Introduction and Learning Objectives
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 generally in plants and specifically in the nonvascular plants. Now you will examine the seedless vascular plants, which in addition to a waxy cuticle and stomata, have a well-developed vascular tissue. This tutorial will also discuss the evolution of seed plants (gymnosperms and angiosperms), and will cover the nonflowering seed plants, the gymnosperms. By the end of this tutorial you should have a working understanding of:
- The evolution of 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 vascular plants
- Alternation of generations in seed plants
- The importance of seeds and pollen to land plants
- The four divisions of nonflowering seed plants (gymnosperms)
- The life cycle of nonflowering seed plants
- Identify the characteristics of vascular plants
- Explain the importance of vascular tissue as an adaptation to drier terrestrial environments
- Identify the characteristic of seedless vascular plants
- Diagram the life cycle of a fern, a homosporous plant, and identify the dominant stage in the life cycle
- Describe the adaptations in seed plants that allow them to be successful in drier terrestrial environments
- Identify the characteristics of non-flowering seed plants (the gymnosperms)
- Explain the changes in the relationship among the Earth’s tectonic plates that occurred at the time the gymnosperms arose
- Summarize the life cycle of a conifer, a heterosporous plant, and identify the dominant stage in the life cycle
- Identify the major characteristic(s) of each group within the gymnosperms and be able to discuss representatives that demonstrate the diversity of the group and the evolutionary trends seen within the gymnosperms
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. 1). 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 1. Drawing of a fossil of Cooksonia (Click to enlarge)
Vascular tissue is the characteristic that distinguishes the 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 2). 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 nutrients. 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.
Figure 2. The vascular tissue in a plant, showing xylem and phloem. (Click to enlarge) (http://www.ucmp.berkeley.edu/IB181/VPL/Ana/AnaP/Ana1l.jpeg)
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 mostly 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.
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.
When one compares the life cycle of a moss to that of a fern (a seedless vascular plant; Fig. 3), 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 3. 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 22, 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. 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.
Figure 4. Equisetum arvense. A horsetail. (Click to enlarge)
Figure 5. Marattia douglasii. A fern. (Click to enlarge)
Seed Plants: Sporophytes More Prominent, Gametophytes More Reduced
By now you should be familiar with the alternation of generations present in all plants, and that the gametophyte (1n) and the sporophyte (2n) exist in multicellular forms. The gametophyte is the dominant form in nonvascular plants; the sporophyte is smaller and is nourished by the gametophyte as it develops. In seedless vascular plants and seed plants the sporophyte is the dominant form. This change in the dominant generation is depicted in Figure 6.
Figure 6. Three Variations of Alternation of Generations. Nonvascular plants, seedless vascular plants, and seed plants exhibit three modifications of alternation of generations. (Click to enlarge)
The second change in the alternation of generations is seen in the seedless vascular plants are most commonly represented by the ferns. For example, the ferns that are observed growing in the forest and the fern fronds (leaves) that are a part of floral arrangements are sporophytes (2n). The spores that are released from the undersides of fern fronds grow into gametophytes (1n), which live independently from the sporophyte. Note, the sporophyte is much larger than the gametophyte (Fig. 6).
Seed plants represent the third modification of alternation of generations. Like the ferns, the sporophyte is the prominent generation. Unlike the ferns, however, seed plants have gametophytes that are surrounded by sporophytic tissue. This sporophytic tissue nourishes the gametophyte; therefore, the gametophyte does not live independently from the sporophyte and is even more reduced in size. (In Figure 1, the line to the gametophyte of the seed plant points to a structure composed of only eight cells.)
Dispersal Mechanisms: Haploid Spores Versus Diploid Seeds
In addition to having more reduced gametophytes, seed plants use a different method to disperse their reproductive cells than do seedless vascular plants. A seedless vascular plant (e.g., a fern) releases haploid spores into the environment. If conditions are suitable, the spore grows into a mature gametophytic generation. If conditions are harsh, the spore will persist without germinating and will lie dormant until favorable conditions are present. The spore will then germinate into a gametophyte which will produce haploid gametes by mitosis. These gametes will then fuse to produce a zygote, which must live in a moist environment as it develops into an embryo and eventually a mature plant.
