Introduction and Goals
By now you should have a good understanding of the advantages and challenges faced by plants living on land. The previous tutorial introduced you to the adaptations to terrestrial environments seen in plants. Now you will examine the nonvascular plants, the most primitive of the living plants lineages, and the seedless vascular plants, which in addition to a waxy cuticle and stomata, have a well-developed vascular tissue. By the end of this tutorial you should have a working understanding of:
- The distinguishing characteristics of nonvascular plants and their life cycles
- The three groups (mosses, liverworts, hornworts) of nonvascular plants
- 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
- Identify the characteristics of non-vascular plants
- Describe the adaptations in members of this group that allow them to be successful in terrestrial environments
- Summarize the life cycle of a moss, a heterosporous plant, and identify the dominant stage in the life cycle
- Identify the characteristics of seedless vascular plants
- Explain the importance of vascular tissue as an adaptation to drier terrestrial environments
- Diagram the life cycle of a fern, a homosporous plant, and identify the dominant stage in the life cycle
Nonvascular Seedless Plants
As the name of this group indicates, plants in this lineage do not have vascular tissue (or if present, it is very reduced). Because they lack substantial vasculature, plants in this lineage are generally small in size, lack significant structural support, grow close to the ground in moist areas, and lack significant water-conducting cells. Plants first evolved in environments that were transitional between the land and the sea, and although modern nonvascular plants are dependent on water to complete their life cycles, they are able to withstand long periods of desiccation. Nonvascular plants include mosses, liverworts, and hornworts.
Nonvascular Plants: Environment and Morphology
Of all the plant lineages, nonvascular plants are the most basal. This means that these lineages diverged before that remaing groups of land plants diversified. Current information suggest that the liverworts diverged first, followed by the mosses then the hornworts (Fig. 1). They are less derived than seedless vascular plants and seed plants, but the nonvascular plants are highly successful in the environments they inhabit.
Figure 1. Phylogenetic tree showing the hypothesized relationships among the lineages of land plants. The three groups that comprise the nonvascular plants are not members of a monophyletic groups, with hornworts more closely related to vascular plants. (Click to enlarge)
As you walk through a wooded area, you will likely find mosses growing on rocks, rotting wood, trees, or on the ground. Nonvascular plants are generally small and do not extend much more than a few inches above the surface they are growing on (Fig. 2). Their appearance can best be described as a "carpet of green." The plant body that is most obvious is the gametophyte generation, which is haploid.Nonvascular plants typically grow in moist environments. Their lack of vascular tissue requires them to maintain close contact with water to prevent desiccation. They do not have true roots, true stems, or true leaves (which are distinguished by the presence of vascular tissue). Rhizoids are the root-like structures that function to anchor them to the surface they are growing on, however, they are not capable of water uptake. Water is absorbed throughout the "leafy" plant body of the gametophyte. They also require a moist environment for successful fertilization. They do not produce pollen grains and have retained the primitive condition of a flagellated sperm. The male gametes are motile in water and must be released into a moist environment so that the sperm can swim to the female gametangium (where the egg cells are located).
Figure 2. Liverworts with sporocarps arising from thalli (http://oregonstate.edu/dept/nursery-weeds/weedspeciespage/liverwort/sporocarp_page.html)
Alternation of Generations: The Moss Life Cycle
To better understand alternation of generations in nonvascular plants, study the moss life cycle in Figure 3. 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.
Figure 3. Bryophyte Life Cycle. (Click image to enlarge)
Evolution of Vascular Plants
The first fossil record of a vascular plant is from the Silurian period, about 425 million years ago. This is a drawing of a fossil of Cooksonia (Fig. 4). 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.
At 5 cm, Cooksonia had grown much taller than the land plants that preceded it. This was due to the vascular tissue that provided the structural support necessary to grow taller 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 common in the forests of Pennsylvania.
Figure 4. Drawing of a fossil of Cooksonia (Click to enlarge)
Seedless Vascular Plants
Seedless vascular plants have a waxy cuticle, stomata, and well-developed vascular tissue. Their vasculature allows them to grow to larger sizes than the nonvascular plants, but they still largely occupy moist habitats. While this lineage is more well adapted to drier habitats than are the nonvascular plants, they still require moisture for reproduction. Although the developing diploid embryo is dependent on the haploid gametophyte for survival (like mosses), the diploid sporophyte is more conspicuous and is the prominent generation of seedless vascular plants. Phylogenetically, seedless vascular plants are basal to the seed plants. The seedless vascular plants include species such as ferns and horsetails.
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. Vascular tissue provides a means for transport and structural support for the body of the plant. Vascular tissue consists of xylem and phloem (Figure 5). Xylem is primarily the vasculature through which 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 and around the plant body to provide 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 soluble organic nutrientsVascular 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.
Figure 5. The vascular tissue in a plant, showing xylem and phloem. (Click to enlarge) (http://www.ucmp.berkeley.edu/IB181/VPL/Ana/AnaP/Ana1l.jpeg)
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 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.
When one compares the life cycle of a moss to that of a fern (a seedless vascular plant; Fig. 6), 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.
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.
Figure 6. The fern life cycle. (Click image to enlarge)
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. Also, another major difference is that the sporophyte and gametophyte live independently for part of the life cycle. In the case of the fern, the gametophyte is photosynthetic, 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 12, 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 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. 7).
The third group of seedless vascular plants is probably the most familiar. These are the ferns or pterophytes (Fig. 8). 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.
Figure 7. Equisetum arvense. A horsetail. (Click to enlarge)
Figure 8. Marattia douglasii. A fern. (Click to enlarge)
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.
This tutorial also 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. While the sporophyte generation begins its life in the protection of the archegonium, the sporophyte and gameotype 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.
After reading this tutorial, you should have a working knowledge of the following terms: