Download 1 2006S Bio153 Lab 6: Gymnosperms and Angiosperms July 24th

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2006S Bio153 Lab 6: Gymnosperms and Angiosperms
July 24th / July 26th
The seed plants: The final step in plants’ conquest of land came with the evolution of
the seed. The seed protected the plant embryo (the young sporophyte) from
desiccation, allowing plants to break free of a dependence on water to complete the life
cycle. All seed-bearing plants are heterosporous: they have microsporangia that
produce male microspores (which develop into the microgametophyte) and female
megasporangia that produce female megaspores (which develop into the
megagametophyte). The megagametophyte is highly reduced and retained inside
the megaspore. The megaspore, in turn, is retained in the megasporangium; the
whole structure is called the ovule. The ovule is enclosed by sporophyte tissue called
the integument. Thus, the whole, vulnerable gametophyte generation that produces
the female gamete is packaged and protected within the tissues of the sporophyte. The
microspore is chaperoned by parent cells in a package called pollen, which is
delivered to the ovule by wind or a pollinator. The gametes do not need water to
produce a zygote.
The first known seed-like structure appeared in the late Devonian, around 370 million
years ago. 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 angiosperms. Thus, seeds of seedferns and of true seed plants are another example of evolutionary convergence.
The seed plants are a monophyletic group consisting of the gymnosperms (cycads,
ginkgo, gnetophytes and conifers) and the angiosperms. Gymnosperms ("naked
seeds") are plants that produce seeds on the surface of modified leaves. Scales of a
pine cone represent modified leaves on which seeds are borne. In contrast, the
flowering plants or angiosperms (“vessel seeds”) are plants that produce seeds within
a fruit.
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The Gymnosperms: While the gymnosperms are considerably more advanced than the
seedless vascular plants, they still possess several limitations. Because they do not
produce flowers, they do not attract animal pollinators and must depend on wind to
deliver the male gamete to the ovule. As well, the time required to complete seed
development in gymnosperms is long; it can take up to 3 years from pollination to
completion of seed development. They do not produce fruit, which in angiosperms is an
effective means of seed dispersal. Because of these limitations, they are a less diverse
group than the angiosperms. However, they are often very successful in extreme
environments.
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. Zamia spp. are
dioecious, meaning that the staminate (pollen-producing) and ovulate cones are
borne on separate plants.
Ginkgo biloba is a deciduous, broad-leaf gymnosperm.
It is called a relictual species, or "living fossil", because it
is the only living member of this group. In fact, the
Ginkgo probably exists today because it was cultivated in
monastery gardens in China. Ginkgo biloba is a
dioecious plant that is frequently planted in urban
areas. Most of trees you will see are males as the
females produce a foul odour!
Most living gymnosperms are conifers (pines,
cedars, spruces etc.). They have narrow, needlelike leaves that have low rates of photosynthesis,
but are very resistant to water loss. Thus, conifers
are often very successful in dry or cold
environments (where water is scarce because it is
frozen much of the year). The name “Conifer”
refers to the reproductive structure (the cone).
Fertilization and development occur on the female
cone.
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The Angiosperms – the flowering plants
The flowering plants are the most diverse, specialized and complex of the plants. They
appear relatively late in the fossil record – although early terrestrial plant fossils date
from about 470 million years ago, angiosperms appear in the late Jurassic (around 140
million years ago). After their appearance, they radiated rapidly and now represent
more than 90% of all plant species. Much of their success is due to their complex
associations with pollinators.
Flowers vary greatly in morphology,
but are made up of a consistent
pattern of structures. In general,
there are two groups of structures
(fertile and sterile). The sterile
structures include the sepals
(collectively the calyx) and the
petals (collectively the corolla).
The sepals are usually green and
support the corolla; the petals are
often brightly coloured to attract
pollinators. Collectively, the calyx
and the corolla are called the
perianth.
The function of the perianth is to attract pollinators. The fertile structures are the
sexual organs (the genitalia of the plant). The male components are collectively called
the androecium (“house of the male”); the female, the gynoecium (“house of the
female”). The androecium is made up of a variable number of stamens. Each
stamen is made up of a filament bearing the anther, which produces and releases
the pollen. The gynoecium is made up of one or several carpels. The carpel is a
modified leaf that contains the ovule. Each carpel has an ovary at its base, a tube
called the style, and a specialized structure at the top of the style called the stigma,
which is designed to receive the pollen. Flowers that contain both male and female
organs are called perfect; those that contain only the androecium or gynoecium are
called imperfect. Flowers may be single or grouped together in an inflorescence.
Angiosperms, like all seed plants, are heterosporous. Microsporangia produce
microgametophytes; megasporangia produce megagametophytes. The
gametophytes are now truly parasites on the sporophyte generation; the male
gametophyte is reduced to just 3 haploid cells: 2 sperm cells plus a cell that forms the
pollen tube. The female gametophyte is just 7 haploid cells, one of which is the egg
cell. The remaining 6 cells contribute to the formation of the seed. The term
angiosperm (“enclosed seed”) reflects a unique character of this group – the enclosed
ovule. The ovule of an angiosperm is the organ that contains the female gametophyte,
and within it, the egg cell. Thus, the egg cell is completely enclosed in layers of tissue.
