Download 1 Bio153H5 Lab 3 Greenhouse Tour: Diversity of Structure in

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Bio153H5 Lab 3
Greenhouse Tour: Diversity of Structure in Vascular Plants
Please read this background material before doing the self-guided tour.
Sign-up sheets for the 1½ - hour tour will be posted by the Bio153 lab
rooms shortly. The assignment sheet will be posted on the web by Jan
31st.
The terrestrial invasion: About 400 million years ago, autotrophic,
photosynthetic organisms known as plants successfully colonized land and
diversified into the many different kinds of land plants that we know today. Early
terrestrial plants were non-vascular; these are represented by the bryophytes
(liverworts, hornworts and mosses) that you saw in Lab 2. Today, we will be
looking at adaptations and diversity in the vascular plants.
For a plant, the great benefit of life on land was free access to sunlight, O2 and
CO2. However, moving to land came at a cost -- how do you stay in light and air
without drying out? Other problems associated with the transition from an aquatic
to a terrestrial lifestyle:
1) Unlike water, air gives no buoyancy or support for the body;
2) Unlike water, air provides no dissolved nutrients;
3) Air temperatures vary a great deal more than do water temperatures;
4) Completing the life cycle outside of water is difficult.
All terrestrial plants (from bryophytes onwards) solve the problem of water loss at
least in part by developing a waxy cuticle over most of their surfaces exposed to
air. This restricts water loss, but also restricts the exchange of gases; thus all land
plants (except for liverworts) have apertures for gas exchange called stomata.
Closing of the stomata prevents excessive water loss.
One important innovation was the development of vascular tissue -- vessels that
conduct water and minerals up from the ground, and transport products of
photosynthesis from the leaves. One type of water-conducting tissue (xylem) is
reinforced by the secondary deposition of a substance called lignin on the cell
wall. Associated with xylem is a delicate food-transporting tissue called phloem.
Xylem and phloem together form the plant's vascular tissue. Although xylem
originated as a transporting tissue it quickly became an important structural
supporting tissue. Without vascular tissue, terrestrial plants maintained their
upright stature only through turgor pressure; this limited not only the habitats they
could occupy, but also the size they could attain. Thus, vascular tissue solved 2 of
the problems associated with terrestrial life:
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1) It gave support to the plant body outside of water;
2) It allowed for the movement of water and nutrients in the body.
In effect, vascular plants act as wicks, drawing a “transpiration stream” of water
from the roots, through the conducting vessels. This facilitated the evolution of
large plants -- because plants are sessile, they cannot move away from each other,
so plants growing close together must compete for light by growing taller! The
clade of the Tracheophyta - represented today by psilophytes, horsetails, ferns,
cycads, ginkgos, conifers, gnetophytes and angiosperms -- show the most
advanced vascular systems, and thus include the largest plants. In the related
clade of the Lycophyta (clubmosses), vascular tissue is somewhat less developed.
But lycophytes were very large in the Carboniferous -- some species were up to
40m tall, and their fossilized remains form much of our coal deposits.
In many ways, the evolutionary history of plants is a story of progressive
adaptation to life on land and in air. In lab 4, we will discuss the other great (but
related) trend in the evolution of terrestrial plants: the reduction of the
gametophyte (the haploid structure giving rise to the gametes by mitosis), and
the predominance of the sporophyte (the diploid structure that gives rise to
spores by meiosis). In non-vascular plants, the gametophyte is the dominant
structure; the sporophyte is short-lived and inconspicuous. In non-seed bearing
vascular plants such as the fern, the frond is a diploid sporophyte; the haploid
gametophyte is generally a small, short-lived and inconspicuous structure clinging
to the ground. In seed plants, the gametophyte is reduced even further: it simply
becomes part of the seed. In the most advanced seed bearing plants (the
angiosperms), the gametophyte is just a few cells.
Organization of Vascular Plants: The vegetative parts of vascular plants
consist of three organs, the stem, the leaf and the root. Stems and leaves
together are called the shoot. These organs normally bear a consistent
organizational relationship to each other, but are often modified. These
modifications usually represent adaptations to a particular lifestyle.
1&2) Stems and leaves:
The shoot is the basic unit of vascular plants. In primitive vascular plants, the stem
is the only vegetative organ, and the site of photosynthesis. In most
tracheophytes, the stem consists of nodes where leaves and branches arise and
internodes (the length of stem between nodes). Stems are able to give rise to
new plants.
Leaves are borne on stems at the node. Each leaf has a bud in its axil (the angle
formed by the leaf and the apical part of the stem) which has the potential to grow
into a branch (or a flower). Normally all branches must arise at this position.
With a few rare exceptions, leaves are unable to give rise to new plants.
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Primitive vascular plants such as Psilotum possess small projections which lack
vascular tissue (prophylls). Lycopods have small leaves that are simple
extensions of the stem, and the single, unbranched strand of vascular tissue that
supplies these leaves does not leave a gap in the stem vascular tissue. Leaves of
this kind are described as microphylls. The leaves of most tracheophytes have
an elaborately branched vascular system, and the vascular tissue that supplies the
leaf leaves a gap in the stem vascular tissue. These leaves are known as
megaphylls. The currently accepted hypothesis is that megaphylls evolved from
branched stems. During the Devonian period, most plants branched
dichotomously (that is, the stem apex divided to produce 2 equal branches. In
some plants, the branches became unequal (accelerated growth?) so only one
continued growing up, while the other remained short and spread horizontally.
Leaves of primitive megaphyllous plants often show dichotomous branching of the
vascular tissue. Megaphylls vary greatly in form and function, as we will see in
this lab.
Modifications of leaves and stems:
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Cladophylls (or cladodes): A stem modified to resemble a leaf.
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Spines: Because there is strong selection to deter herbivory, there is a
tremendous diversity of sharp structures on plants, arising from a variety of
organs. Some spines are modified leaves, while others are stipules (leaf
like structures that develop at the base of a leaf). Spines may also be
formed as part of the stem, or from modified epidermal hairs. Thorns are
modified branches.
• Water storage: In some xerophytes (plants adapted to living in dry conditions),
leaves are highly reduced and the stem performs the functions of leaves. The
stem is thick and fleshy for water storage, is photosynthetically active, and bears
stomata. In these plants, leaves may be modified as spines. Other xerophytes
store water in the leaves.
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Food storage and perennation: Some underground stems and leaves
are modified for food storage and perennation (tissue that can
regenerate the deciduous parts of the plant). Examples include tubers,
bulbs, rhizomes and corms. You will look at these more closely in Lab
4.
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Climbing organ: Some climbing plants produce structures known as
tendrils to aid in support. Some tendrils originate as stems (e.g. in
grapevines), others are modified leaves (e.g. in legumes such as peas) or
leaf tips.
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• Water collection: Some leaves (e.g. in the bromeliads) are modified to collect
rain water, rather than to store it internally.
• Carnivory: Leaves can be modified to catch, digest and absorb insects.
3) Roots:
The root is the (usually) descending axis of a plant, which is normally underground.
It anchors the plant and absorbs water and minerals. Primary roots arise from
the embryo root found in the seed; adventitious roots grow out from a stem.
Roots do not bear leaves, and they produce branches in an irregular manner. Roots
normally cannot give rise to new plants, although there are a few exceptions: for
example, the root of a dandelion is able to produce a new stem.
Modification of roots:
Roots do not show the comparable diversity that we see in stems and leaves,
however, some modifications include:
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Food storage: Swollen roots commonly act as food storage, and
many "root vegetables" are such structures.
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Climbing organ: A few climbing plants such as English Ivy (Hedera)
and Trumpet Vine (Campsis) have roots that aid in climbing.
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Aerial roots: These are adventitious roots produced from above
ground structure. They can act to support, or prop the plant (e.g. in
corn); where they contact the soil they function in absorption of
water. Some orchids have special aerial roots capable of absorbing
moisture from the air (dew or rain). These roots can give rise to new
stems.
