Introduction to Seed Plants:
Gymnosperms

Chapter 22

. Evolution of the Seed Habit    

In the land plants considered so far there are two limitations that are evident;-  

Water requirement of the gametophyte: 

The sporophyte can only grow where the gametophyte can succeed. The gametophyte is the weak link in the life cycle. 

.Lack of genetic variability:  In many of these plants it is quite possible for male gametes produced by a gametophyte to fertilize eggs produced by the same gametophyte. In such cases, the only source of variation is the crossing over events of meiosis during spore production. This is problematic as the fitness of a species relies on there being variation in a population on which natural selection can act. 

. Heterospory promotes outbreeding and genetic variability and it is not surprisingly therefore that heterospory evolved independently in at least four different plant groups - the ferns, club mosses, horsetails and seed plants. Some of these plants even had mixed sporangia with both microspores and megaspores in one sporangium!

. Producing a few large megaspores invested with greater food reserves would ensure the female gametophyte could not only produce eggs but nurture the young sporophyte in its early parasitic phase. 

. Seed plants evolved from heterosporous, endosporous vascular plants

. Imagine such a plant, producing only one megaspore per megasporangium. The megaspore is not released until the female gametophyte has fully developed within it, complete with archegonium and egg.

. Imagine that the whole megasporangium (with its single megaspore) is released. Imagine too that the megasporophyll and ligule (A, B) extend and enwrap the megasporangium, protecting it. This composite structure is an ovule

. Imagine that this protected megasporangium (with its single megaspore and internal gametophyte) is retained on the parent sporophyte until the egg in its archegonium is fertilized and forms an embryo.

. Only when this has happened, and food reserves are laid down with the embryo, and the outer protective integuments harden, is this structure released. This is what we call a seed. 

. This is an oversimplification but it was by this kind of progression that the seed habit is thought to have evolved. 

The evolution of the seed reduced the vulnerable gametophyte phase of the life cycle drastically. 

Whereas before dispersal depended on spores taking gametophytes (and ultimately sporophytes) to new habitats, the seed is the new means of dispersal.  

The first seed plants were gymnosperms (or "naked-seed" plants) and first appeared in the Devonian period, soon after the first land plants. 

Outline

Overview

Phylum Pinophyta

Phylum Ginkgophyta

Phylum Cycadophyta

Phylum Gnetophyta

Human Relevance of Gymnosperms

Conifers

Other Gymnosperms

Plants

The History of Gymnosperms

Gymnosperms descended from Devonian progymnosperms.

·     Adaptive radiation during the Carboniferous and Permian periods led to today's divisions.

During the Permian, Earth became warmer and drier; therefore, lycopods, horsetails and ferns (previously dominant) were largely replaced by conifers and their relatives, the cycads

Plants

This large change marks the end of the Paleozoic era and the beginning of the Mesozoic era.

Human Relevance of Gymnosperms

Conifers

Edible Seeds

Crates, Boxes, Matchsticks, Furniture

Telephone Poles

Turpentine and Rosin (Resin)

Fuel (Pitch)

Pulpwood

Ornamentals

Pharmaceuticals (Taxol)

Human Relevance of Gymnosperms

Other Gymnosperms

Ginko Seeds - Food

Ephedra - Mormon tea

Overview

Gymnosperm refers to the exposed nature of the seeds.

Produced on the surface of sporophylls or similar structures instead of being enclosed within a fruit as in flowering plants.

Female gametophyte is produced inside an ovule that contains a nucellus.

Enclosed in integument.

Four living Phyla

Exposed versus Enclosed Seeds

. The evolution of the seed plants - the Gymnosperms

. Gymnosperms

1. The transition to life on dry land has been marked by the reduction of the gametophyte generation and the development of heterospory. The female gametophyte developed in such a way as to provide a degree of protection as the zygote started its existence as the next sporophyte

generation.

. There was still, however, dependence on free liquid water to carry out fertilization.

Additional developments were needed to increase the probability of success for the most vulnerable parts of the life cycle:

.  

A. These brought a loss of dependence on water for fertilization.

B. Additional protection (increased shielding and nutrition) for the developing embryo -culminating in the appearance of seeds.

.  

