On the under side of the prothallus two kinds of organs are borne. These are the =archegonium= (containing egg-cells) and the =antheridium= (containing 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. The sporophyte of seed plants, or the “plant” as we know it, produces two kinds of spores--one kind becoming =pollen-grains= and the other kind =embryo-sacs=. The pollen-spores are borne in sporangia, which are united into what are called =anthers=. The embryo-sac, which contains the egg-cell, is borne in a sporangium known as an =ovule=. _A gametophytic stage is present in both pollen and embryo sac: fertilization takes place, and a sporophyte arises. Soon this sporophyte becomes dormant, and is then known as an_ =embryo=. The embryo is packed away within tight-fitting coats, and the entire body is the =seed=. When the conditions are right the seed grows, and the sporophyte grows into herb, bush, or tree. The utility of the alternation of generations is not understood.