Part 192. The ferns (Class Filicinae). Our most primitive ferns are represented by adder-tongues (Ophioglossum) and grape-ferns (Botrychium, Fig. 357).

Fig. 357. Adder tongue, A, and grape fern, B (Ophioglossum vulgatum and Botrychium Lunaria, Adder tongue Family, Ophioglossaceoe). Sporophytes showing roots (w), stem (st), leaf stalk (bs), point (z) at which leaves branch to form a foliage blade (b) and a spore bearing division (f). Two thirds natural size. (Sachs.) Not very common but widely distributed in mostly open ground.

Fig. 357.-Adder-tongue, A, and grape-fern, B (Ophioglossum vulgatum and Botrychium Lunaria, Adder-tongue Family, Ophioglossaceoe). Sporophytes showing roots (w), stem (st), leaf-stalk (bs), point (z) at which leaves branch to form a foliage-blade (b) and a spore-bearing division (f). Two-thirds natural size. (Sachs.)-Not very common but widely distributed in mostly open ground.

Unfortunately their life-histories are not yet fully known owing to peculiar difficulties in tracing the germination of the spores. The gametophyte is subterranean (Fig. 358) and at least when mature it is saprophytic. Except for its lack of chlorophyll it is not a little like the gametophyte of Riccia. The gametophyte of certain ferns closely related to the above more nearly resembles that of Anthoceros, and is holophytic, as we may suppose to have been the case with the original fern-ancestor. When we compare the sporophytes of an adder-tongue and a horned liverwort, however, so many striking differences appear, that it may at first seem hopeless to think of homologizing the parts. Indeed, we have in ferns true leaves, stems, and roots, no trace of which appear in any liverwort. But we have sporangia in both, and in the growing zone of Anthoceros we have a cylindrical meristematic organ suggesting possibilities of much further differentiation. If the sporangium of Anthoceros were enlarged and instead of elaters produced sterile tissue between groups of spores forming two rows on either side of the columella, the resulting organ would be a flat spike of sporangia like that of Ophioglossum (Fig. 359). What may have happened is that in very ancient times, before the age when coal plants flourished, a liverwort something like an Anthoceros did evolve a root from the lower end of its growing zone, which made possible an expansion of the green tissue above, while this in turn helped to bring about the formation of two rows of globular sporangia making a flat cluster as already described. Such an expanded member

Fig. 358. Grape fern. A, gametophyte (prothallus) cut vertically to show the antheridia (an), the archegonia (ac), and the pseudo roots (w), 50/1. B, lower part of a young sporophyte dug up in September, cut vertically to show the stem (st) and leaves (b, b', b), 20/1.

Fig. 358.-Grape-fern. A, gametophyte (prothallus) cut vertically to show the antheridia (an), the archegonia (ac), and the pseudo-roots (w), 50/1. B, lower part of a young sporophyte dug up in September, cut vertically to show the stem (st) and leaves (b, b', b"), 20/1. (Hofmeister.)

Fig. 359. Adder tongue. Upper part of spore bearing division of leaf (6/1), cut vertically to show the tip (s), the spore cavities (sp), the places (r) where a slit is formed to free the spores, and the woody strands or fibrovascular bundles (g) which strengthen and conduct sap.

Fig. 359.-Adder-tongue. Upper part of spore-bearing division of leaf (6/1), cut vertically to show the tip (s), the spore-cavities (sp), the places (r) where a slit is formed to free the spores, and the woody strands or fibrovascular bundles (g) which strengthen and conduct sap. (Sachs.)

The striking differences between liverworts and ferns of any kind have so impressed not a few botanists as to have made them doubt the likelihood of ferns having originated in the manner above suggested; and this doubt has gained strength from the fact that the most ancient fossil ferns are of highly complex organization, being often tree-like in form, and so even less like liverworts than the presumably degenerate ferns with which we are most familiar to-day. Moreover, if modern liverworts are also to be regarded as degenerate plants-a view, as we have seen, for which there is some evidence-the gap which separates them from ferns is even wider. It may well be true that ferns evolved directly from seaweeds in which a clearly marked alternation of generations had developed as in certain rather highly organized red algae living to-day. On this supposition, however, we are still left with the difficulty of imagining the stages through which a seaweed could pass in fitting itself for life on land as a tree. Here fossils cannot help us, for we have none at all intermediate between seaweeds and ferns. Since, however, there are undoubted fundamental resemblances between a Coleochaete, an Anthoceros, and an Ophio-glossum, these may offer at least a possible clue as to how the great changes in question may have taken place.

Grape-ferns would be readily derivable from adder-tongues by further branching of the two leaf branches, which in the fertile or sporangial segment might result in each sporangium being borne on a little stalk or branchlet of its own. We may well imagine that wonderful possibilities of development lay before such a type as this as soon as it established itself on the edges of swamps or on land where food and moisture abounded. It could then afford to delay the production of spores until it had built a thick, tall stem, by means of leaves made larger and larger year after year and devoted entirely to making food so that a surplus might be stored in the stem. Finally, a very large number of sporangia might be produced upon much-branched spore-sac-leaves; and these, held high in the air, could scatter their spores most effectively.