BIOLOGICAL DIVERSITY: NONVASCULAR PLANTS AND NONSEED VASCULAR PLANTS Table of Contents Evolution of Plants | The Plant Life Cycle | Plant Adaptations to Life on Land Bryophytes | Tracheophytes: The Vascular Plants | Vascular Plant Groups | The Psilophytes | The Lycophytes The Sphenophyta | The Ferns | Learning Objectives | Terms | Review Questions | Links The plant kingdom contains multicellular phototrophs that usually live on land. The earliest plant fossils are from terrestrial deposits, although some plants have since returned to the water. All plant cells have a cell wall containing the carbohydrate cellulose, and often have plastids in their cytoplasm. The plant life cycle has an alternation between haploid (gametophyte) and diploid (sporophyte) generations. There are more than 300,000 living species of plants known, as well as an extensive fossil record. Plants divide into two groups: plants lacking lignin-impregnated conducting cells (the nonvascular plants) and those containing lignin-impregnated conducting cells (the vascular plants). Living groups of nonvascular plants include the bryophytes: liverworts, hornworts, and mosses. Vascular plants are the more common plants like pines, ferns, corn, and oaks. The phylogenetic relationships within the plant kingdom are shown in Figure 1. Figure 1. Phylogenetic reconstruction of the possible relationships between plant groups and their green algal ancestor. Note this drawing proposes a green algal group, the Charophytes, as possible ancestors for the plants. 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. Evolution of Plants | Back to Top Fossil and biochemical evidence indicates plants are descended from multicellular green algae. Various green algal groups have been proposed for this ancestral type, with the Charophytes often being prominently mentioned. Cladistic studies support the inclusion of the Charophytes (including the taxonomic order Coleochaetales) as sister taxa to the land plants. Algae dominated the oceans of the precambrian time over 700 million years ago. Between 500 and 400 million years ago, some algae made the transition to land, becoming plants by developing a series of adaptations to help them survive out of the water. Table 1. Photosynthetic pigments of algae and plants. Prokaryote groups are shown in red, protists in blue, and vascular plants in purple. Taxonomic Photosynthetic Pigments Group chlorophyll a, chlorphyll c, Cyanobacteria phycocyanin, phycoerythrin Chloroxybacteria chlorophyll a, chlorphyll b Green Algae chlorophyll a, chlorphyll b, (Chlorophyta) carotenoids Red Algae chlorophyll a, phycocyanin, (Rhodophyta) phycoerythrin, phycobilins chlorophyll a, chlorphyll c, Brown Algae fucoxanthin and other (Phaeophyta) carotenoids Golden-brown chlorophyll a, chlorphyll c, Algae fucoxanthin and other (Chrysophyta) carotenoids Dinoflagellates chlorophyll a, chlorphyll c, (Pyrrhophyta) peridinin and other carotenoids chlorophyll a, chlorphyll b, Vascular Plants carotenoids Vascular plants appeared by 350 million years ago, with forests soon following by 300 million years ago. Seed plants next evolved, with flowering plants appearing around 140 million years ago. This pattern is shown in Figure 2. Figure 2. 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. The Plant Life Cycle | Back to Top Plants have an alternation of generations: the diploid spore-producing plant (sporophyte) alternates with the haploid gamete-producing plant (gametophyte), as shown in Figure 3. Animal life cycles have meiosis followed immediately by gametogenesis. Gametes are produced directly by meiosis. Male gametes are sperm. Female gametes are eggs or ova. Figure 3. Typical alternation of generations life cycle, such as occur in some protistans and plants. 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. The plant life cycle has mitosis occurring in spores, produced by meiosis, that germinate into the gametophyte phase. Gametophyte size ranges from three cells (in pollen) to several million (in a "lower plant" such as moss). Alternation of generations occurs in plants, where the sporophyte phase is succeeded by the gametophyte phase. The sporophyte phase produces spores by meiosis within a sporangium. The gametophyte phase produces gametes by mitosis within an antheridium (producing sperm) and/or archegonium (producing eggs). These different stages of the flowering plant life cycle are shown in Figure 4. Within the plant kingdom the dominance of phases varies. Nonvascular plants, the mosses and liverworts, have the gametophyte phase dominant. Vascular plants show a progression of increasing sporophyte dominance from the ferns and "fern allies" to angiosperms. Figure 4. The life cycle stages of a flowering plant. The above image is reduced from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Angiosperm/Angiosperm_life_cycle. Follow that link to view a larger image. Homospory and Heterospory Plants have two further variations on their life cycles. Plants that produce bisexual gametophytes have those gametophytes germinate from isospores (iso=same) that are about all the same size. This state is referred to as homospory (sometimes referred to as isospory). A generalized homosporous plant life cycle is shown in Figure 5. Homosporous plants produce bisexual gametophytes. Ferns are a classic example of a homosporous plant. Figure 5. A typical homosporous life cycle. Note the production of a single type of bisexual gametophyte that will eventually produce the antheridia (sperm bearing structures) and archegonia (egg bearing structures). 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. Plants that produce separate male and female gametophytes have those gametophytes germinate from (or within in the case of the more advanced plants) spores of different sizes (heterospores; hetero=different). The male gametophyte produces sperm, and is associated with smaller or microspores. The female gametophyte is associated with the larger or megaspores. Heterospory is considered by botanists as a significant step toward the development of the seed. A generalized heterosporous life cycle is shown in Figure 6. Figure 6. Typical heterosporous life cycle. Note the production of separate, unisexual male and female gametophytes. 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. Plant Adaptations to Life on Land | Back to Top Organisms in water do not face many of the challenges that terrestrial creatures do. Water supports the organism, the moist surface of the creature is a superb surface for gas exchange, etc. For organisms to exist on land, a variety of challenges must be met. 1. Drying out. Once removed from water and exposed to air, organisms must deal with the need to conserve water. A number of approaches have developed, such as the development of waterproof skin (in animals), living in very moist environments (amphibians, bryophytes), and production of a waterproof surface (the cuticle in plants, cork layers and bark in woody trees). 2. Gas exchange. Organisms that live in water are often able to exchange carbon dioxide and oxygen gases through their surfaces. These exchange surfaces are moist, thin layers across which diffusion can occur. Organismal response to the challenge of drying out tends to make these surfaces thicker, waterproof, and to retard gas exchange. Consequently, another method of gas exchange must be modified or developed. Many fish already had gills and swim bladders, so when some of them began moving between ponds, the swim bladder (a gas retention structure helping buoyancy in the fish) began to act as a gas exchange surface, ultimately evolving into the terrestrial lung. Many arthropods had gills or other internal respiratory surfaces that were modified to facilitate gas exchange on land. Plants are thought to share common ancestry with algae. The plant solution to gas exchange is a new structure, the guard cells that flank openings (stomata) in the above ground parts of the plant. By opening these guard cells the plant is able to allow gas exchange by diffusion through the open stomata. 3. Support. Organisms living in water are supported by the dense liquid they live in. Once on land, the organisms had to deal with the less dense air, which could not support their weight. Adaptations to this include animal skeletons and specialized plant cells/tissues that support the plant. 4. Conduction. Single celled organisms only have tyo move materials in, out, and within their cells. A multicellular creature must do this at each cell in the body, plus move material in, out, and within the organism. Adaptations to this include the circulatory systems of animals, and the specialized conducting tissues xylem and phloem in plants. Some multicellular algae and bryophytes also have specialized conducting cells. 5. Reproduction. Organisms in water can release their gametes into