Evolution of Gymnosperms
The fossil plant Elkinsia polymorpha, a "seed fern" from the Devonian period—about 400 million years ago—is considered the earliest seed plant known to date. Seed ferns (Figure) produced their seeds along their branches, in structures called cupules that enclosed and protected the ovule—the female gametophyte and associated tissues—which develops into a seed upon fertilization. Seed plants resembling modern tree ferns became more numerous and diverse in the coal swamps of the Carboniferous period.
Fossil records indicate the first gymnosperms (progymnosperms) most likely originated in the Paleozoic era, during the middle Devonian period: about 390 million years ago. The previous Mississippian and Pennsylvanian periods, were wet and dominated by giant fern trees. But the following Permian period was dry, which gave a reproductive edge to seed plants, which are better adapted to survive dry spells. The Ginkgoales, a group of gymnosperms with only one surviving species—the Ginkgo biloba—were the first gymnosperms to appear during the lower Jurassic. Gymnosperms expanded in the Mesozoic era (about 240 million years ago), supplanting ferns in the landscape, and reaching their greatest diversity during this time. The Jurassic period was as much the age of the cycads (palm-tree-like gymnosperms) as the age of the dinosaurs. Ginkgoales and the more familiar conifers also dotted the landscape. Although angiosperms (flowering plants) are the major form of plant life in most biomes, gymnosperms still dominate some ecosystems, such as the taiga (boreal forests) and the alpine forests at higher mountain elevations (Figure) because of their adaptation to cold and dry growth conditions.
Seeds and Pollen as an Evolutionary Adaptation to Dry Land
Bryophyte and fern spores are haploid cells dependent on moisture for rapid development of multicellular gametophytes. In the seed plants, the female gametophyte consists of just a few cells: the egg and some supportive cells, including the endosperm-producing cell that will support the growth of the embryo. After fertilization of the egg, the diploid zygote produces an embryo that will grow into the sporophyte when the seed germinates. Storage tissue to sustain growth of the embryo and a protective coat give seeds their superior evolutionary advantage. Several layers of hardened tissue prevent desiccation, and free the embryo from the need for a constant supply of water. Furthermore, seeds remain in a state of dormancy—induced by desiccation and the hormone abscisic acid—until conditions for growth become favorable. Whether blown by the wind, floating on water, or carried away by animals, seeds are scattered in an expanding geographic range, thus avoiding competition with the parent plant.
Pollen grains (Figure) are male gametophytes containing just a few cells and are distributed by wind, water, or an animal pollinator. The whole structure is protected from desiccation and can reach the female organs without depending on water. After reaching a female gametophyte, the pollen grain grows a tube that will deliver a male nucleus to the egg cell. The sperm of modern gymnosperms and all angiosperms lack flagella, but in cycads, Ginkgo, and other primitive gymnosperms, the sperm are still motile, and use flagella to swim to the female gamete; however, they are delivered to the female gametophyte enclosed in a pollen grain. The pollen grows or is taken into a fertilization chamber, where the motile sperm are released and swim a short distance to an egg.