2. We will examine a bit of the picture with regard to the evolution of the seed plants and

highlight some of the important adaptations that allowed a successful move to dry land.

. A. Lines of early vascular plants gave rise to the Lycophyta, in which heterospory arose, the Psilophyta, the Pteridophyta and the Sphenophyta.

B. Another branch led to the Rhyniophyta and then to the Trimerophyta. From the

Trimerophytophyta came the progenitors of modern seed plants, the Progymnosperms in the

Paleozoic Era (408 to 360 mya

. The flora that existed in the Upper Silurian to Lower Devonian was probably an assemblage of marsh inhabitants with structural feature transitional between aquatic non-vascular and land vascular plants.

. In these very primitive land plants in which there was little differentiation between the different parts. They had simple shoots rise from a creeping axis which hardly differs in structure from the upright shoots themselves.  These shoots or stems often branched in a simple manner, forking into two, and then into two again, and sometimes the shoots terminated in spore capsules called sporangia.

These ancient plants lacked leaves, seeds, and flowers. Instead of roots they had horizontal stems, connected with the soil by root hairs. The whole plant was generally less than 50 cm in height.

. Trimerophyta specimens are believed to have evolved directly from Rhyniophyta and are found in the fossil record of 360 million years ago.

. Included in this group are

the first trees. While all species of the Progymnosperms are now extinct, examination of the fossil record allows us to see that they illustrate several important trends .

. 1. Secondary growth (formation of secondary conducting cells from cells produced in a meristematically-active vascular cambium) had originated in the Lycophytes and the Sphenopyhta (plants like Equisetum), but the cell divisions were only in one plane

(periclinal).

. Production of a great amount of secondary xylem (which would, of

necessity, increase the tree's circumference) was limited because the circumference (i.e.,

completeness of the ring of cells) of the vascular cambium was limited.

. However, in the Progymnosperms, cambial cells evolved the ability to perform radial, longitudinal (anticlinal) divisions. This reduced the amount of tangential growth that the secondary cells had to carry out to accommodate increasing stem (trunk) circumference and so increased the strength and water-conducting capacity of the xylem tissue because there was a more complete production of wood.

. Water conducting cells were all tracheids; the open-ended vessel elements had still not evolved.

2. Megaphyll type leaf structure appeared (we have already seen this in the ferns, but ferns are off of the main line of seed plant evolution from the green algae).

. 3. In some heterosporous lines a single megaspore was produced by each megasporangium.

The megaspore was retained and contained by sporangium-protecting tissuesof sporophyte origin (leaves or modified leaves). These integuments were apparently adapted sterile telomes (perhaps forerunners of megaphylls).

. Fertilization occurred when a microspore was caught in the protective structure surrounding the megagametophyte and released its sperm. The sperm used trapped liquid water to swim the short distance to

the egg. The protective layers of the integuments and the megagametophyte surrounding the developing zygote constituted a forerunner to the modern seed.

. C. Out of the Progymnosperms evolved the modern Gymnosperms (literally naked [gymno] seeds [sperm]). There are only about 900 species of Gymnosperms today. In the Mesozoic era (245-65 mya) they were much more diverse. In fact, the Mesozoic Era was the era of the

Dinosaurs and the Gymnosperms.

. It was a time when continental drift caused shifting of large land masses and the raising of mountains. This led to dramatic temperature gradients following gradients in elevation and extensive arid areas in the "rain shadows" of the mountains. Plants, like the Gymnosperms, that were adapted to lower temperatures and drier conditions, were very successful.

. The Gymnosperms are linked by being woody plants (i.e., they contain secondary xylem) with a dominant sporophyte generation (trees, shrubs or vines), bearing naked seeds (i.e., seeds not enclosed in additional sporophyte-derived structures), and having separate pollen (male gametophyte-producing) and seed (female gametophyte-producing) cones.

. The name Gymnosperm describes the seed-bearing habit of these plants. From a strict evolutionary perspective, the Gymnosperms are not a single taxonomic grouping (i.e., all Gymnosperms do not share a single evolutionary predecessor). There are four living Divisions of Gymnosperms.

. These are:

1. Cycadophyta - cycads: have short, thick stems, large compound leaves, and separate male and female plants. Relatively few species exist today, but there were many cycads in the Age of Dinosaurs. They resemble palm trees (sago "palms" found in Florida).

