Fig. 262.—Prothallus of a Fern. Enlarged:
Archegonia at a; antheridia at b.
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.
The spores of ferns are borne on leaves; the spores of seed-bearing plants are also borne amongst a mass of specially developed conspicuous leaves known as flowers; therefore these plants have been known as the flowering plants. Some of the leaves are developed as envelopes (calyx, corolla), and others as spore-bearing parts, or sporophylls (stamens, pistils). But the spores of the lower plants, as of ferns and mosses, may also be borne in specially developed foliage, so that the line of demarcation between flowering plants and flowerless plants is not so definite as was once supposed. The one definite distinction between these two classes of plants is the fact that one class produces seeds and the other does not. The seed-plants are now often called spermaphytes, but there is no single coordinate term to set off those which do not bear seeds. It is quite as well, for popular purposes, to use the terms phenogams for the seed-bearing plants and cryptogams for the others. These terms have been objected to in recent years because their etymology does not express literal facts (phenogam signifying “showy flowers,” and cryptogam “hidden flowers”), but the terms represent distinct ideas in classification. The cryptogams include three great series of plants—the Thallophytes or algæ, lichens, and fungi; the Bryophytes or moss-like plants; the Pteridophytes or fernlike plants.
Fig. 263.—Diagram to explain the Terminology of the Frond.
Suggestions.—186. The parts of a fern leaf. The primary complete divisions of a frond are called pinnæ, no matter whether the frond is pinnate or not. In ferns the word “pinna” is used in essentially the same way that leaflet is in the once-compound leaves of other plants. The secondary leaflets are called pinnules, and in thrice, or more, compound fronds, the last complete parts or leaflets are ultimate pinnules. The diagram (Fig. [263]) will aid in making the subject clear. If the frond were not divided to the midrib, it would be simple, but this diagram represents a compound frond. The general outline of the frond, as bounded by the dotted line, is ovate. The stipe is very short. The midrib of a compound frond is known as the rachis. In a decompound frond, this main rachis is called the primary rachis. Segments (not divided to the rachis) are seen at the tip, and down to h on one side and to m on the other. Pinnæ are shown at i, k, l, o, n. The pinna o is entire; n is crenate-dentate; i is sinuate or wavy, with an auricle at the base; k and l are compound. The pinna k has twelve entire pinnules. (Is there ever an even number of pinnules on any pinna?) Pinna l has nine compound pinnules, each bearing several entire ultimate pinnules. The spores.—187. Lay a mature fruiting frond of any fern on white paper, top side up, and allow it to remain in a dry, warm place. The spores will discharge on the paper. 188. Lay the full-grown (but not dry) cap of a mushroom or toadstool bottom down on a sheet of clean paper, under a ventilated box in a warm, dry place. A day later raise the cap.