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Biology 110 - Basic Concepts and Biodiversity

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Plants II - Vascular Non-flowering Plants

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Like all plants, vascular plants have a gametophytic generation and a sporophytic generation. Recall, the sporophytic generation is the diploid part of the life cycle and produces haploid spores through meiosis. Remember that the moss life cycle is characterized by two types of haploid spores, male and female. We call this condition heterosporous ("hetero" meaning different and "sporous" referring to the spores). In this case, the sporophyte produces (via meiosis) megaspores and microspores. Haploid megaspores develop into haploid female gametophytes, which then produce eggs. Likewise, haploid microspores develop into male gametophytes, which then produce sperm. Haploid gametes then join to form sporophytes.

In seedless vascular plants, both the heterosporous condition described above and the homosporous condition ("homo" meaning same) result in a single type of spore that develops into bisexual gametophytes. The fern life cycle figure, which can be viewed on the next page, depicts this condition. Bisexual gametophytes can produce both male and female gametes (sperm and eggs). Note, sperm and eggs are still separate and must join during fertilization, just as in the heterosporous condition.

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Transcript for Plants II - part I

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Life Cycle

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The seedless vascular plants can be divided into three groups: Lycophyta (lycophytes or club mosses), Sphenophyta (horsetails), and Pterophyta (ferns). Lycophytes appeared during the Devonian period but split into two lines during the Carboniferous period. One line became the huge extinct trees that thrived some 300 million years ago, and a good portion of the carbon they fixed was fossilized and is now burned as coal. The other line of lycophytes are small nonwoody plants. These extant lycophytes are usually found in either temperate forest floors or tropical areas. One species, Lycopodium, can be found in the forests around Pennsylvania.

Except for one existing species (Equisetum), the group whose members are commonly called horsetails is also extinct. Equisetum occurs in damp locations and is an example of a homosporous plant (Fig. 4).

The third group of seedless vascular plants is probably the most familiar. These are the ferns or pterophytes (Fig. 5). Most of us have seen ferns growing on a forest floor or as cut fronds in a flower arrangement. There are about 12,000 species of ferns in existence today, and they are found in tropical and temperate regions.

While the vasculature of seedless vascular plants has allowed them to grow to larger sizes than nonvascular plants, they still usually occupy moist habitats.

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Figure 4. Equisetum arvense. A horsetail. (Click to enlarge)
Figure 5. Marattia douglasii. A fern. (Click to enlarge)

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Transcript for Plants II - Part II

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Seed Plants: Sporophytes More Prominent, Gametophytes More Reduced

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Pine trees, firs, spruces, larches, yews, junipers, cedars, cypresses, and redwoods are all conifers. Most of these are evergreens, however, there are a few deciduous (trees that drop their leaves each fall) conifers (e.g., the cypress trees in the Florida everglades or the larch trees in central PA). The name conifer comes from the Latin word meaning cone bearing. Conifers can be either monoecious or dioecious. That is, their male and female reproductive structures reside on the same or different plants, respectively. Unlike other nonflowering seed plants, their sperm are not flagellated; they are delivered directly via the pollen tube.

Conifers date back to the Mesozoic period. Unlike the other tropical nonflowering seed plants, most conifers are found in the forested parts of the Northern Hemisphere. They are by far the most economically and ecologically important members of the gymnosperms. You probably are familiar with the 2X4's used in construction. These boards, as well as many others, are made from pine trees. The wood of pine trees is softer than that of flowering seed plant trees, therefore, it is easier to hammer nails into this wood.

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Transcript for Plants III - Part I

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The Nonflowering Seed Plant Life Cycle

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As the eggs are developing, two sperm cells are developing within the pollen grain. A third cell in the pollen grain begins to grow as the pollen tube moves toward the megagametophyte. Once the pollen tube reaches the megagametophyte, the sperm cells fertilize the egg cells. Note that pollination occurred when the pollen grain reached the ovule but fertilization did not occur until the sperm reached the egg. In most cases, fertilization does not happen until at least one year after pollination.

Only one fertilized egg will survive and develop into an embryo. The embryo is diploid, therefore, it becomes the sporophyte of the next generation. In seedless plants the fertilization and development of the next-generation sporophyte takes place separate from the first-generation sporophyte. However, in this life cycle, the female gametophyte remained within the parental sporophytic tissue.

The embryo is made up of a rudimentary root and several embryonic leaves. The seed consists of three types of tissue: the new generation sporophyte or diploid embryo; the haploid female gametophytic tissue that stores nutrients; and the parent sporophytic tissues of the seed coat. The processes of gamete formation, pollination, fertilization, and germination are often very slow, and the life cycle can take two to three years from beginning to end.

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Transcript for Plants III - Part II

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Summary

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This tutorial began our exploration of vascular plants. In particular, we examined the significance of their vascular tissue. Vasculature provides plants with a means to transport materials and aids upright growth in the terrestrial environment. Ferns were used as a representative of seedless vascular plants to examine their life cycle. We learned that the sporophyte is the dominant generation and that this diploid condition can provide plants with an advantage against the damaging effects of the sun. While the  sporophyte generation begins its life in the protection of the archegonium, the sporophyte and gametophyte live separately for part of the life cycle. During this time the gametophyte is either photosynthetic or has a symbiotic relationship with a fungus that provides its nutrition.

This tutorial also examined the evolution of the seed plants.  The seed plants show adaptations to drier environments.  First, we considered changes in the alternation of generations during land plant evolution. Then, we learned the importance of pollen and seeds in the development of land plants. We explored the diversity of extant members of the nonflowering seed plants (gymnosperms), as well as their evolutionary past.  By looking at the life cycle of a pine, we compared and contrasted the life cycles of seedless plants and nonflowering seed plants.

 

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Questions?  Send your instructor a message through ANGELCanvas!

 

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