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 taining sperm-cells). These organs are minute specialized parts of the prothallus. Their positions on a particular prothallus are shown at a and b in Fig. 262, but in some ferns they are on separate prothalli (plant diœcious). The sperm-cells escape from the antheridium and in the water that collects on the prothallus are carried to the archegonium, where fertilization of the egg takes place. From the fertilized egg-cell a plant grows, becoming a "fern." In most cases the prothallus soon dies. The prothallus is the gametophyte (from Greek, signifying the fertilized plant).

The fern plant, arising from the fertilized egg in the archegonium, becomes a perennial plant, each year producing spores from its fronds (called the sporophyte); but these spores—which are merely detached special kinds of cells—produce the prothallic phase of the fern plant, from which new individuals arise. A fern is fertilized but once in its lifetime. The "fern" bears the spore, the spore gives rise to the prothallus, and the egg-cell of the prothallus (when fertilized) gives rise to the fern.

A similar alternation of generations runs all through the vegetable kingdom, although there are some groups of plants in which it is very obscure or apparently wanting. It is very marked in ferns and mosses. In algæ (including the seaweeds) the gametophyte is the "plant," as the non-botanist knows it, and the sporophyte is inconspicuous. There is a general tendency, in the evolution of the vegetable kingdom, for the gametophyte to lose its relative importance and for the sporophyte to become larger and more highly developed. In the seed-bearing plants the sporophyte generation is the only one seen by the non-botanist. The gametophyte stage is of short duration and the parts are small; it is confined to the time of fertilization.