It is thought that the enclosed ovule evolved as a mechanism to prevent herbivory.
Fertilization occurs via the pollen tube which grows out of the pollen (somewhat like
the hypha of a fungus) and burrows through the style to deliver the sperm nuclei.
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Following fertilization, the ovule develops into a seed and the ovary wall forms a
fruit. Thus, a fruit is a fertilized, mature ovary. Fruits have evolved as specialized
structures for seed dispersal.
Because plants are not motile,
there success depends on being
able to exploit other means to
move offspring away from the
parent plant to prevent direct
competition between parent and
offspring for the same resources.
Some fruits are fleshy; seeds are
dispersed after the fruit is eaten
and the seeds pass unharmed
through the digestive tract of the
frugivore (fruit-eating animal).
Dry fruits are inedible and are
dispersed by wind, by clinging to
the exterior of animals, by
floating, etc.
Pollination
Early angiosperms were probably wind pollinated, which has several disadvantages.
First, vast quantities of pollen must be produced to increase the chances of pollen
reaching a conspecific. Second, the chance of inbreeding is high, because most pollen
falls close to the parent plant, possibly landing on a closely-related individual. Wind
pollination also doesn’t work well in moist environments, because moisture weighs down
the pollen grain. To increase the probability of trapping pollen grains that float by on
the wind, the ovule exudes sap. In the transition from wind pollination to insect
pollination, insects were attracted to this sap as a food source. By visiting many plants,
they transferred pollen from one to the other. This created selection for the plant to
develop mechanisms to attract insects, and eventually led to the evolution of nectaries
(nectar-producing bodies) and conspicuous flowers. By the Cenozoic, other types of
pollinators, including vertebrates (bats and birds) arrived on the scene, contributing to
the diversity of flowering plants.
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Coevolution occurs when 2 species exert selective pressures on each other, resulting
in evolutionary changes in both species. Certain flower morphs have evolved
independently several times in response to the characteristics of the pollinators that visit
them. These morphological types are often referred to as “pollinator syndromes”.
Table 1. Some examples of pollinator syndromes.
Pollinator
Bees
- don’t see red
- see in UV spectrum
- diurnal
- land to feed
Butterflies
- good vision
- poor sense of smell
- diurnal
- long proboscis
Moths
- nocturnal
- good sense of smell
- hover feeders
Bats
- nocturnal
- good sense of smell; good vision!
- large head
Birds
- esp. hummingbirds
- see red better than blue
- poor sense of smell
Flies
- poor vision; attracted to carrion, feces
Flower Type
- usually blue or yellow (bees don’t see red)
- often have UV guides (markings visible in the UV
spectrum)
- sweet scent
- landing platform
- narrow floral tube to accommodate the proboscis
- open in daytime
- often red, odorless
- long, narrow floral tube
- landing platform
- often white (visible at night)
- strong smell (esp. at night)
- plants flat or bent backwards
- white
- musty smelling
- sturdy flowers (accommodate bat’s head)
-
bright, often red
little smell
long tube
no landing platform
brownish red, often inconspicuous
smell like rotting meat, feces
The relationship between plants and pollinators is mutualistic, in that both receive a
benefit from the interaction. At the simplest level, the benefit to the plant is directed
transfer of pollen, and the benefit to the pollinator is a nutritive reward – nectar and/or
pollen. (There are other rewards that we will discuss in lecture.) The plant provides the
reward to ensure that the pollinator will be motivated to visit another plant of the same
type. However, there are costs to both parties in participating in this relationship. For
the plant, the production of nectar (and extra pollen, if the pollinator is a bee) is
energetically costly. For the pollinator, visiting flowers and handling them (especially
while it is learning how to handle the flower) is also energetically costly. Both parties
are trying to maximize their returns and minimize their energetic expenditure, which can
create conflict: the plant “wants” the pollinator to visit many plants of the same type;
the pollinator would “like” to visit only a few plants that provide substantial rewards.
Thus, the plant must balance the benefits of producing a substantial reward with the
costs (not only is it expensive to produce, but a large reward may produce satiation,
discouraging the pollinator from visiting another flower and depositing pollen!). This
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cost/benefit asymmetry can have interesting implications for the coevolutionary paths of
plants and pollinators.
Another consideration is that of specialist and generalist pollinators. A plant
benefits from having a specialist pollinator – if the pollinator only visits plants of a
particular species, pollen is always being delivered to the appropriate type of plant.
Sometimes it is also beneficial to a pollinator to specialize on one type of plant – it may
lessen competition with other pollinators. It has been hypothesized that some plants
have evolved extremely complex morphologies with hard-to-access nectaries specifically
for this purpose: by being difficult to handle, they ensure that only one species of
pollinator (or even a small subset of a species that learn the “trick”) will visit the flower.
In these cases, the reward may be high to motivate these pollinators to specialize
exclusively on this type of flower. Yet again, however, there may be conflict: it may be
advantageous for the pollinator to be more generalist, so it can feed on many types of
flowers without having to travel long distances to find food.