In the seed plants, the diploid zygote and the embryo that develops from it are contained within a seed. Many seeds are able to remain dormant until conditions are optimal for germination and growth. For example, some pine seeds actually require the heat of a fire to trigger germination. This is adaptive because just after a fire the seedling can grow quickly without competition from taller trees. Therefore, spores and seeds are similar because they both are resistant to harsh conditions.
However, spores and seeds differ in their structure and composition. Spores are unicellular, haploid, and contain little storage tissue. They are also very small and relatively simple. Seeds are multicellular, larger, and can contain a large amount of storage material.
Another major difference between the spores of seedless plants and seeds is that the haploid spores of seedless plants are released by the parent and develop independently, whereas seeds develop within the parental sporophytic tissue (Figure 7). Seed plants have female spores (megaspores) and male spores (microspores). The male microspore of a seed plant produces sperm within pollen grains, which are transported to the female megaspore. Seed plants have a haploid megaspore that is contained within a fleshy solid mass contained within an ovule. A seed is a fertilized mature ovule. There are also tissues in the ovule (integuments) that become the seed coat.
Therefore, while seedless vascular plants disperse their offspring via haploid spores, seed plants disperse their offspring via diploid seeds.
Figure 7. Seed development in a flowering plant. Seeds develop within the parental sporophytic tissue in seed plants. (Click to enlarge)
Importance of Pollen
The sperm from seedless plants have flagella that propel them through water to reach the egg cell. This mechanism works well for plants in moist environments, however, these plants have a difficult time reproducing in drier environments. The pollen grain is an important adaptation to dry environments. Pollen is a tough structure that contains the precursor to sperm cells. The tough outer coat of the pollen grain is able to survive very harsh conditions, therefore, it can protect the sperm cells for years. When the pollen grain finally lands on the female structure of a plant, it germinates and sperm cells travel to the egg cell through a pollen tube. Thus, the male gamete is protected (rather than open to the environment, as in seedless plants). This adaptation allows seed plants to live in such dry and harsh conditions as deserts. Compare Figures 8 and 9 to see the difference in fertilization between seedless and seeded plants.
Figure 8. Fern Life Cycle. A seedless plant. (Click to enlarge)
Figure 9. Seed Plant Life Cycle. (Click to enlarge)
Nonflowering Seed Plants
The nonflowering seed plants (the gymnosperms) probably arose from a fern relative, appropriately named a progymnosperm, sometime between 409-363 million years ago. The nonflowering seed plants were the predominant land plants by about 225 million years ago. They were the primary vegetation available to herbivorous dinosaurs. Currently, the nonflowering seed plants are comprised of four major groups: cycads, ginkgos, gnetophytes, and conifers.
Nonflowering seed plants are able to grow larger than seedless vascular plants because of their woody stems. The vascular tissue in most members is highly lignified, which adds strength to their cell walls. The strengthened wood allows them to achieve great heights.
The members of this group are very diverse, however, they share one distinctive feature; they all have "naked" seeds. This means that they lack ovaries. As you will learn, nonflowering seed plants and flowering seed plants have ovules in which their seeds develop. In flowering seed plant, these ovules are contained within an ovary; an ovary is not present in nonflowering seed plants.
Cycads are slow-growing and long-lived perennials (they live and reproduce year after year.) They are considered woody, even though their wood does not look like that of a pine or oak tree. The leaves of cycads are large and appear feather-like, much like those of palm leaves. These leaves are arranged spirally at the top of the stem. Cycads are dioecious plants; that is, their male and female reproductive structures (cones) reside on separate plants (Fig. 10). One feature retained in cycads is motile sperm. Remember, nonvascular plants and seedless vascular plants have sperm equipped with flagella for motility. However, like all nonflowering seed plants, the cycads have "naked" seeds.
Cycads have a special type of root called a corraloid root that develops early in the cycad’s life. These roots are ultimately colonized by cyanobacteria (genus Nostoc). What do you think the role of these cyanobacteria is? What type of relationship exists between the cycad and these cyanoacteria?
