Gymnosperm – Plants, Examples, Phylogeny
qiazhan English name: Gymnosperm
Sub Title:
gymnosperm, any vascular plant that reproduces by means of an exposed seed, or ovule—unlike angiosperms, or flowering plants, whose seeds are enclosed by mature ovaries, or fruits. The seeds of many gymnosperms (literally, “naked seeds”) are borne in cones and are not visible until maturity. Taxonomists recognize four distinct divisions of extant (nonextinct) gymnospermous plants—Pinophyta, Cycadophyta, Ginkgophyta, and Gnetophyta—with 88 genera and more than 1,000 species distributed throughout the world.
Gymnosperms were dominant in the Mesozoic Era (about 252.2 million to 66 million years ago), during which time some of the modern families originated (Pinaceae, Araucariaceae, Cupressaceae). Although since the Cretaceous Period (about 145 million to 66 million years ago) gymnosperms have been gradually displaced by the more recently evolved angiosperms, they are still successful in many parts of the world and occupy large areas of Earth’s surface. Conifer forests, for example, cover vast regions of northern temperate lands, and gymnosperms frequently grow in more northerly latitudes than do angiosperms.
General features
In all living gymnosperm groups, the visible part of the plant body (i.e., the growing stem and branches) represents the sporophyte, or asexual, generation, rather than the gametophyte, or sexual, generation. Typically, a sporophyte has a stem with roots and leaves and bears the reproductive structures. As vascular plants, gymnosperms contain two conducting tissues, the xylem and phloem. The xylem conducts water and minerals from the roots to the rest of the plant and also provides structural support. The phloem distributes the sugars, amino acids, and organic nutrients manufactured in the leaves to the nonphotosynthetic tissues of the plant.
In most gymnosperms the male pollen cones, called microstrobili, contain reduced leaves called microsporophylls. Microsporangia, or pollen sacs, are borne on the lower surfaces of the microsporophylls. The number of microsporangia may vary from two in many conifers to hundreds in some cycads. Within the microsporangia are cells which undergo meiotic division to produce haploid microspores.
The gametophyte phase begins when the microspore, while still within the microsporangium, begins to germinate to form the male gametophyte. A single microspore nucleus divides by mitosis to produce a few cells. At this stage the male gametophyte (called a pollen grain) is shed and transported by wind or insects.
Female ovulate cones, called megastrobili, may be borne on the same plant that bears microstrobili (as in conifers) or on separate plants (as in cycads and Ginkgo). A megastrobilus contains many scales, called megasporophylls, that contain megasporangia. Within each megasporangium, a single cell undergoes meiotic division to produce four haploid megaspores, three of which typically degenerate. The remaining megaspore undergoes mitosis to form the female gametophyte. As the number of free nuclei multiplies, the megasporangium and megaspore wall expand. At this stage the ovule is ready to be fertilized.
Before fertilization can take place, however, the mature male gametophyte (the pollen grain) must be transported to the female gametophyte—the process of pollination. In many gymnosperms, a sticky “pollination droplet” oozes from a tiny hole in the female megasporangium to catch pollen grains. The droplet is then resorbed into the megasporangium for fertilization. In other species, the pollen grain settles on the surface of the megasporangium, where the male gametophyte develops further. A pollen tube emerges from the grain and grows through the megasporangium toward the multicellular egg-containing structure called the archegonium. The egg and sperm continue to mature, the nucleus of the latter undergoing additional divisions resulting in two male gametes, or sperm. Interestingly, cycads and Ginkgo are the only seed plants with flagellated sperm. By the time the pollen tube reaches the archegonium, both the egg and sperm are fully mature, and the egg is ready to be fertilized.
In gymnosperms, when the nuclei of the two sperm meet the egg cell, one nucleus dies and the other unites with the egg nucleus to form a diploid zygote. The fertilized egg undergoes mitosis to begin the development of a new sporophyte generation—the multicellular embryo of the seed. Food for the developing embryo is provided by the massive starch-filled female gametophyte that surrounds it. The time interval between pollination and maturation of the embryo into a new sporophyte generation varies among different groups, ranging from a few months to over one year (in pine, for example).
In gymnosperms such as cycads and Ginkgo, the seed coat is known as the sarcotesta and consists of two layers. The sarcotesta is often brightly coloured in cycads, and the sarcotesta of Ginkgo seeds is foul-smelling when ripe. The seeds of some conifers have a thin winglike structure that may assist in the distribution of the seeds. The seeds of other conifers, such as yews, have a fleshy structure, known as an aril, surrounding them. The cones of juniper are fleshy and commonly eaten by birds.
At maturity, a gymnosperm embryo has two or more seed leaves, known as cotyledons. Cycads, Ginkgo, and gnetophytes have two cotyledons in the embryo; pine and other conifers may have several (eight is common; some have as many as 18). In cycads and Ginkgo the cotyledons remain within the seed and serve to digest the food in the female gametophyte and absorb it into the developing embryo. Conifer cotyledons typically emerge from the seed and become photosynthetic.
