You should have a working knowledge of the following terms:
Introduction and Goals
By now you should have a good understanding of the advantages and challenges to plants living on land. You should also know that the nonvascular plants do not have vascular tissue or seeds, however, they do have a waxy cuticle, stomata, and protected embryo. The last tutorial introduced you to the seedless vascular plants, which in addition to a waxy cuticle and stomata, have a well-developed vasculature. Now we will examine the function of vascular tissue in the seedless vascular plants. By the end of this tutorial you should have a working understanding of:
- The evolution of seedless vascular plants
- The importance of vascular tissue
- The advantages of a dominant sporophyte
- The life cycle of seedless vascular plants
- The three groups (lycophytes, sphenophytes, and pterophytes) of seedless plants
Evolution of Vascular Plants
The first fossil record of a vascular plant is from the Silurian period, about 420 million years ago. The drawing on the right depicts a fossil of Cooksonia. There are many well-preserved fossils, some of which clearly show the vasculature that had begun to form, even in this early plant.
Notice the bulbous projections at the tips of some of the stems. These are thought to have been the spore-producing structures; therefore, this is considered the sporophytic generation of the plant. This is important because it shows that the transition to a more prominent sporophyte had already occurred by this point.
Figure. 1(Click image to enlarge)
At 5 cm, Cooksonia had grown much taller than the land plants that preceded it. This was due to the vasculature that provided the structural support necessary to grow on land. Seedless vascular plants went on to dominate the land through the Carboniferous period, about 300 million years ago. At that time, they grew to heights of 15 meters or more. The period is named Carboniferous because it was characterized by swamps filled with tree ferns and other seedless vascular plants that subsequently became the coal that is mined today. The seedless vascular plants now in existence are much smaller, and they are very prevalent in the forests of Pennsylvania.
Vascular tissue is the characteristic that distinguishes the seedless vascular plants from those plants that preceded them. While protected gametes allowed plants to move onto land, it was vascular tissue that allowed plants to dominate the landscape. Vasculature provides a means for fluid transport and structural support for the body of the plant. Vascular tissue consists of xylem and phloem. Xylem is primarily the vasculature where water and minerals travel from the roots up the stems to the various parts of the plant. Phloem transports the sugars made during photosynthesis down to the roots for energy. Thus, there is a net movement of water up and nutrients down. These two types of vessels differ structurally and functionally. The xylem is made of nonliving cells that tend to be more fortified for plant strength. The phloem consists of living cells that are modified to allow the flow of fluids.
Vascular tissue gives support to plants. Lignin is embedded in plant cell walls between the cellulose matrix, and it is a very stable molecule that does not break down easily. We are able to use wood in construction because of the strength that lignin provides. Lignin plays the same role in plants.
Like all plants, seedless vascular plants have a gametophytic generation and a sporophytic generation. Recall, the sporophytic generation is the diploid part of the life cycle and, via meiosis, haploid spores are produced. Remember from the last tutorial, 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.
When one compares the life cycle of a moss (a nonvascular plant; below, left) to that of a fern (a seedless vascular plant; below, right), the most notable difference is the relative sizes of the sporophyte and gametophyte. In mosses the haploid gametophyte is the dominant generation, whereas in ferns the diploid sporophyte is the dominant generation.
Figure. 2(Click image to enlarge)
Figure. 3(Click image to enlarge)
What was life like when the early plants were colonizing land? Remember, one of the advantages of moving onto land was the new abundance of light energy that plants could access. However, light can damage DNA and induce mutations. Recall, most mutations are deleterious, and haploid organisms that suffer a lethal mutation have no wild-type copy to "rescue" them from lethal mutations. For this reason, the haploid stage is more sensitive to genetic insult than is the diploid stage. Likely, the transition from a prominent haploid stage to a prominent diploid stage was adaptive for the relatively high mutation rate suffered by terrestrial plants.
Take another look at the moss and fern life cycles. Both have flagellated sperm. This means that they are both dependent on water for fertilization. Mosses are already very small and low to the moist ground, but ferns have vascular tissue and are much taller. This could be another reason for the dominance of the sporophyte. In the case of ferns, the gametophyte is much smaller and lower to the ground, where moisture is more available for fertilization.
The other similarity between mosses and ferns is that both have antheridia and archegonia. Recall from tutorial 31, these structures are the specialized gametophytic tissue where gametes are produced. The archegonium is also where the egg is fertilized once the sperm from the antheridium swims through water to reach it.
Classification of Seedless Vascular Plants
The seedless vascular plants can be divided into three groups: Lycophyta (lycophytes), 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 lycophtes 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.
The third group of seedless vascular plants is probably the most familiar. These are the ferns or pterophytes. 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 also generally occupy moist habitats.
This tutorial examined seedless 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. This sporophyte generation begins its life in the protection of the archegonium.