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Biology 110 - Basic Concepts and Biodiversity

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Plants I - Evolution and Diversity, Nonvascular Plants

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The major challenge for early plants first migrating onto land was the lack of water. In an aquatic environment, desiccation is generally not a problem and there is no need for any protective covering to prevent water loss. Lacking any protection from the dry terrestrial environment, early plants probably dried out very quickly and would have been limited to very moist environments.

The ancestors of early plants were dependent on water, not only to maintain their moisture content but also for structural support. The buoyancy of water supports upright growth of giant marine seaweeds (e.g., kelp, Fig. 6) Consider the seaweeds that are often found washed up on the beach. Although these algae are no longer alive, when held beneath the water their upright form is restored. In a terrestrial environment, the surrounding media is air rather than water. Air does not provide any support for upright growth. The transition to land required changes in structural features, and, as will be discussed later in this tutorial, adaptations for structural support are key features used in plant classification.

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Figure 6. Kelp forest off California coast (http://bio.research.ucsc.edu/people/carr/nereo-lit.htm)

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PlantsI_partI.txtTranscript for Plants I - Part I

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Adaptive Features of Plants

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The most recent adaptations to the terrestrial environment were the evolution of flowering plants and the production of fruit as a means for seed dispersal. Flowering plants produce their seeds within a fruit that provides a functional "packaging" around the seed(s). The fruit can be edible, such that the digested seeds are then deposited with the feces of the animal that consumed the fruit. Other fruits are suitable for transport on air currents, water currents, or on the fur of different animals. You will learn more about flowering seed plants (and their remarkable adaptations to life on land) in Tutorial 15.

The first evidence of seed plants in the fossil record occurs approximately 305 million years ago. Seed production enabled plants to reproduce more successfully because the embryos had a much better chance of surviving the dry terrestrial environment than did the embryos of more primitive plants that were still dependent on the parent plant body. Just think about the advantage to plants whose offspring could be widely dispersed and were protected (within the seed) until conditions were suitable for growth. Seed plants are so dominant in the world today that we have to remind ourselves that there are numerous plants in existence that do not produce seeds.

Fruit production by flowering plants is a more specialized adaptation to life on land because it reflects not only the environment, but also the other life forms that exist there. Although the first flowering plants occur in the fossil record only 175 million years ago, the success of fruit production is marked by the huge radiation of flowering plants.

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Transcript for Plants I - Part II

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Plant Phylogeny: Lineages are Defined by Major Adaptive Features

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To better understand alternation of generations in nonvascular plants, study the moss (bryophyte) life cycle in Figure 15. It depicts the life cycle of a moss and helps to distinguish the haploid and diploid stages. Note the prominent form of the moss, the gametophyte, which is haploid (1n). This is the plant body that is most often observed. In the figure there are separate male and female gametophytes; however, the gametophyte can also be bisexual (male and female gametangia are located on the same plant body). In step 1, the gametophyte is the generation that produces gametes; sperm are produced in the male gametangium, the antheridium (plural, antheridia), and eggs are produced in the female gametangium, the archegonium (plural, archegonia). In step 2, the motile sperm has reached the egg, which is retained in the archegonium and fertilization takes place. (Remember, water is required for fertilization because the flagellated sperm must swim to the egg.) In step 3, the diploid (2n) zygote undergoes mitosis and begins to develop into the embryo (also 2n). In step 4, the embryo matures into the sporophyte, the diploid (2n) plant body. The sporophyte is the small, brown, stalked structure that one sometimes sees held above the main body of the moss. In step 5, meiosis takes place in the sporangium of the mature sporophyte and haploid spores are produced. In steps 6 and 7, the haploid spores are dispersed and each spore undergoes mitotic cell division to create a haploid multicellular gametophyte. The prominent haploid gametophyte is then ready to produce gametes (back to step 1).

An important feature of the moss life cycle is that the developing embryo is retained on the gametophyte plant body. This is an adaptation to the terrestrial environment because the embryo is protected from desiccation throughout its development into the sporophyte. (Remember the description of plants as embryophytes?) If you think about haploid and diploid stages in terms of the animal life cycle, it will be difficult to make sense of the plant life cycle. As noted above, the plant life cycle includes alternation of generations, with a multicellular haploid stage. Review again, step 5 of the life cycle (where meiosis takes place). Note that meiosis does not occur again when gametes are produced. Once you recognize these differences, you should begin to feel more comfortable thinking about plant life cycles.

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 Figure 15. Bryophyte Life Cycle. (Click image to enlarge) 

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Transcript for Plants I - Part III

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Summary

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The diversity of plants existing today is the result of 475-700 million years of evolution and adaptation to the terrestrial environment. Plants arose from a lineage of green algae and their closest protest relatives are the Charophytes. Both have cellulosic cell walls, form cell plates during cytokinesis, store carbon in the form of starch, have chlorophyll b as an accessory pigment, and share similar RNA and DNA sequences for particular genes. Charophytes are aquatic organisms, and it is highly likely that the earliest plants occupied transitional environments between the sea and the land. The transition to the terrestrial environment was advantageous for plants because there was direct access to sunlight and little to no herbivore activity. Early plants were ill-equipped for life out of the water, and desiccation was a major challenge to a land-based existence.
Adaptations to the terrestrial environment enabled generation after generation of plants to successfully live out of the water. The waxy cuticle and stomata are effective in reducing water loss and preventing desiccation. Vascular tissue further reduces the problem of desiccation because it allows transport of water and nutrients throughout the plant. Upright growth for improved access to sunlight is also an advantage conferred by vascular tissue because it also functions in internal support of the plant body. Protection of gametes and developing embryos is the role of jacketed gametangia, and later, the seed. More recently in plant history, adaptive features have been influenced by other organisms in the terrestrial environment; some plants produce specialized flower structures and fruits that attract insects and other animals that aid in pollination and seed dispersal.
The nonvascular plants (including mosses, liverworts and hornworts) are highly successful and can be found the world over. They are resistant to desiccation, but prefer a moist environment due to their lack of vascular tissue and motile gametes. Nonvascular plants, like other plants, are embryophytes, and their life cycles are based on alternation of generations. The prominent generation of nonvascular plants is the multicellular haploid gametophyte. The diploid sporophyte generation is completely dependent on the gametophyte for its survival. In the more derived plant lineages, the gametophyte is greatly reduced. You should also know that the nonvascular plants do not have vascular tissue or seeds, however, they do have a stomata, a protected embryo, and most have a waxy cuticle. Seedless vascular plants are more derived than nonvascular plants and are defined by their lack of seed production and presence of vascular tissue. The more derived lineages, nonflowering seed plants and flowering seed plants, both produce seeds, but only the flowering seed plants produce flowers and fruits. Life cycles and the major characteristics of the vascular plants will be covered in the next two tutorials.

 

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