Roots require oxygen for respiration, which is why most plants cannot live in soil
that is badly drained and lacks air spaces. In some trees growing in swampy areas,
the roots may emerge from the water to aerate the root system (e.g. mangroves).
Many special adaptations of roots are found among epiphytes -- plants that grow
on other plants but are not parasites (orchids get a bad rep!). The root epidermis
is often waterproofed, and may be capable of photosynthesis.
The plants we have selected for you to examine during the greenhouse tour are
just a small sample of what is being grown. The greenhouse is normally open
from 9:00 to 5:00 every day, so please come back later and see the rest of the
specimens!
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Display material
The display material is fair game on the lab practical, so take notes! Pay
particular attention to any structures in bold type. The background material
may be helpful with lecture material concerning the evolution and diversification
of plants.
1) Lycopodium (clubmosses)
Lycopods evolved in the Devonian and were the dominant terrestrial plant form
in the Carboniferous; most had become extinct by the Permian. The living
clubmosses are not mosses (which are bryophytes); while superficially
resembling mosses, the moss-like structure is a sporophyte (in true mosses,
this is a gametophyte). Most extant species are tropical and epiphytic;
temperate clubmosses live on the forest floor and are conspicuous because they
remain green year round.
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Examine the Lycopodium specimen, noting the microphyll
leaves. Why do you think that temperate clubmosses can remain
photosynthetically active during winter?
2) Primitive megaphylls: the ferns (Pterophyta)
The most primitive living megaphyllous plants are the ferns. These usually
possess large, elaborately divided leaves (fronds) that may be much larger than
the rest of the plant. Commonly, the fronds are compound: the lamina is divided
into leaflets or pinnae which are attached to the rachis (the extension of the
leaf stalk). In some genera, the leaves climb like vines. Some tropical ferns can
have leaves that are 3-5 m in length; this huge flat surface for photosynthesis is
a major evolutionary step forward from the small microphylls seen in more
primitive land plants. In fact, most of what you see of a fern is its leaves; stems
are usually short and often subterranean. Many tropical ferns are epiphytes.
The fern “body” is the diploid sporophyte that gives rise to haploid spores by
meiosis. The sporangia (hollow uni- or multicellular structures that produce the
spores) are found on the margins or undersides of leaves, on specially modified
leaves, or on separate stalks. The sporangia often occur in button-like clusters
called sori. Almost all ferns are homosporous, meaning that meiosis produces
one type of haploid spore that develops into a bisexual gametophyte that
produces both male and females gametes. You may see the gametophtye at the
base of some of the ferns on display – it is a small, cryptic, short-lived flat
structure looking like a tiny, often heart-shaped “leaf”. You will have an
opportunity to see these structures in greater detail in Lab 4.
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Examine the ferns on display, noting the leaf structure and
occurrence of sporangia.
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If specimens of Camptosorus rhizophyllous or Asplenium viviparum are on
display, you may see new plants forming vegetatively on the leaves. (The ability
to form new plants from leaf tissue is not unique to ferns. The kalanchoë, a
common houseplant also on display in the greenhouse, is a succulent called the
“maternity plant”, due to its ability to produce plantlets (clonally produced
“baby” plants) from meristematic tissue located in notches along the margins of
the leaves). Examine the staghorn fern. This epiphytic fern superficially does
not resemble most ferns with which you may be familiar, but the presence of
sporangia on its leaves indicates its true identity. In the staghorn fern, the
gametophyte is large and permanent, clinging to the host tree.
