or Vi Filicales

B.

Lycopodiales Ligulatae. Selaginella comprises some 500 species, widely spread through the tropics, some native on tem perate hill stations. The latter, chiefly of exposed habit, have radial symmetry, but most Selaginellas are dorsiventral, and live under shade. The genus shares the leading features of Lycopodium, but it differs in the presence of a ligule, and in the fact that all the species are heterosporous. Various Selaginellas are favourite greenhouse plants, and the fan-like spread of the delicate branches with their dimorphic leaves is well known (fig. 9, B) ; also the strange rhizophores springing from points of branching of the shoot, which turning downwards give rise to the true roots, being themselves organs of indeterminate morphological nature. It will be unnecessary to describe the vegetative organs or their anatomy in detail; the chief comparative interest lies in the propagative process The sporangia are borne on radially-constructed distal cones. A single sporangium similar to that of Lycopodium, is borne just above the insertion of each sporophyll, with the ligule protecting it from without. The microsporangia are brownish when ripe and the megasporangia pale in colour, and both may be borne on a single cone. They appear all alike up to the stage when the numerous spore-mother-cells are formed. If all the spore-mother cells undergo tetrad-division numerous microspores resembling those of Lycopodium result. But in a inegasporangiurn only one, or at most a few of them form tetrads, and the resulting spores are large with a rugged wall; the number matured in a single sporangium may vary from one to f our, or some multiple of four. On germination each microspore produces a small number of spermatOzoids from a very reduced prothallus ; but the large megaspore forms a more bulky prothallus, which, projecting from the disrupted wall, bears archegonia. One of these on fertiliza tion develops an embryo with a suspensor The essentials of the process are as in Lycopodium, though the details are different. As the sporeling develops its leafy shoot grows upwards, and its root downwards ; with the megaspore attached laterally the whole has the appearance of a seedling of some flowering plant (fig, 9, A). This is clearly an advance upon Lycopodium.

The other genus Isoetes is peculiar in habit and in habitat, yet shares many of the characteristics of Selaginella. It contains about 50 species of tufted herbs, mostly living at the bottom of fresh water lakes, though a few are amphibious or terrestrial. The plant consists of a short massive lobed stock, bearing crowded awl shaped leaves of considerable length. Each bears a ligule on its upper surface, and when fertile as any one of them may be, a large cake-like sporangium lies between this and the axis. Roots with dichotomous branching arise from furrows between its lobes. The sporangia are heterosporous, and propagation is essentially similar to that in Selaginella; but there is no suspensor, and the spermatozoids are multi-ciliate.

A chief interest in these Ligulate types lies in their comparison with the Lepidodendraceae and Sigillariaceae, for these are also ligulate and heterosporous (see PALAEOBOTANY). These fossils attained dendroid dimensions, and though the primary vascular system was not unlike that of the modern Lycopods, they often differed in having secondary growth with an active cambium. In

Isoetes there is a sluggish secondary growth in the short stock, which itself shows certain analogies with the Stigmarian trunks of the gigantic fossils. Since any of the leaves of Isoetes may be fertile the whole plant appears as a strobilus of the same nature as Lepidostrobus, seated upon a Stigmarian base. In fact Isoetes is like a telescoped, but still living, fossil.

VI. Filieales.—The Filicales may be held as comprising all the living Megaphyllous Pteridophytes, together with such fossils as show essentially similar characters. But the mere fact that their leaves are relatively large in proportion to the stem that bears them is not a sufficient diagnosis. Some Lycopods (lsoetes, Sigillaria) share this character, and megaphylly is possible in any of Pteridophytes. But as a matter of fact, excepting Isoetes, none such are now living other than the Filicales. The most distinctive feature of ferns, however, is that on the relatively large leaves many sporangia are borne, either singly or in groups (sori).

Ferns are represented at the present day by about 15o genera, and 6,000 species. Some are minute, others attain considerable size as tree ferns; but none can be reckoned among the largest of living plants, nor is there fossil evidence that ferns ever attained extreme dimensions. Their geographical spread is general ; some few are arctic, but ferns increase in numbers both of species and of individuals towards the Equator. Most are mesothermal hygrophytes, i.e., they flourish under moist conditions with a moderate temperature; and the majority are shade-loving. Hence their headquarters are in the mountains of the tropics, where they form a considerable part of the undergrowth below the forest canopy. But their habitat is variable; some specialized types are actually aquatic, while others are able to withstand conditions of moderate, some even of extreme drought. Ferns are much richer in genera, species and individuals than any other living Pteridophytes. They present the climax of successful development in homosporous vascular plants. They show also a high degree of variety both in their vegetative and their prop agative characters ; these provide good diagnostic features for their classification. They have a full and long palaeontological history that stretches back to Palaeozoic times. The geological record can therefore be used as a valid check upon the conclusions drawn from the comparison of living types.

It has been said that the Palaeozoic period was the age of ferns, and it is true that "fern-like" leaves were then common. But it has now been shown that many of these belonged to seed plants ranked as Pteridosperms, a class long since extinct, which also had fern-like leaves (see PALAEOBOTANY). It is not improb able that they represent a stock more distinct from ferns than the similarity of their foliage would suggest, for they had advanced early to seed-formation. It may be left as an open question whether or not both may have had far back in their evolution some common origin.