Figure 10. Cycas angulata from Australia. Plant on left is an adult cycad. Top right is a pollen cone and bottom right is a seed cone. Photos from The Cycad Pages (http://plantnet.rbgsyd.nsw.gov.au/cgi-bin/cycadpg?taxname=Cycas+angulata)(Click image to enlarge)
There is only one extant (living) species of ginkgo, appropriately named Ginkgo biloba. The genus name comes from the Chinese word meaning "silver apricot" (gin=silver, kyo=apricot). The species name is Latin for "double leaf" (bi=double, loba=leaf). The leaves are uniquely fan-shaped, with a split in the middle that makes them appear to have two lobes. Like cycads, ginkgos are dioecious and have motile sperm. Due to their broad leaves, ginkgos are often mistaken for a flowering seed plant. However, the pattern of veins (dichotomous venation) in the leaves is unlike any found in flowering seed plants. Look closely and you will see that each vein splits in two as it passes across the leaf (Fig. 11).
Figure 11. A Gingko leaf. (Click image to enlarge)
The most common place in America to find a ginkgo tree is along the sidewalk. They are frequently used to add color and shade to urban settings. There are many ginkgos on Penn State’s University Park Campus (Fig. 12). The ginkgo tree is particularly resistant to disease, insects, and air pollution. In addition, the leaves turn a beautiful yellow in autumn, just before they fall. Considered to be a living fossil, Ginkgo-like fossils have been found dating back over 270 million years.
Figure 12. A male ginkgo tree located near the library on the University Park Campus (on the right), and a female tree located near Chandlee building (on the left). (Click to enlarge)
Figure 13. The female trees produce reproductive structures that look like fruits. You will be asked to think about these in the case study at the end of this tutorial. (Click to enlarge)
There are three genera of gnetophytes: Weltwitschia (Fig. 14), Ephedra (Fig. 15), and Gnetum (Fig. 16). These are probably the least familiar gymnosperms. The relationships among the groups of gymnosperms are still not known with certainty. Some molecular studies suggest that they are a monophyletic group, while others support the hypothesis that the gnetophytes and the conifers (below) form a group that is more closely related to flowering plants. Still other studies suggest that only the gnetophytes are more closely related to angiosperms. The gnetophytes are the only gymnosperms to undergo a process known as double fertilization. In double fertilization, two sperm cells enter the ovule; one fertilizes the egg and the other fertilizes another cell within the ovary. This process is found in all angiosperms, but could also be the result of convergent evolution. Many groups of researchers are pursuing phylogenetic studies of the gymnosperms, so stay tuned!
Figure 14. Welwitschia mirabilis.
Figure 15. Ephedra sp.
Figure 16. Gnetum gnemon.
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.
The Nonflowering Seed Plant Life Cycle
We will discuss the life cycle of nonflowering seed plants, using pines as an example (Figure 17). The male cones produce haploid pollen grains (spores) by meiosis. The pollen grains develop into microgametophytes (male gametophytes). The female cones have scales that each contain two ovules. Each ovule has one opening called the micropyle. When the ovule is ready to accept pollen, it secretes a liquid to which the pollen grain can adhere. As the liquid dries, the pollen is pulled into the ovule through the micropyle. At this point, a specialized cell within the ovule goes through meiosis to produce four haploid cells (megaspores). Only one megaspore survives, growing and dividing to produce the immature megagametophyte (female gametophyte). Several eggs can develop within the megagametophyte.
Figure 17. The nonflowering seed plant lifecycle. (Click to enlarge)
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.
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.
After reading this tutorial, you should have a working knowledge of the following terms:
A Real Life Example: How accurate is this Ohio State web site about Ginkgos?
The following link is from Ohio State’s Horticulture and Crop Science in Virtual Perspective web site. This page provides information about the tree Ginkgo biloba:
There are two big mistakes on this page. Use what you know about the biology of ginkgos and find the mistakes.
- What are the two major mistakes?
- Do you think this site has been peer-reviewed?
Questions? Send your instructor a message through ANGEL!