3) The cycads: early seed plants
The seed was one of nature’s great innovations, equivalent to the cleidoic
(closed) eggs of reptiles: it protected the embryo (the young sporophyte,
formed by union of male and female gametes produced by the gametophyte)
from desiccation, solving one of the great challenges of reproduction in a
terrestrial environment. In the Devonian, progymnosperms gave rise to 2
distinct lineages of seed-bearing plants. The seed-ferns (which were not ferns,
but superficially resembled them) flourished in the Carboniferous and lasted into
the Triassic. These were not the ancestors of “true” seed plants, five of which
are still extant – the cycads, the ginkgo, the gnetophytes, the conifers and the
flowering plants. Thus, seeds of seed-ferns and of true seed plants are another
example of evolutionary convergence.
Cycads appeared in the Permian and flourished in the Mesozoic; this period is
often called the Age of Dinosaurs and Cycads. These palm-like plants are mostly
found in tropical and subtropical regions, and are relatively unchanged during
their long evolutionary history. Most are large, and the central part of their
trunks is a mass of pith. The stems are often underground. Many are highly
toxic, producing neurotoxins and carcinogens, but some have edible pith known
as sago. They harbour cyanobacteria, which help to fix nitrogen in the soil.
Cycads may have been the first plants to be pollinated by insects – the pollen
was likely spread by beetles. The pollen and ovulate cones are borne on
separate plants.
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Examine the cycads on display. Note the staminate
(pollen-producing) and ovulate cones on Zamia.
4) Modified leaves and stems: the xerophytes
If the evolution of vascular plants is a story of adaptation to life away from
unlimited water, xerophytes exhibit an extreme in their ability to cope with dry
conditions. In the string of pearls plant, the leaves are almost spherical
(maximizing surface area: volume ratio!) to minimize water loss. Plants such as
Crassula and Glottiphylum have numerous large leaves on thick stems.
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On the other hand, Lithops has only two leaves which are level with the
surface of the ground. Aloe and Agave are plants that show large leaves and a
reduced stem. (Note: Although they occupy a different biome, arctic plants such
as saxifrage also show reduction of the stem relative to the leaves. Why?). You
may remember that we discussed agave (the century plant) in class as an
example of a semelparous organism. While called the century plant, flowering
often occurs after 15 - 20 years of vegetative growth. The mother of this plant
flowered in spring of 1999 -- the inflorescence reached all the way to the ceiling of
the greenhouse! Seeds germinated on the ceiling and baby plants rained down
onto the floor of the greenhouse. This plant is one of those babies, due to
reproduce somewhere around 2015 to 2020….
Other xerophytes show significant reduction in the leaves, and elaboration of the
stems. The stem is thick and fleshy, acting as water storage, and bears stomata.
Most xerophytes lose water at the same rate as mesophytes (plants occupying
moderate habitats) if their stomata are open; thus, the best way to minimize
water loss is to “shut down” (i.e. go dormant) during dry periods. Many
xerophytes open the stomata at night to reduce water loss. Leaves of xerophytes
may be deciduous (the leaves are shed or dehisce when necessary to minimize
water loss), may be very small, or may be modified as spines. These modified
leaves (spines) can be found on several varieties of cactus, such as Opuntia.
Both cacti and euphorbs exhibit a thick stem and spines, but these characters are
not derived from a common ancestor. Their similarities are an example of
convergent evolution. One clue that euphorbs and cacti do not share a recent
common ancestor but have adapted to similar conditions is that the spines of the
euphorb are not modified leaves, but stipules (a structure arising from the stem
at the base of a leaf).
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Examine the various succulents, making note of which ones show
water storage in leaves and/or in stems. Which spines are leaves,
and which are stipules? What is the function of the hairs covering
many succulents? Look also at the kalanchoë, and see if you can
see plantlets forming on the leaves.
Other shoot modifications:
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Cladophylls (or cladodes): A stem modified to resemble a leaf.
An example is Ruscus (Butcher's Broom), or the filmy, leaf-like
branches of asparagus. Look for the minute white scale leaves and
for the flower buds, or even flowers, on the cladodes
(cladophylls).
