BIOLOGICAL DIVERSITY: SEED PLANTS

BIOLOGICAL DIVERSITY: SEED PLANTS
Gymnosperms are Seed Plants

Gymnosperms have seeds but not fruits or flowers. Gymnos means naked, sperm means seed: thus the term gymnosperm = naked seeds. Gymnosperms developed during the Paleozoic Era and became the dominant seed plant group during the early Mesozoic Era, as shown in Figure 1. The ancestors of gymnosperms were some now-extinct type of heterosporous fern or related group. There are 700 living species of gymnosperms placed into four divisions: conifers (such as pines and spruce), cycads (such as the sago palm, Cycas revoluta), ginkgos (the maidenhair tree, Ginkgo biloba), and gnetophytes (such as Mormon tea, Ephedra).

Gymnosperms are undoubtedly the group from which the angiosperms developed, although, as Charles Darwin noted in Origin of Species, which group "remains an abominable mystery". Numerous gymnosperm groups have been proposed as flowering plant ancestors over the past century.
Cycads

Cycads are placed in the Division Cycadophyta. They retain several fern-like features, notably pinnate leaves and circinate vernation. However, they usually produce cones of nonphotosynthetic reproductive structures, a distinctively unfernlike feature. Cycads, like all seed plants, are also heterosporous, unlike the ferns which are all homosporous. Cycad cones are unisexual, in fact the plants producing them are dioecious, having separate male and female plants. Cycads also produce free-swimming sperm (a feature found only in ginkgoes among living seed plant groups).

Cycads were much more prominent in the forests of the Mesozoic than they are today. Presently, they are restricted to the tropics. Zamia floridana is the only cycad occurring natively in the continental United States. Several species of Cycas, notably C. revoluta (shown in Figure 2), are commonly encountered cultivated plants in warm, moist areas. Cycas revoluta leaves are often used in Palm Sunday services in some churches, both for their feathery appearance and ease of obtaining from local greenhouses.
Ginkgos

The ginkgoes also were a much more prominent group in the past than they are today. The sole survivor of this once robust and diverse group is Ginkgo biloba, the maidenhair tree shown in Figure 3. Extensively used as an ornamental plant, Ginkgo was thought extinct in the wild until it was discovered growing natively in a remote area of China. Ginkos are dioecious, with separate male and female plants. The males are more commonly planted since the females produce seeds that have a nasty odor. Pollination is by wind. Recently, Ginkgo has become the current herbal rave, although scientific studies have debunked the claim that the herbal supplement made from ginkgoes improves memory. I think.

Precise systematic placement of the ginkgoes has yet to bet determined. Ginkgoes have motile (swimming) sperm, a rarity among living seed plants (only ginkgoes and cycads have this feature today), although the vegetative anatomy of ginkgoes is more conifer-like (long shoot and short shoot morphology discussed below; structure of their wood). Ginkgoes, like the cycads, are dioecious, and also have similar seed features to cycads.

Plants possibly allied to the modern ginkgoes have been found in Permian-aged and later rocks. These plants have been classified in the leaf-genera Ginkgoites (shown in Figure 4) and Baiera, although recent studies suggest these genera are really morphological variants and that the modern genus Ginkgo should be used to include these fossils. During the Mesozoic ginkgoes were worldwide in their distribution and important elements in the gymnosperm forests that dominated the land.
Conifers

The conifers remain a major group of gymnosperms that include the pines, spruce, fir, bald cypress and Norfolk Island Pine (Araucaria). The division Pinophyta contains approximately 550 species of conifers. The conifers are cone producing trees and shrubs that usually have evergreen needle-like leaves. Needles have a thick cuticle, sunken stomates, and a reduced surface area. The conifers, as a group, are well adapted to withstand extremes in climate and occur in nearly all habitats from the equator to the subpolar regions. The taiga biome consists largely of various conifer species.
Auracarias

Members of this group of conifers have numerous small, scale-like leaves spiraling around their stems. Araucaria, a major genus that gives its name to the group, is a common ornamental because of the symmetry and beauty of its growth form. The monkey puzzle tree, shown in Figure 5, is a species of Araucaria.

The fossil record of Auracarias and similar plants is quite good. The fossil genus Auracarioxylon that grew in Arizona during the Triassic Period comprises the largest group of petrified wood in the Petrified Forest National Park of Arizona.
Taxodiaceae: Sequoias and more

Members of this group include some of the largest trees, and have been significant members of the forests of the world since the Mesozoic. Sequoia, shown in Figure 6, and Sequoiadendron are major genera in this group.


The Pine Life Cycle

Pines have an interesting life cycle, shown in Figure 7, that takes two years to complete. Not all seed plants have such a long time span to complete their life history: some flowering plants manage to do it in as little as a few weeks.

