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Gymnosperms
Gymnosperms are plants with vascular tissue and are considered more advanced than both bryophytes and ferns because, among other things, they produce seeds. The word "gymnosperm" is a combination of two Greek words meaning naked (gymnos) seed (sperma). The ability to produce seeds has enabled gymnosperms, along with the only other seed-bearing group, the flowering plants, to become the major vegetation on Earth today. Gymnosperms are woody and may be trees, shrubs, or even vines. Examples include the needle-leaved pines, hemlocks, spruces, and firs, the awl- and scaled-leaved junipers, the fernlike-leaved cycads, and the broad-leaved Ginkgo.
Gymnosperm Reproduction [Illustration]
After a number of years of strictly vegetative growth from the seed and seedling stages, young pine trees begin to produce strobili, the sexual reproductive structures that are cone shaped in many gymnosperms. Pines are monoecious, with microstrobili (strobili that produce male spores) and megastrobili (strobili that produce female spores) on the same plant. However, other gymnosperms, such as cycads and Ginkgo, are dioecious, with separate male and female plants. In addition, all gymnosperms are heterosporous, meaning that they produce two types of spores, small male spores called microspores and large female spores called megaspores -- a condition also true of certain ferns, Selaginella (the spike mosses), Isoetes (the quillworts), and all flowering plants. (The bryophytes and many ferns and fern-allies are homosporous-that is, they produce a single type of spore.)
Microstrobili, or male cones, occur in clusters just below the tips usually of lower branches as the buds unfold at the beginning of the growing season. The minute megastrobili, or female cones, are borne on short side branches of the elongating central stalks of uppermost buds on the higher branches of the same plant. The megastrobili are visible only as these buds elongate in the spring.
A microstrobilus, or male cone, is composed of a central stalk bearing microsporophylls, which are modified leaves, each with two elongated microsporangia on its lower surface. Early in the spring, each of the microsporocytes, or cells capable of producing male spores within each microsporangium, undergo meiosis and produce groups of four haploid microspores; these separate into individual microspores with winged cell walls. At this stage, each microspore looks like a ball with "mouse ears" attached. The exact time at which meiosis and microspore formation occur varies according to season and latitude.
Megastrobili, or female cones, are structurally more complex and much larger than the rather short-lived microstrobili. A single megastrobilus is composed of a central stalk with bracts. Each of these bracts bears ovule-bearing appendages known as ovuliferous scales. Each ovuliferous scale, in turn, bears two megasporangia or ovules. The ovule is covered with a specialized tissue known as the integument, which becomes the seed coat following fertilization, and includes a passageway known as the micropyle. After pollination, within each ovule a single megasporocyte undergoes meiosis, forming a row of four megaspores. The three megaspores nearest the micropyle deteriorate, leaving one viable megaspore, which develops into a female gametophyte over a period of several months.
The microspores of pines begin developing into male gametophyte-bearing pollen grains before being shed from the microsporangia, usually in the spring. The microsporangia release clouds of minute pollen once they have developed into four-celled gametophytes containing a generative cell, a tube cell, and two sterile or prothallial cells. At the same time, the ovuliferous scales of the current year's megastrobili begin to separate slightly following the elongation of the central stalk. This results in the formation of cracks between adjacent ovuliferous scales through which a sticky substance, called pollen drops or droplets, is exuded. Pollination occurs when some of the airborne pollen grains land on these droplets and are drawn down into the ovule through the micropyle.
After pollination, the scales of the megastrobili close and protect the developing ovules. As the female gametophyte matures, two or three archegonia are formed at the end of the ovule where the micropyle is located. It takes more than a year for the female gametophytes in pines to mature.
During this time, the pollen grains adhering to the pollination droplets each form a pollen tube-an outgrowth that slowly "digests" the tissues of the megasporangium or ovule and eventually delivers the sperm to the mature archegonia. The generative cell, one of the cells in a pollen grain, divides, forming a sterile cell and a spermatogenous cell. Before reaching the archegonia, the spermatogenous cell divides to form two sperm. It is at this stage, when it consists of the pollen grain and tube with its two sperm, that the male gametophyte has reached maturity.
Once the pollen tube reaches the now mature, egg-containing archegonium, the two sperm are released, and one unites with the egg to form a zygote. The second sperm degenerates.
A single female gametophyte normally contains two or three archegonia, the eggs of all of which may be fertilized. However, only a single embryo normally completes development. As the embryo develops, the integument hardens, forming the seed coat. Likewise, the ovuliferous scales harden, and a thin layer of each scale becomes the "wing" of a seed. Each pine seed is an embryonic sporophyte.
The mature, winged seeds are shed from the female cone as its appendages spread apart. In pines, seeds typically are released the autumn of the year after pollination. In some species, female cones do not open until they are exposed to extreme heat, such as that generated by forest fires. If seeds land in a favorable location, they may germinate.