Page:EB1911 - Volume 22.djvu/622

Rh the ventral canal cell and one or more canal cells. When the archegonium has opened by the separation of the terminal cells of the neck, the disintegration of the canal cells leaves a tubular passage, at the base of which is the ovum (fig. 9, b). Down this canal the spermatozoid, which in the Ferns has been shown to be attracted by reason of its positive irritability to malic acid, passes and fuses with the ovum. After fertilization the latter surrounds itself with a cell-wall and develops into the sporophyte. The early segmentation of the embryo differs in the several groups, but usually the first leaf or leaves, the apex of the stem and the first root are differentiated early, while a special absorbent organ (the foot) maintains for some time the physiological connexion between the sporophyte and the prothallus. The sporophyte is always highly organized both as regards form and structure. Root, stem and leaf can be distinguished even in the simplest forms, and the plant is traversed by a well-developed vascular system. The reproductive organs of the sporophyte are the sporangia, within which the spores are produced; the sporangia are often borne on or in relation to leaves, which may be more or less distinct from the foliage leaves in form and structure (cf. fig. 2). The cells of the wall of the sporangium are usually so constructed as to determine the dehiscence of the sporangium and the liberation of its spores. The spores produced in each sporangium vary from very many to a single one in the case of some heterosporous forms. These latter bear spores of two kinds, microspores and megaspores, in separate sporangia. From the microspore an extremely reduced male prothallus and from the megaspore the female prothallus, develops (cf. fig. 1, e). The spores of the homosporous Vascular Cryptogams are usually of small size; the prothalli produced from them usually bear both antheridia and archegonia, though under special conditions an imperfect sexual differentiation may result. The complete life-history, with its regular alternation of gametophyte and sporophyte, is now known in all except a few rare genera of recent Pteridophyta, and will be described in connexion with the several groups. A cytological difference of great importance between the two generations can only be mentioned in passing. The nuclei of the cells of the sexual generation possess a definite number of chromosomes and this number is also characteristic of the sexual cells. On fertilization the number is doubled and all the cells of the spore-bearing generation have the double number. On the formation

of the spores a reduction to the number characteristic of the gametophyte takes place.

The systematic arrangement of the Vascular Cryptogarns for the purposes of identification and description necessarily

remains unchanged, while the comparative morphology is being more fully worked out. But modifications in the order of placing the natural groups are of importance in expressing the results of such investigations. Such a scheme may be laced here in a tabular form before entering on the consideration of the life-history, natural history, morphology, and classification of the several groups:—

These main subdivisions are of unequal size and importance. The Sphenophyllales are only known in a fossil state, while the Equisetales, Lycopodiales and Filicales include both living and extinct representatives. The small groups of recent plants forming the Psilotales and Ophioglossales are given independence in this scheme of classification owing to their exact affinities with the other phyla being at present doubtful.

I. .—The plants of the single living genus Equisetum, which vary in height from a few inches to 40 ft., have subterranean rhizomes, from which the erect shoots arise. The habit of the plant depends on the degree of branching rather than upon the foliage. The internodes are elongated and hollow. The leaves are borne in whorls, those of each whorl cohering, except at their extreme tips, to form a sheath. The leaves of successive whorls alternate with one another, and this applies also to the branches which arise in the axil of the leaf sheath. In most species many of these buds, which alternate with the leaves, remain dormant, but in others the aerial shoots are copiously and repeatedly branched. In some species branches of the rhizome with tuberous internodes are formed, which serve as a means of vegetative reproduction. The roots which arise from the base of the lateral buds remain undeveloped on the aerial stem. The vascular bundles equal in number the leaf-teeth from which they enter the stem and form a single ring. Each bundle runs downwards through one internode and then divides into two branches which insert themselves on the alternating bundles entering at this node. The young stems, and the older stems of certain species, are clearly monostelic; but in other species an inner and outer endodermis may be present, or an endodermal layer surrounds each bundle. The vascular bundles themselves are collateral, the xylem consisting of the protoxylem, towards the centre of the stem, and two groups of xylem, between which the phloem is situated; the protoxylem elements soon break down, giving rise to the carinal canal. Only the median or carinal strand of xylem is common to stem and leaf; the lateral cauline strands possibly represent the remains of a centripetally developed mass of primary xylem. There is no secondary thickening except at the node in E. maximum, where some short tracheides, arranged in radial rows, arise from a cambium. The stems, the surface of which exhibits a number of ridges with intervening furrows, perform the greater part of the work of assimilation. The chlorophyll-containing tissue reaches the surface at the sides and base of the furrows,