Page:EB1911 - Volume 22.djvu/626

Rh The several orders of Lycopodiales described above, while presenting a number of features in common, are distinctly isolated from one another. A natural classification of such specialized plants can only be obtained when the extinct forms are more fully known. What is known at present, while it does not indicate the phylogeny of the Lycopodiales, at least shows that such living orders as Lycopodiaceae and Selaginellaceae cannot be regarded as forming a linear series. The difficulty is increased when it is borne in mind that the small surviving forms probably have a long geological history, and may have coexisted with the Lepidodendraceae. For these reasons no attempt has been made to arrange the orders in larger divisions, since such a division as that of the ligulate and eligulate forms, while convenient for practical purposes, may not express the phylogeny of the group. The Psilotaceae, formerly included in the Lycopodiales, have been described separately owing to-their resemblance to the Sphenophyllales. It remains to be mentioned that the Isoëtaceae have been regarded as more nearly allied to the Filicales than to the former, near which they are here placed.

V. —The peculiarities of this small order of Pteridophyta render their systematic position a matter of doubt, especially in the absence of evidence as to their geological history, and justify their separation for the present from the other main natural groups. In the three genera, Ophioglossum, Botrychium and Helminthostachys, there is an underground rhizome, from which one leaf or a few leaves with sheathing bases are produced annually; the roots arise in more or less definite relation to the insertion of the leaves. The latter are simple, or irregularly lobed in Ophioglossum, more or less compoundly pinnate in Botrychium and palmately pinnate in Helminthostachys. The fertile branch or branches are situated on the adaxial surface of the leaves, and may be simple, as in Ophioglossum (fig. 2, d), or more or less compound, the degree of branching in the sterile and fertile segments exhibiting a general parallelism. The stem is monostelic, the arrangement of the xylem and phloem being collateral. The endodermis and pericycle surround the whole stele in Botrychium and Helminthostachys; in Ophioglossum each bundle has a separate sheath. Well-marked secondary thickening occurs in Botrychium. In the roots of Ophioglossum and Botrychium and in the first formed roots of Helminthostachys an endophytic fungus is present, forming a mycorhiza—the stele in the larger roots has the usual radial arrangement of xylem and phloem; monarch roots occur in Ophioglossum. The morphology of the fertile spike is a disputed question, upon the answer to which the systematic position of the Ophioglossaceae largely rests. The spike is most simple in Ophioglossum, where it bears on each side a row of large sporangia, which hardly project from the surface, the vascular bundles occupying a central position. In the young spike, which arises when the leaf is still very small, a band of tissue derived from superficial cells is distinguishable along either side; this sporangiogenic band gives rise to the sporogenous groups, the sterile septa between them, and the outer walls of the sporangia. The spike of Helminthostachys corresponds to that of Ophioglossum, but in it the sporangia are borne on two lateral rows of branched sporangiophores. The sporangia themselves resemble those of Botrychium, which project from the ultimate subdivisions of the branched spike; each is developed from a number of cells, the sporogenous tissue arising from a single cell. Two diverse views of the morphology of the fertile spike in these plants have been entertained. The older view was that it was a fertile segment of the leaf; and though its ventral position presents a difficulty, this must be regarded as a possible explanation; the occasional occurrence of sporangia on the lamina in Botrychium has been regarded as supporting it. On the other hand, the spike has been explained as due to the elaboration of a single sporangium occupying a similar position with regard to the leaf as in the Lycopodiales, and evidence of considerable weight has been brought forward in support of this interpretation. The important bearing of this question on the relationship of the

Ophioglossaceae to the phyla of the Filicales and Lycopodiales will be obvious.

The position of the fertile spike in relation to the leaf corresponds to that of the synangium or sporangiophores in the Psilotales and Sphenophyllales. The Ophioglossaceae are homosporous, and the prothalli, which are known in species of all three genera, are subterranean and saprophytic (fig. 1, f, g). The prothallus of O. pedunculosum, as observed by Mettenius, subsequently reached the surface and produced green lobes; those of the other species known are wholly saprophytic, and contain an endophytic fungus. Those of Ophioglossum are cylindrical, while the dorsiventral prothallus of Botrychium bears the sexual organs on the upper surface. They present a general, but probably homoplastic, resemblance to the saprophytic prothalli of certain Lycopodia. Important points of difference exist, however, in the apical position of the meristem of the Ophioglossaceous prothalli, in the presence of a basal cell to the archegonium, and in the multiciliate spermatozoids. In these respects, in the megaphyllous habit and in certain anatomical features, the Ophioglossaceae approach the Filicales. Some species of Botrychium have recently been found to have embryos provided with a suspensor. The position of the Ophioglossaceae can at present only be regarded as an open question, in considering which the possible antiquity of the group must be borne in mind.

VI. —This group of Pteridophyta differs from the others in being well represented in our present flora by forms, many of which can be regarded not as archaic types which have persisted to the present day, but as having been evolved in comparatively recent periods. The Ferns exhibit a wide range in size from the minute epiphytic Hymenophyllaceae, with leaves barely a centimetre in length, to gigantic tree-ferns 80 ft. or more in height. A general characteristic of their habit is the large size of the leaves, which are often highly compound, relatively to the stern. Some ferns have a longer or shorter erect stem often clothed by the persistent bases of the leaves; in others the stem creeps on the surface of the substratum or is subterranean. Its surface is clothed with filamentous or scaly hairs (paleae), which protect the growing point; and adventitious roots spring from it. The position of the branches varies in the group; they are only exceptionally axillary (Hymenophyllaceae, Botryopterideae). The anatomy of the stele in the stem exhibits on the whole a progression from a solid protostele through a tubular solenostele to one or more circles of separate steles derived by the breaking up of the solenostele. The leaf-traces usually interrupt the continuity of the stele of the axis on their departure. The sporangia are borne in groups (sori) on the under surface of the leaves; sometimes the fertile leaves differ more or less from the purely vegetative ones. The form of the sorus and the structure of the sporangium are of great systematic importance. The sorus is frequently protected by an outgrowth from the surface or margin of the leaf called the indusium. Heterospory is only known in the Hydropterideae. The prothallus developed from the spore is green and in most cases dorsiventral, bearing the archegonia and antheridia on the under surface.

Some of the more striking adaptive modifications in the gametophyte and sporophyte, and certain effects of altered external conditions which have been ascertained experimentally, may be briefly mentioned. The dorsiventrality of the prothallus has been shown to depend mainly on the illumination, the filamentous form being retained in feeble light; a similar result is obtained when the prothalli are cultivated in water. These facts may have a bearing on the filamentous prothalli of some Hymenophyllaceae. The reproduction of the prothallus by gemmae in species of Trichomanes, Vittaria and Monogramme is another interesting adaptation; the prothallus of Gymnogramme