Phylum Cycadophyta

Phylum Ginkgophyta

. 2. Ginkgophyta - Ginkgo biloba is the only living species. It is deciduous (losing its leaves in Winter) and the leaves are dichotomously- veined. There are separate male and female trees. Trees are found on campus near Hunt Hall. The female tree bears seeds which have a fleshy coat and reek of butyric acid. Ginkgo extracts are popular now because they are thought to improve memory.

Plants

Ginkgo biloba is the only living species of an ancient gymnosperm lineage, the Ginkgoopsida, which dates from 270 million years ago (the Jurassic).

Highly revered in its native Asia and cultivated around temples, ginkgo is nearly extinct in the wild. Plants are dioecious (separate female and male), and female trees produce edible gametophytes known as ginkgo nuts.

These are surrounded by a fleshy covering, which putrifies when ripe and smells like rancid butter. Ginkgo extracts are valued in traditional medicine for reducing memory loss and slowing aging, and in western medicine in the treatment of Parkinson's Disease. Ginkgo is also known as "maidenhair tree", because its leaves resemble the fronds of maidenhair fern (Adiantum).

3. Gnetophyta - includes three rather strange groups of plants

a. Gnetum, which looks like a flowering vine or tree

b. Ephedra, shrubs or bushes with reduced leaves. These are found in desert regions of Mexico and the US southwest. Extracts are used as "herbal" remedies ("Ephedrin")

Phylum Gnetophyta

Phylum Gnetophyta

c. Welwitschia, a bizarre plant with two long, ever-growing straplike leaves.Found in Nature only in deserts of South Africa.

. The plant you are looking at resembles something from outer space, especially as it is one of the few species that can survive in this severe desert in Namibia. Welwitschia mirabilis is the most unusual gymnosperm in the world and grows only in a narrow strip paralleling the cold Atlantic,

. the sole source of moisture (as fog) in a region with an average rainfall of zero inches.

This is a male plant, the orange male strobili are evident on the right. Only two leaves are produced but as the plant grows and the leaves split, this peculiar feature of the plant is obscured. March 2003.

.  Xylem with vessel elements is found in the Gnetophyta. It is thought that the vessels in this Division are derived from tracheids with bordered pits. Since it is thought that the vessel elements of flowering plants are derived from scalariform tracheids, the vessels of the Gnetophyta must represent yet another example of convergent evolution.

Phylum Pinophyta

The Conifers

Pines

Largest genus of conifers.

Over 100 living species.

Include world’s oldest known living organisms (Bristlecone Pines).

Phylum Pinophyta

Plants

·   Thick cuticle covers the leaf.

·   Stomata are in pits, reducing water loss.

Despite the shape, needles are megaphylls, as are leaves of all seed plants.

Phylum Pinophyta

Most wood varies considerably in hardness.

Most gymnosperm wood consists primarily of tracheids.

No vessel members or fibers.

Soft Wood

Mycorrhizal fungi associated with the roots of most conifers.

Pine Stem Cross Section

4.. Pinophyta (also called Coniferophyta - cone bearers, but some Pinophyta do not have

woody cones) .

Include pines, redwoods and junipers . They are widespread and are important ecologically (covering a great deal of the Earth's northern regions and accounting for an estimated 10% of its photosynthesis) and economically

(being the source of lumber and paper).

. The waterconducting cells in all other Divisions of the Gymnosperms are tracheids.

Pines are heterosporous, they produce male and female cones which are borne on the same plant. Unlike the Cycapophta and Ginkgophyta, the Pinophta (and Gnetophyta) do not

have swimming sperm.

Plants

. The Life History of a Pine

The life cycle of pine, a representative conifer, is characterized by the following:

The multicellular sporophyte is the most conspicuous stage; the pine tree is a sporophyte, with its sporangia located on cones.

Plants

·        The multicellular gametophyte generation is reduced and develops from haploid spores that are retained within sporangia.

The male gametophyte is the pollen grain; note that there is no antheridium.

The female gametophyte consists of multicellular nutritive tissue and an archegonium that develops within an ovule.

