The origin of seeds

The first plants with seeds are known from Late Devonian rocks, and seed bearers rose to prominence during the Carboniferous. After the end of the Carboniferous, and the extinction of arborescent lycopsids, ferns and horsetails, seed-bearing plants, or gymnosperms, took an increasingly dominant role in floras around the world.

Seeds in gymnosperms are naked, that is, they are not enclosed in ovaries as they are in flowering plants (angiosperms). Seeds follow from the fertilization of an ovule, the structure containing the egg. The gymnosperm ovule (Fig. 18.12) consists of the megasporan-

integument archegonia

Figure 18.12 A typical gymnosperm seed, the ovule of Pinus, the pine, showing the archegonia (fertile female structures) surrounded by a substantial food store. Sperm enter through a narrow gap in the protective integument, and pass through pollen tubes to the archegonia.

gium, the site of the female reproductive structures, and an outer protective layer, the integument, with an open end through which the pollen grains enter. The pollen grains settle on the ovule, and may send pollen tubes into the tissues of the ovule, through which sperm head for the fertile female structures, the archegonia. Upon fertilization, the ovule becomes a seed, containing a viable embryo that develops within the seed coat, and feeding on the nutritive material that composed the bulk of the ovule.

Seed-bearing plants evolved from forms that lacked seeds, and produced spores that were scattered freely. Spores are commonly encountered as fossils and they form the basis of palynological studies for research and commercial purposes (Box 18.4). Free-sporing plants, such as ferns and horsetails, expel their microspores and megaspores, which develop into male and female gametophytes, respectively. The male gametes (sperm) are motile and must swim to fertilize the female egg. This is a risky business that requires at least a veneer of surface water, limiting sexual reproduction to damp conditions. Seed plants retain the egg in a watertight capsule (seed) that is fed by the vascular system. They produce sperm in tiny watertight capsules (pollen) that are blown onto the seed. After fertilization, the benefit is that the embryo has a ready-made water and food supply system (when attached to parent plant) and after dispersal is housed in a drought-resistant shell that only bursts open when soil conditions are optimum in terms of wetness and warmth. Hence seed plants can colonize drier environments.

Gymnosperms are said to have owed their success in the Carboniferous to the fact that they retained their ovules, and that the developing embryo had extra protection from the parent plant. In addition, the free-living game-

tophyte phase was eliminated, and water was not required for the sperm to swim through, so that gymnosperms could inhabit dry upland habitats. Gymnosperms may have had adaptive advantages in certain situations as a result of seed bearing, but it would be wrong to assume that they always prevailed. Ferns,

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