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5) Behavioural modifications in leaves: nastic movements
In general, plants have limited scope for behaviour, but some types of plant
movements can be striking. Nastic movements are movements that occur in
response to a stimulus, but the direction of movement is independent of the
direction of the stimulus. Nyctinastic movements (meaning “night closure”) are
common; leaves in many groups of plants are oriented vertically at night, but open
horizontally during the day. Here we will examine thigmonastic movements
(movements in response to mechanical stimulation) in 2 plants – the mimosa
(Mimosa pudica, the” sensitive plant”) and the Venus fly trap (Dionaea muscipula).
In mimosa, the leaflets or entire leaf will droop in response to touching, shaking, or
changes in temperature. The movement is caused by a drop in turgor pressure in
cells at the base of each leaflet and leaf. The stimulus can travel from one leaflet
to others via electrical and chemical mechanisms. In the Venus fly trap, the rapid
closing of the leaf appears to be driven by changes in turgor pressure in the
mesophyll cells under the epidermis. When the trap is open, the mesophyll cells
are compressed and under pressure; sudden change in tugor pressure causes the
trap to fold in. The leaf will not close unless 2 of the 3 sensitive hairs on the trap
are stimulated, or one is stimulated twice. Insects are attracted to the nectar on
the surface of the leaf; once the trap is closed, the insect is pressed against
digestive glands on the surface of the trap. Not all carnivorous plants exhibit nastic
movements. The pitcher plant (Sarracenia purpurea) lives in nitrogen-poor soils,
and meets its nitrogen demands by capturing and consuming insects. The inside of
the tubular shaped leaf is lined with downward pointing hairs. Once an insect falls
into the plant, these hairs prevent it from crawling out and escaping. The fluid in
the bottom of the tube contains digestive juices. However, some insects coexist
with the pitcher plant: mosquito larvae can grow in the “pitcher” and emerge (fly
out) as adults.
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Examine the mimosa, and stimulate the thigmonastic movement by
gently touching one leaflet.
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Examine the Venus fly trap, but DO NOT TOUCH!!*** Closing
requires the stimulus of more than one of the hairs. Why do you
think this stimulus threshold is adaptive?
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Examine the pitcher plant, noting the modified tubular leaves and
the downward pointing hairs.
*** We ask that you not touch the Venus fly trap because it takes several days for
a leaf to reopen, and repeated touching stresses the plant. Please don’t harass the
plants! Remember how many of you will be passing through the greenhouse over
the next two weeks…
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6) Epiphytes
Plants living in tropical rainforests generally have abundant moisture and a
favourable temperature regime for growth. However, the density of biomass in
the forest creates other problems – access to light being one of the foremost.
Thus, epiphytes (plants that grow on other plants) are common, and include
such groups as ferns (e.g. the staghorn fern), orchids and bromeliads.
These groups come from separate lineages; thus epiphytic habit is another
example of evolutionary convergence. Epiphytes are not parasites because they
do not derive nutrition from the host plant; they obtain water from the humid air
of the canopy, and minerals from dust. Flowering epiphytes also have access to
the numerous pollinators (insects, birds and mammals) that live in the forest
canopy. Bromeliads have leaves that merge at the base to form watertight
collection tanks; some large species may store 45 litres of water! These pools of
water may harbour entire communities – bacteria, protists, insects and insect
eaters. An extremely unusual modification of both leaves and roots is found in the
“flower pot plant”, Dischidia rafflesiana. Some of the leaves of this epiphyte form
hollow cups -- the “flower pots” -- that collect rain and debris. Roots, formed at
the node above the modified leaf grow down and into the pot, absorbing water and
nutrients! Orchids make up the largest family of flowering plants (at least 24,000
species!). Orchids often have aerial roots that allow for vegetative propagation
of the plant. These roots have a thick epidermis called velamen, which in some
species is the only photosynthetic tissue.
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Examine the bromeliads and note the shape of the leaves.
Examine the orchids and note the aerial roots.
The assignment is due on Monday, Feb 28th, and is worth 4% of your final
grade. You are welcome to come back to the greenhouse at a later date, but
please note that not all specimens will be labelled or on view after Feb 18th.