a triploid food tissue, the endosperm that
nourishes the embryo. In Ephedra and Gnetum, the two sperm entering the female gametophyte
each fuse with a single nucleus, producing diploid embryos. Only one embryo matures. The
surviving embryo is nourished, as in other gymnosperms, by the food stored outside the
archegonium in the other cells of the female gametophyte
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Laboratory Exercise 06- Seed Plant Synapomorphies
and gymnosperm diversity
Exercise:
The goal of this lab is to observe the main seed plant synapomorphies and representatives of
the major lienages of non-flowering seed plants (the “gymnosperms”: cycads, Ginkgo, gnetales
and conifers). You will study in lab and in the greenhouse:
1- Ginkgo seed dissections (a seed is a fertilized ovule)
2- Pollen grains (endosporic male gametophytes, at maturity).
3- Examples of secondary growth (wood).
4- Cycad diversity and reproductive structures.
5- Gnetales diversity and reproductive structures.
6- Conifer diversity and reproductive structures.
1-Ginkgo seed dissection:
We may have mature seeds (from trees around Seattle) or immature seeds (picked off the trees in
midsummer). If the seeds are mature, you will know immediately by the odor (see demo article
giving the organic constituents of Ginkgo gas). Ovules are found on stalks that develop in the
axils of short shoot leaves, they occur in pairs. One of the paired ovules usually aborts and can
still be seen. Make a diagram below
1. If a stalk is present, gently break it off the seed keeping track of the micropylar end of the
ovule (away from the stalk).
2. The ovule integument has 3 layers. Peel off the outer fleshy layer and wash the now stony seed
in water.
3. Try to peel off the stony middle layer. You may need a hammer to crack (not crush!) this hard
layer so you can remove it. Do not section the seed with a razor - leave it intact. Working
under the dissecting scope, find two papery layers beneath the stony middle layer of the
integument.
4. The outermost papery layer is the third or innermost layer of the integument. Note that it is
fused to the other papery layer (which is the megaspore wall) at one end of the seed. The other
end of the seed is the micropylar end. Carefully peel off the papery layer of integument,
leaving the megaspore wall intact.
5. Holding the seed upright under the dissecting scope, use a needle to peel away the megaspore
wall at the micropylar end, revealing a reddish spot. Immediately under the reddish spot, as
you peel, you will discover the "tent pole" (a raised section of the female gametophyte- see
diagram below). On either side of the "tent pole", you will see a tiny opening. These are the
archegonial necks. What lies inside each archegonium? ___________________________
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6. Slice the gametophyte longitudinally with a razor through the archegonial necks to reveal the
archegonia. Notice that the female gametophyte is green - a unique feature in seed plants.
Depending on whether the ovule had been fertilized, you may have cut through an embryo in
sectioning the gametophyte.
Male reproductive structures of Ginkgo biloba:
Examine the male strobili in the (previously frozen) material provided. Note catkin-like strobilus
with spirals of paired sporangia ("catkins" are long, thin stalks bearing numerous small, windpollinated reproductive structures). Make a diagram below
2. Pollen Grains
Make a wet mount of pollen grains from Ginkgo or one of the conifers. Note the two sac-like
structures emerging from the pollen wall, can you think of a function for these?
______________________________________
Illustrate a pollen grain below
What part of the life cycle does the pollen grain represent? What does it carry?
_________________________________________________________________
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3. Secondary Growth
Secondary growth is a synapomorphy of seed (and woody) plants that occurs as the result
of lateral meristematic activity, and produces an increase in the girth of an organ. The two most
common lateral meristems in conifers and woody dicots are the bifacial vascular cambium that
produces secondary xylem (wood, to the inside) and secondary phloem (to the outside), and the
cork cambium that produces only cork to the outside, or both cork and phelloderm (to the
inside). The secondary xylem (wood) of a plant provides a permanent record of vascular cambial
activity throughout the life of the plant. By contrast, the secondary phloem is constantly being
displaced further-and-further out into the bark, and is eventually sloughed off with the old bark.
Wood
Examine a block of wood. There are usually three planes in which wood is cut for
examination. If the cut is at right angles to the long axis of the stem or root, it is a cross or
transverse section. Note the growth rings in the transverse section. A growth ring is the amount
of secondary xylem deposited by the vascular cambium during one growing season. The xylem
elements formed in the spring are larger than those formed later in the growing season. Note the
radiating lines in transverse section. These structures are vascular rays.
A longitudinal cut along a radius produces a radial section. A longitudinal slice parallel with
a tangent but not along a radius is a tangential section.
What is the appearance of the vascular rays in radial section?
In tangential section?
Can you identify the age of the tree when it was cut down?
In which years did the tree grow more?
Can you identify the heartwood and the sapwood?
How thick is the bark?
Can you distinguish the cork?
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Obtain a slide of pine wood (Pinus). Compare what you see in the slide to what you saw in the
block of wood. You can recognize a transverse section from the configuration of the growth
rings. Most of the xylem cells look like thick-walled squares or rectangles in this section. These
are the tracheids. Extending along radii are elongated cells, which appear to have cell contents.
