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Biological Diversity: Seed Plants

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Biological Diversity: Seed Plants BIOLOGICAL DIVERSITY: SEED PLANTS Table of Contents Gymnosperms are Seed Plants | Cycads | Ginkgos | Conifers | Gnetales | Angiosperms are Flowering Plants | Flowers | Trends in Plant Evolution | Learning Objectives | Terms | Review Questions | Links Gymnosperms are Seed Plants | Back to Top 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). Figure 1. The fossil records of some protist and plant groups. The width of the shaded space is an indicator of the number of species. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission. 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 | Back to Top 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. Figure 2. Cycas revoluta from Hawaii, Top: female cones bearing large, orange seeds. Image from http://www.botany.hawaii.edu/faculty/carr/images/cyc_rev_f.jpg; Bottom: male cone bearing numerous pollen-producing sporangia. Image from http://www.botany.hawaii.edu/faculty/carr/images/cyc_rev_m.jpg. Ginkgos | Back to Top 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. Figure 3. Ginkgo biloba, close-up of young shoot. Image from http://www.dinoworld.net/ginkgo.JPG. 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. Figure 4. Ginkgoites leaf from the Triassic of North Korea. Although this fossil is not the typical fan shape of most modern ginkgo leaves, modern ginkgoes do produce leaves of this general shape. Image from http://www.dges.tohoku.ac.jp/museum/fosgal19.html. Conifers | Back to Top 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. Figure 5. Image of Araucaria sp. Note the large female comes at the tips of branches. Image from http://www.botany.hawaii.edu/faculty/carr/images/araucar_sp3.jpg. 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. Figure 6. Top: Habit photograph of Sequoia sempervirens. Note the tall, woody stem with leaves borne at the top of the stem. Image from http://www.botany.hawaii.edu/faculty/carr/images/seq_sem_hab.jpg. Bottom: Close-up of Sequoia sempervirens leaf and cone. Image from http://www.botany.hawaii.edu/faculty/carr/images/seq_sem_cu.jpg. 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, as shown in Figure 8. Figure 7. The pine life cycle. Note: to view these four segments in correct sequence you will need to increase your browser window width as much as possible. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission. Figure 8. Pine pollen from 38,000 year old sediments in the White Mountains of eastern Arizona. Image from http://www.smu.edu/geology/palynolab.htm. Gnetales | Back to Top The Gnetales, shown in Figure 9, 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. Figure 9. Representative gnetalean plants. Top: Ephedra female plant. Image from http://www.botany.hawaii.edu/faculty/carr/images/eph_sp_f.jpg. Middle: Welwitschia mirabilis habit shot of a cultivated specimen. Image from http://www.botany.hawaii.edu/faculty/carr/images/wel_mir.jpg.
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