Plants

Conifer life cycles are heterosporous; male and female gametophytes develop from different types of spores produced by separate cones.

·        Trees of most pine species bear both pollen cones and ovulate cones, which develop on different branches.

Plants

Pollen cones have microsporangia; cells in these sporangia undergo meiosis producing haploid microspores, small spores that develop into pollen grains - the male gametophytes.

Plants

Ovulate cones have megasporangia; cells in these sporangia undergo meiosis producing large megaspores that develop into the female gametophyte. Each ovule initially includes a sporangium (nucellus) enclosed in protective integuments with a single opening, the micropyle.

Plants

It takes nearly three years to complete the pine life cycle, which progresses through a complicated series of events to produce mature seeds.

Windblown pollen falls onto the ovulate cone and is drawn into the ovule through the micropyle.

The pollen grain germinates in the ovule, forming a pollen tube that begins to digest its way through the nucellus.

Plants

·       A megaspore mother cell in the nucellus undergoes meiosis producing four haploid megaspores, one of which will survive; it divides repeatedly by mitosis producing the immature female gametophyte.

·        Two or three archegonia, each with an egg, then develop within the multicellular gametophyte.

Plants

More than a year after pollination, the eggs are ready to be fertilized; two sperm cells have developed and the pollen tube has grown through the nucellus to the female gametophyte.

·       Fertilization occurs when one of the sperm nuclei unites with the egg nucleus. All eggs in an ovule may be fertilized, but usually only one zygote develops into an embryo.

Plants

The pine embryo, or new sporophyte, has a rudimentary root and several embryonic leaves. It is embedded in the female gametophyte, which nourishes the embryo until it is capable of photosynthesis. The ovule has developed into a pine seed, which consists of an embryo (2n), its food source (n), and a surrounding seed coat (2n) derived from the parent tree.

Plants

Scales of the ovulate cone separate, and the winged seeds are carried by the wind to new locations. Note, that with the seed plants, the seed has replaced the spore as the mode of dispersal.

A seed that lands in a habitable place germinates, its embryo emerging as a pine seedling.

Plants

. A. Male (pollen or staminate) cones are small and are found, generally in clusters, lower down in the canopy. The cone has an axis (continuation of a branch) that bears papery leaves which carry two microsoporangia on their lower surface (because they bear sporangia these papery leaves should be thought of as sporophylls).

. Cells inside this sporangium undergo meiosis to produce 4 haploid microspores. Many microspores are produced in a given microsporangium. While the microspores are still within the sporangium, the haploid nucleus of each undergoes 2 mitotic divisions. Thus, within its spore coat, each microspore contains four haploid nuclei.

. Two of these are described as representing the relictual vegetative gametophyte and play no further role

in the reproductive process. The other two nuclei will be important when the immature male gametophyte develops further. The microspore cell wall develops into a thick, resistant wall which bears two wing-like projections.

. This immature microgametophyte (4 haploid nuclei enclosed in the wing-bearing wall) is a pollen grain. A pine tree will produce a great amount of

pollen in a single season.

. B. Female (seed or ovulate) cones are found higher in the canopy. When they are immature they are smaller than the male cones, but as they develop, over a period of a few years, they become much larger and more conspicuous. They are more complex than male cones. There is a central axis.

. The axis bears modified leaves (bracts) and the axillary bud of each of the bracts has grown out to form a modified branch, the ovuliferous cone scale. The upper surface of each scale bears 2 ovules.

. 1. At the center of each ovule is a megasporangium that contains a giant diploid cell (megaspore mother cell or megasporocyte) which will eventually undergo meiosis.

2. This is surrounded by an outgrowth of the scale, the integument. The integuments effectively surround the megasporangium, except for a point at its end which is oriented toward the cone's central axis. This gap in the integuments is called the micropyle.

.

Pine Ovule - Longitudinal Section

. Note: because the megaspoirangium sits on a modified branch, and not on the bract which is a modified leaf, the bract should not be thought of as a sporophyll.

. C. Pollen grains are of light weight and, when released because of the splitting of the microsporangium wall, are readily scattered by the wind. The fact that female cones will be borne higher up in the tree will tend to increase cross pollination (i.e., pollen from one tree will interact with the female parts of another tree).