The sporophyte, as in all other vascular plant groups, is the dominant, photosynthetic part of the life cycle: when you are holding pine needles in your hand you are holding sporophyte parts. Pines have specialized reproductive structures in which meiosis occurs: pine cones. Pollen grains are produced in the male cones, and contain the male gametophyte (which consists of only a very few cells). Pollen released from the male cones is carried by wind to the female cones, where it lands. The cones close and the next year the pollen grain germinates to produce a pollen tube that grows into the female gametophyte. The sperm cell (from the pollen grain) and egg cell fuse, forming the next generation sporophyte. The sporophyte develops into an embryo encased within a seed. The seed is later released to be transported by the wind to where (hopefully) it lands and germinates. If you have seen a large pine tree you realize there are hundreds or more female cones on such a tree. Pine pollen has been noted to travel great distances from the plant that produced it, if the wind is strong enough. To aid this transport pine pollen has two air sacs, and thus is quite distinctive.

Gnetales |

The Gnetales, are an odd group: they have some angiosperm-like features but are not themselves angiosperms. Cladistic analyses support placement of the gnetales (or some portion of them) as outgroups for the flowering plants. Three distinctive genera comprise this group: Welwitschia, Gnetum, and Ephedra. Ephedra occurs in the western United States where it has the common name "Mormon tea". It is a natural source for the chemical ephedrine, although there is no evidence the Mormons in Utah (where the plant is extremely common) ever used it for tea. Welwitschia is limited to coastal deserts in South Africa, although fossil leaf, cuticle and pollen evidence indicates plants of this type were widespread during the Mesozoic Era. Welwitschia is noted for its two long, prominent leaves. Gnetum has leaves that look remarkably like those in angiosperms, as well as vessels in the xylem, generally considered an angiosperm characteristic.

Among the gnetalean plants, Ephedra is perhaps the best known. One folkloric name for the plant is "Mormon tea". This is a misnomer as there appears little or no evidence that members of a religion that bans stimulants such as caffeine ever brewed a tea from the plant. However, the plant does produce the drug ephedrine, a stimulant lately linked to deaths of athletes.

Welwistchia is a very bizarre plant natively growing only in the coastal deserts of South Africa. The plant produces two long leaves and a crown of reproductive cones rimming a brown, central body. Pollen resembling Welwitschia has been found in many parts of the world, indicating a formerly more widespread distribution of this enigmatic plant.
Angiosperms are Flowering Plants

Flowering plants, the angiosperms, were the last of the seed plant groups to evolve, appearing during the later part of the of the Age of Dinosaurs (the beginning of the Cretaceous, 140 million years ago). All flowering plants produce flowers. Within the female parts of the flower angiosperms produce a diploid zygote and triploid endosperm. Fertilization is accomplished by a variety of pollinators, including wind, animals, and water. Two sperm are released into the female gametophyte: one fuses with the egg to produce the zygote, the other helps form the nutritive tissue known as endosperm.

The angiosperms (angios = hidden) produce modified leaves grouped into flowers that in turn develop fruits and seeds. There are presently 235,000 known living species. Most angiosperms also have larger xylem cells known as vessels that improve the efficiency of their vascular systems.

Whence came the angiosperms? This was Darwin's "abominable mystery". Clearly angiosperms are descended from some group of Mesozoic-aged gymnosperm seed plant....but which one? Click here to view an online lab exercise in phylogeny and try to figure things out!

The classical view of flowering plant evolution suggests early angiosperms were evergreen trees that produced large Magnolia-like flowers. Click here to view an illustration of suggested paths of floral evolution. However, this view has recently been contradicted by the oldest fossil yet found, a 140 million year old plant found by David Dilcher and his associates.

The angiosperms underwent an adaptive radiation during the Cretaceous, and for the most part escaped the major extinctions at the end of the Cretaceous.
Flowers

Flowers are collections of reproductive and sterile tissue arranged in a tight whorled array having very short internodes. Sterile parts of flowers are the sepals and petals. When these are similar in size and shape, they are termed tepals. Reproductive parts of the flower are the stamen (male, collectively termed the androecium) and carpel (often the carpel is referred to as the pistil, the female parts collectively termed the gynoecium). Lily flowers (shown in Figure 10) demonstrates these concepts.

