Introduction and Learning Objectives
Nonvascular plants and seedless vascular plants were presented in the previous tutorial. The next two tutorials will present the two groups of seed plants, also known as the gymnosperms and angiosperms. This tutorial will focus on the evolution of seed plants and, in particular, nonflowering seed plants. By the end of this tutorial you should have a basic working understanding of:
- 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
- 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
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 1.
Figure 1. 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. 1).
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 2). 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 2. 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 3 and 4 to see the difference in fertilization between seedless and seeded plants.
Figure 3. Fern Life Cycle. A seedless plant. (Click to enlarge)
Figure 4. 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. 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 5. 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.
Figure 6. 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 Univeristy Park Campus (Fig. 7). 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 7. A male ginkgo tree located near the library on the University Park Campus (on the left), and a female tree located near Chandlee building (on the right). (Click to enlarge)
Figure 8. 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. 9), Ephedra (Fig. 10), and Gnetum (Fig. 11). 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 9. Welwitschia mirabilis.
Figure 10. Ephedra sp.
Figure 11. 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 12). 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 12. 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 examined the nonflowering seed plants (sometimes referred to as gymnosperms) 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 this group, 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:
Questions? Either send your instructor a message through ANGEL or attend an online office hour (the times are posted on ANGEL).