Elementary Botany

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.

Classification of the Pteridophytes.

Of the living pteridophytes four classes may be recognized.

CLASS FILICINEÆ.[34]

This class includes the ferns. Four orders may be recognized.

600. Order Ophioglossales. (One Family, Ophioglossaceæ).—This order includes the grapeferns (Botrychium), so called because of the large botryoid cluster of sporangia, resembling roughly a cluster of grapes; and the adder-tongue (Ophioglossum), the sporangia being embedded in a long tongue-like outgrowth from the green leaf. Botrychium and Ophioglossum are widely distributed. The roots are fleshy, nearly destitute of root hairs, and contain an endophytic fungus, so that the roots are mycorhiza. The gametophyte is subterranean, and devoid of chlorophyll. In Botrychium virginianum, an endophytic fungus has been found in the prothallium. Another genus (Helminthostachys) with one species is limited to the East Indies.

601. Order Marattiales (One Family, Marattiaceæ).—These are tropical ferns, with only four or five living genera (Marattia, Danæa, etc.). They resemble the typical ferns, but the sporangia are usually united, several forming a compound sporangium, or synangium.

The Ophioglossales and Marattiales are known as eusporangiate ferns, while the following order includes the leptosporangiate ferns.

602. Order Filicales.—This order includes the typical ferns. Eight families are recognized.

Family Osmundaceæ.—Three genera are known in this family. Osmunda has a number of species, three of which are found in the Eastern United States; the cinnamon-fern (O. cinnamomea), the royal fern (O. regalis), and Clayton’s fern (O. claytoniana). No species of this family are found on the Pacific coast.

Family Gleicheniaceæ.—These ferns are found chiefly in the tropics, and in the mountain regions of the temperate zones of South America. There are two genera, Gleichenia containing all but one of the known species.

Family Matoniaceæ.—One genus, Matonia, in the Malayan region.

Family Schizæceæ.—These are chiefly tropical, but two species are found in eastern North America, Schizæa pusilla and Lygodium palmatum, the latter a climbing fern.

Family Hymenophyllaceæ.—These are known as the filmy ferns because of their thin, delicate leaves. They grow only in damp or wet regions, mostly in the tropics, but a few species occur in the southern United States.

Family Cyatheaceæ.—These are known as the tree ferns, because of the large size which many of them attain. They occur chiefly in tropical mountainous regions, many of them palm-like and imposing because of the large trunks and leaves. Dicksonia, Cyathea, Cibotium, Alsophila, are some of the most conspicuous genera.

Family Parkeriaceæ.—There is a single species in this family (Ceratopteris thalictroides), abundant in the tropics and extending into Florida. It is aquatic.

Family Polypodiaceæ.—This family includes the larger number of living ferns and many genera and species are found in North America. Examples, Polypodium, Pteridium (= Pteris), Adiantum, etc.

603. Order Hydropterales (or Salviniales).—The members of this order are peculiar, aquatic ferns, some floating on the water (Azolla, Salvinia), while others are anchored to the soil by roots (Marsilia, Pilularia). They are known as water ferns. The sporangia are of two kinds, one containing large spores (macrospores) and the other small spores (microspores). They are therefore heterosporous ferns.

Family Salviniaceæ.—There are two genera, Salvinia and Azolla.

Family Marsiliaceæ.—Two genera, Marsilia and Pilularia. In this family the sporangia are enclosed in a sporocarp, which forms a pod-like structure.

CLASS EQUISETINEÆ.[35]

604. Order Equisetales.—The single order contains a single family, Equisetaceæ, among the living forms, and but a single genus, Equisetum. There are about twenty-four species, with fourteen in the United States (see Chapter XXIX).

CLASS LYCOPODIINEÆ.[36]

605. Order Lycopodiales.—The first two families of this order include the homosporous Lycopodiineæ, while the Selaginellaceæ are heterosporous.

Family Lycopodiaceæ.—There are two genera. Lycopodium (club moss) includes many species, most of them tropical, but a number in temperate and subarctic regions. The gametophyte of many species is tuberous, lacks chlorophyll, and in some there lives an endophytic fungus. Phylloglossum with one species is found in Australia.

Family Psilotaceæ.—There are two genera. Psilotum chiefly in the tropics has one species (P. triquetrum) in the region of Florida.

Family Selaginellaceæ.—These include the little club mosses, with one genus, Selaginella (see Chapter XXX).

CLASS ISOETINEÆ.

606. Order Isoetales, with one family Isoetaceæ and one genus Isoetes (see Chapter XXXI). There are about fifty species, with about sixteen in the United States.