Unlike the dead tracheids, these cells are alive in wood that still functions in conduction. These
radially elongated cells make up the vascular rays.
Can you determine in which direction in your slide is the center of the stem? Can you see growth
rings?
In some places you will note circular areas, which are surrounded with thin-walled
parenchyma cells. These areas are vertical resin canals, common in many conifers.
Obtain one slide from the wood of an angiosperm species. You will notice the wood
differs from pine even at first glance. Probably the most conspicuous features in a transverse
section are the large perforations or vessels. Each vessel element has perforation plates at each
end. In angiosperm wood, the perforation plates are on oblique end walls, and are scalariform.
That is to say, they have several bars that dissect the opening or perforation. A vessel is not a
cell, but consists of a vertical series of cells with the end walls missing. What would you
conclude about the efficiency of such structures in water and mineral conduction? You will
notice that in a transverse section, there are many cells with extremely thick walls and very tiny
cell cavities. These cells are wood fibers, which serve to strengthen the wood. Did you see wood
fibers in pine? Why is pine one of the so-called “soft woods”, while oak is one of the “hard
woods”?
Can you find cells that have cytoplasm and nuclei in them? These are xylem parenchyma cells,
which are actually vertical chains of somewhat elongated cells.
What cell types found in the wood of an angiosperm are missing from a gymnosperm (pine)?
_______________________________________
4. Cycads
A. Seed Cones and Megasporophylls. Observe demos and illustrate below:
1. Cycas megasporophylls. Note the resemblance of megasporophylls to compound leaves.
Cycas is the only genus that does not produce a seed cone.
2. Dioon megasporophylls. Although Dioon produces a definite cone, note that the
megasporophylls are still somewhat leaf like.
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3. Zamia seed cone. Note the shape of the megasporophylls (not at all leaf like). They are
peltate like the sporophylls of Equisetum.
B. Pollen cones. Observe demos and illustrate below:
Zamia pollen cones.
a) Whole cones: Observe dried cones and any living cones present on potted plants.
b) Microsporophylls: In a petri dish you will find a cone that has been broken up. Pick
up one or two microsporophylls to study under the dissecting scope. Note abaxial
position and approximate number of microsporangia.
5. Gnetales: This lineage consists of three genera: Ephedra, Gnetum and Welwitschia. The three
are morphologically so distinct that no one, at a glance, would suspect they were even remotely
related. But detailed study of their vegetative and reproductive structures and processes, as well
as DNA sequences, reveals a close relationship.
Examine and diagram living representatives of these 3 lineages.
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6. Conifers:
Reproductive structures
Pollen Cones. Pollen cones are always simple (i.e. have one kind of cone appendage, called
microsporophyll). In Deodar cedar (Cedrus deodara, Pine family) both seed and pollen cones
form at the apex of spur shoots. At this time of year, Cedrus pollen cones have mostly abscised
and are lying on the ground in large numbers. Obtain a detached, mature pollen cone for study.
Pick off one or two scales. Observe with the dissecting scope.
How many microsporangia per scale? ____________
This number is constant throughout the Pinaceae, and highly variable in other conifers.
Make a diagram below.
Seed Cones.
The structure of seed cones varies enormously depending on the conifer family. In the Pinaceae,
the compound nature of seed cones is obvious. Recall that in compound cones, the cone axis
bears appendages called bracts. Each bract has in its axil or on its adaxial surface an
ovuliferous scale. The ovuliferous scales are interpreted to be highly reduced branches. In other
conifer families, bract and scale are completely or partially fused.
Take a longitudinal section slide of a young Pinus seed cone back to your bench to observe.
Note bracts and ovuliferous scales.
The diagram to the left will help
you interpret the material, you may
want to sketch your own as well below.
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Demo of mature cones of various other Pinaceae (Pine, Doug Fir, etc.). The relative sizes of
bract and ovuliferous scale may change as the cones grow. Which of the two units (bract or
scale) is largest in the mature cone depends on the particular taxon. Both units are quite large and
easily seen in Pseudotsuga menziesii (Doug Fir) because the conspicuous 3-pronged bracts stick
out from between scales. In Cedrus the bracts are tiny and difficult to find in the mature cone.
Diagram a few different cone types below.
Female gametophyte of conifers: We will focus on Pinus, the number of archegonia and other
details differ from family to family. Pinus mature female gametophyte (on dissecting scope).
Label the diagram below, indicate the ploidy of the innermost and outermost structures.
Compare to the Ginkgo ovule/seed you dissected.
Ploidy
Ploidy
Pine nut dissection: remove the seed coat (what part of the ovule is it derived from?______
What is its ploidy level?_________). Using a razor blade, make a longitudinal section and
observe under the dissecting scope. Identify the embryo with its cotyledons and central
vascular strand (stele). Using a dissecting needle, remove the embryo from the seed and
separate out the cotyledons (how many?_____).
Draw your observations beside the above diagram.
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