Anatomy of Plants

Gymnosperms (Cyced, Ginko, Conifers)

The gymnosperms produce naked seeds; that is, the seeds are not enclosed by fruit. There are four divisions of gymnosperms (Cycadophyta, Ginkgophyta, Gnetophyta, and Coniferophyta; see Figure 19). Cycads are cone-bearing, palmlike plants found today mainly in tropical and subtropical regions. Only one species of ginkgo, the maidenhair tree, survives today. The gnetophyta has only three genera left. The largest group of gymnosperms is the cone-bearing conifers, which include pine, cedar, spruce, fir, and redwood trees. These trees have needlelike leaves that are well adapted to not only hot summers but also cold winters and high winds. Most gymnosperms are evergreen trees.

Figure 19 Gymnosperms
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The sporophyte is dominant in the pine life cycle. Typically, the male pine cones are quite small and develop near the tips of lower branches. Each scale of the male cone has two or more microsporangia on the underside. Inside the microsporangia are microspore mother cells that undergo meiosis and develop into mature pollen grain (with two lobular wings), which is a sperm-bearing male gemetophyte. The female pine cones are larger and located near the top of the tree. Each scale of the female cone has two ovules that lie on the upper surface. Within the ovule, a megaspore mother cell undergoes meiosis and develops into mature female gametophyte, which has 2 - 6 archegonia, each containing a single, large egg lying near the ovule

opening. During pollination, pollen grains are transferred from the male cone to the female cone. Once enclosed within the female cone, the pollen grain develops a pollen tube that slowly grows toward the ovule. The pollen tube discharges two nonflagellated sperms. Only one of the sperms fertilizes an egg in the ovule 15 months after pollination. After fertilization, the ovule matures and becomes the seed composed of the embryo, its stored food, and a seed coat. Finally, in the third season, the female cone, by now woody and hard, opens to release its seeds, whose wings are formed from a thin, membranous layer of the cone scale. When a seed germinates, the sporophyte embryo develops into a new pine tree, and the cycle is complete (Figure 20).

Figure 20 Conifer Life Cycle
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As shown in Figure 21, the outermost layer of the conifer seed is the seed coat. It originates from the mothe tree and is diploid. The seed coat has three layers: the outer layer; the thicker, tough stony or middle layer; and the inner layer. Some species of conifers have resin vesicles in the middle or outer layers of the seed coat. These resin vesicles may play a role in seed coat dormancy, protecting dehydration, and deterring seed herbivory. Immediately inside the cell wall is the nucellus, a papery layer surrounding the megaspore cell wall. Inside the megaspore cell wall is the megagametophyte, the haploid nutritional tissue found in gymnosperm seeds. Early in the development of cones, megagametophytes produce egg cells which are fertilized by male gemetes to produce zygotes. Zygotes develop into embryos. The megagametophyte then plays its second functional role, surrounding the embryo, protecting and nourishing it. The megagametophyte in Douglas-fir is 60% lipids, 16% proteins, and 2% sugars, making it a high-energy and nutrient tissue both for the embryos it contains and for a plethora of seed predators

including small mammals, birds, and insects. The embryo is found in the corrosion cavity, a pit in the centre of the megagametophyte that is fully filled by the embryo in mature seeds. It consists of the cotyledons (first leaf), shoot apical meristem, root apical meristem, root cap and suspensor. The cotyledons and shoot apical meristem point towards the wider end of the seed; while the radicale (embryonic root) and suspensor are at the more pointed end. The suspensor is found at the base of the root cap and plays a role early in embryo development by pushing the embryo into the megagametophyte.