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?
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.
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. 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.
There are three genera of gnetophytes : Weltwitschia , Ephedra , and Gnetum . 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!
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.
Plants II Part 3 VoiceThread Transcript
We will discuss the life cycle of nonflowering seed plants, using pines as an example. 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.
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.