Flowers may be complete, where all parts of the flower are present and functional, or incomplete, where one or more parts of the flower are absent. Many angiosperms produce a single flower on the tip of a shoot (like the lily pictured in Figure 10, or tulips). Other plants produce a stalk bearing numerous flowers, termed an inflorescence, such as is seen in many orchids. Many flowers show adaptations for insect pollination, bearing numerous white or yellow petals. Others, like the grasses, oaks, and elms, are wind pollinated and have their petals reduced and often inconspicuous.
Angiosperm Life Cycle

Flowering plants also exhibit the typical plant alternation of generations, shown in Figure 11. The dominant phase is the sporophyte, with the gametophyte being much reduced in size and wholly dependant on the sporophyte for nutrition. The is not a unique angiosperm condition, but occurs in all seed plants as well. What makes the angiosperms unique is their flowers and the "double fertilization" that occurs. Technically this is not double fertilization, but rather a single egg-sperm fusion (fertilization proper) plus a fusion of the second of two sperm cells with two haploid cells in the female gametophyte to p[produce triploid (3n) endosperm, a nutritive tissue for the developing embryo. More details on this aspect of the flowering plants are available in the FLOWERING PLANT REPRODUCTION: Fertilization and Fruits chapter.
Angiosperm Systematics

The flowering plants, the division Magnoliophyta, contain more than 235,000 species, six times the number of species of all other plants combined. The flowering plants divide into two large groups, informally named the monocots and the dicots. The techjnical names for these groups are the class Magnoliopsida for dicots and the class Liliopsida for monocots.

The dicotyledons are in the class Magnoliopsida and have these features: either woody or herbaceous, flower parts usually in fours and fives, leaves usually net-veined, vascular bundles arranged in a circle within the stem, and produce two cotyledons (seed leaves) at germination. Prominent dicot families include the mustards, maples, cacti, peas and roses. Several dicot families are noteworthy because of the illegal drugs (shown in Figure 12) derived from them: the Cannabinaceae (marijuana) and Papaveraceae (poppies from which opium and heroin are derived). Erythroxylum coca (in the dicot family Erythroxylaceae) is the plant from which the illegal drug cocaine is extracted.

Not all dicot plants are misused to produce illegal drugs. Notable dicot families with legitimate uses include the pea family, which includes the crop plants beans, clover, and peas as well as many ornamental landscape plants such as Acacias. Beans are an excellent source of nonanimal protein as well as fiber. Another dicot of enormous use is cotton, Gossypium, shown in Figure 13. Chocolate and cola are products of the plant family Sterculiaceae. Coffee is produced from Coffea arabica, a plant in the family Rubiaceae, while tea comes from Camelia sinensis (Theaceae), a plant native to China.

The class Liliopsida has plants that are herbaceous (a majority are, only palms and bamboo stand out as monocot trees), flower parts are in threes, leaves are usually parallel-veined, vascular bundles are scattered within the stem, and produce one cotyledon (seed leaf) at germination. Monocot families include lilies, palms, orchids, irises, and grasses.

The monocot family Poaceae (known previously as the Gramineae) includes the grasses such as corn, oats, wheat, rye, and rice that are staple food products as well as ornamental plants such as crabgrass and tiff grass. The importance of this plant family to modern civilization cannot be overstated, as the first six plants mentioned in the previous sentence provide 75% of our food, either directly as food we eat or indirectly as food for animals we eat.
Trends in Plant Evolution

Several evolutionary trends within the plant kingdom have been noted. The monophyletic nateure of this kingdom is not in dispute, with the first major division being between vascular and nonvascular plants. Wihin the vascular plants we see increasing changes in the relationship between sporophyte and gametophyte, culminating in flowering plants.

Developing from green algal ancestors, plants show a trend for reduction of the complexity, size, and dominance of the gametophyte generation. In nonvascular plants the gametophyte is the conspicuous, photosynthetic, free-living phase of the life cycle. Conversely, the angiosperm gametophyte is reduced to between three and eight cells (hence it is very inconspicuous) and is dependent on the free-living, photosynthetic sporophyte for its nutrition.

Plants also developed and refined the root-shoot-leaf axis with its specialized conducting cells of the xylem and phloem. The earliest vascular plants, such as Cooksonia and Rhynia, were little more than naked (unleafed) photosynthetic stems. Some plants later developed secondary growth that produced wood. Numerous leaf modifications are known, including "carnivorous" plants such as the Venus flytrap, as well as plants that have reduced or lost leaves, such as Psilotum and the cacti.

A third trend is the development of the seed to promote the dormancy of the embryo. The seed allows the plant to wait out harsh environmental conditions. With the development of the seed during the Paleozoic era plants became less prone to mass extinctions.

The fourth trend in plant evolution is the encasing of a seed within a fruit. The only plant group that produces true fruit is the flowering plants, the angiosperms. Fruits serve to protect the seed, as well as aid in seed dispersal.

All text contents ©1995, 1999, 2000, 2001, 2003, 2004, by M.J. Farabee. Use of text for educational purposes is encouraged.

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