596. Comparison of selaginella and isoetes with the ferns.—On comparing selaginella and isoetes with the ferns, we see that the sporophyte is, as in the ferns, the prominent part of the plant. It possesses root, stem, and leaves. While these plants are not so large in size as some of the ferns, still we see that there has been a great advance in the sporophyte of selaginella and isoetes upon what exists in the ferns. There is a division of labor between the sporophylls, in which some of them bear microsporangia with microspores, and some bear macrosporangia with only macrospores. In the ferns and horsetails there is only one kind of sporophyll, sporangium, and spore in a species. By this division of labor, or differentiation, between the sporophylls, one kind of spore, the microspore, is compelled to form a male prothallium, while the other kind of spore, the macrospore, is compelled to form a female prothallium. This represents a progression of the sporophyte of a very important nature.
597. On comparing the gametophyte of selaginella and isoetes with that of the ferns, we see that there has been a still farther retrogression in size from that which we found in the independent and large gametophyte of the liverworts and mosses. In the ferns, while it is reduced, it still forms rhizoids, and leads an independent life, absorbing its own nutrient materials, and assimilating carbon. In selaginella and isoetes the gametophyte does not escape from the spore, nor does it form absorbing organs, nor develop assimilative tissue. The reduced prothallium develops at the expense of food stored by the sporophyte while the spore is developing. Thus, while the gametophyte is separate from the sporophyte in selaginella and isoetes, it is really dependent on it for support or nourishment.
598. The important general characters possessed by the ferns and their so-called allies, as we have found, are as follows: The spore-bearing part, which is the fern plant, leads an independent existence from the prothallium, and forms root, stem, and leaves. The spores are borne in sporangia on the leaves. The prothallium also leads an independent existence, though in isoetes and selaginella it has become almost entirely dependent on the sporophyte. The prothallium bears also well-developed antheridia and archegonia. The root, stem, and leaves of the sporophyte possess vascular tissue. All the ferns and their allies agree in the possession of these characters. The mosses and liverworts have well-developed antheridia and archegonia, and the higher plants have vascular tissue. But no plant of either of these groups possesses the combined characters which we find in the ferns and their relatives. The latter are, therefore, the fern-like plants, or pteridophyta. The living forms of the pteridophyta are classified as follows into families or orders. ([See page 295].)
599.
TABLE SHOWING RELATION OF GAMETOPHYTE
AND SPOROPHYTE IN THE PTERIDOPHYTES.
| GAMETOPHYTE. (Becoming smaller, mostly independent. In selaginella and isoetes becoming dependent on the sporophyte.) | SPOROPHYTE (Largest part of the plant. The fern plant. Independent of, and more hardy than, the gametophyte. Usually perennial.) | Beginning of Gametophyte. | ||||||
|---|---|---|---|---|---|---|---|---|
| Vegetative Part. | Sexual Organs. | Beginning of Sporophyte. | Vegetative Part. | Fruiting Part. | ||||
| Ferns. (Polypodiaceæ.) | A green, thin, expanded, heart-shaped growth, with rhizoids. | Usually both kinds on the same prothallium. | Fertilized egg. (Develops into fern plant.) | Root, stem, leaf. | Sporangia on leaf. All of one kind. Sporangium contains .... | Spores. | ||
Antheridia with spermatozoids. | Archegonia, each with egg. | |||||||
| Equisetum. | A green, thin, expanded, lobed growth, with rhizoids. | Usually the two kinds on different prothallia. | Fertilized egg. (Develops into equisetum plant.) | Root, stem, leaf. | Sporangia on sporophylls. All of one kind. Sporangium contains .... | Spores. | ||
Antheridia, on small male prothallia, with spermatozoids. | Archegonia on larger female prothallia, each with an egg. | |||||||
| Isoetes. | Colorless, rounded mass of cells, inside of spore wall, usually no rhizoids, or but few. Two kinds. | On different prothallia. | Fertilized egg. (Develops into isoetes plant.) | Root, stem, leaf. Stem very short. Leaves bear sporangia in cavities at base; outer leaves usually bear macrosporangia, inner ones microsporangia. | Sporangia of two kinds. Small ones contain .... | Microspores. | ||
Small ones, male. Developed into small prothallial cell, and antherid cell while still in sporangium. | Large ones, female. Developed from nutriment stored in macrospore from sporophyte. | One antheridium, much larger than the single prothallial cell. Antheridium with spermatozoids. | Few archegonia, in apex of oval colorless, female prothallium, each with egg. | Large ones contain .... | Macrospores. | |||
| Selaginella. | Colorless, rounded mass of cells inside of spore wall, no rhizoids, or but few. Two kinds. | On different prothallia. | Fertilized egg. (Develops into selaginella plant.) | Root, stem, leaf. Spore-bearing leaves grouped on the end of stem in a spike. Lower ones bear macrosporangia, upper ones bear microsporangia. | Sporangia of two kinds. Small ones contain .... | Microspores. | ||
Small ones, male. Developed into small prothallial cell, and antherid cell while in sporangium. | Large ones, female. Developed while still in sporangium and dependent on sporophyte. | One antheridium, much larger than the single prothallial cell. Antheridium with spermatozoids. | Few archegonia, in apex of oval, colorless, female prothallium, each with egg. | Large ones contain .... | Macrospores. | |||
[Classification of the Pteridophytes.]
Of the living pteridophytes four classes may be recognized.
CLASS FILICINEÆ.[34]