. The scales of the seed cone are sufficiently separated that pollen grains can be blown between them, settling in the "axil" of the scale, near the central axis of the cone, where the micropyle is located. This opening is filled with a sticky fluid (the pollination droplet) which traps and orients the pollen grain at the point where the megasporangium is exposed via the micropyle.

. The wings of the pollen grain apparently help orient the grain properly. When a pollen grain is oriented against the megasporangium the integuments close down and trap it in a pollen chamber.

D. Once the pollen grain is trapped within the pollen chamber, the pollen grain and ovule ("immature" male and female gametophytes, respectively) complete their development.

. 1. The single megasporocyte in each ovule undergoes meiosis to produce four haploid nuclei, and three of the four then die. The fourth cell divides mitotically several times to establish a megagametophyte containing a mass of storage cells and two archegonia,

each with a large egg.

. 2. Two of the four cells (nuclei) in the pollen grain are relatively inactive, the other two are very important. One (tube cell) begins to form the pollen tube that elongates by tip growth (like a fungal hypha) and digests a path through the micropyle, the megasporangium and megagametophyte wall to reach the egg in its archegonium.

Pine Life Cycle

. The other cell (generative cell) divides once mitotically to form two non-motile sperm cells which are

moved through the tube (probably) by contractile elements of the cytoskeleton of the tube. Eventually the tip of the tube opens, sperm are released and fertilization occurs.

Note: This has been accomplished without free, liquid water!

.E. The resulting zygote undergoes mitotic divisions to establish a group of cells (suspensor) which elongate to force the remaining cells (proembryo) deep into the nutrient-filled tissue of the old megagametophyte. Then an embryo (with radicle, hypocotyl and epicotyl, and several cotyledons) forms.

. The integuments (which are diploid cells from the previous sporophyte

generation) harden to form the seed coats. The resulting seed contains the seed coats, a dormant embryo (diploid, it will develop into the next sporophyte generation) and food reserves (haploid cells from the megagametophyte].

. Dormant means that the embryo is in a "holding pattern". It is fully formed but it is not yet ready to germinate and start the next life cycle. Does dormancy have importance?

. F. While the development of the pollen grain with its pollen tube has eliminated the need for liquid water, it should be clear that the pollination process rather inefficient. Pollen dispersed by wind will fly in all directions. Only a small proportion will land in the spaces between the

bracts of a seed cone and end up near the micropyle and be trapped in the pollination droplet.

. Most of the pollen is wasted. Clearly an adaptation that increases the efficiency of pollen delivery to the female would be an additional improvement in the reproductive process.

Pine Life Cycle

. Wollemia nobilis The Wollemi Pine

Wollemia nobilis is a tree conifer in the plant family Araucariaceae with its closest relatives being the Kauri, Norfolk Island, Hoop, Bunya and Monkey Puzzle pines. The discovery of the Wollemi Pine in 1994 created great excitement amongst the botanical world as it was presumed to have been extinct, only known to botanists through its appearance in fossils dating back 91 million years and then disappearing around two million years ago.

Plants

Coniferophyta (Conifers)

Coniferophyta is the largest division of gymnosperms:

Most are evergreens and include pines, firs, spruces, larches, yews, junipers, cedars, cypresses, and redwoods

Plants

.

. From a seed no bigger than one from a tomato, California's coast redwood (Sequoia semperviren) may grow to a height of 367 feet (122 m) and have a width of 22 feet (7 m) at its base. Imagine a 35-story skyscraper in your city and you have an inkling of the trees' ability to arouse humility.

. A combination of longitude, climate, and elevation limits the redwoods' range to a few hundred coastal miles. The cool, moist air created by the Pacific Ocean keeps the trees continually damp, even during summer droughts. These conditions have existed for some time, as the redwoods go back 20 million years in their present range.

. Exactly why the redwoods grow so tall is a mystery. Theories continue to develop but proof remains elusive.The trees can reach ages of 2,000 years and regularly reach 600 years.

Resistance to natural enemies such as insects and fire are built-in features of a coast redwood. Diseases are virtually unknown and insect damage insignificant thanks to the high tannin content of the wood. Thick bark and foliage that rests high above the ground provides protection from all but the hottest fires.

.