304 MOSSES AND FERNS chap. 



of the Osmundaceae, is also found at the apex of the later roots 

 and always in the stem. In size the sporophyte varies ex- 

 tremely. In some of the smaller Hymenophyllaceae the 

 creeping stem is not thicker than a common thread, and the 

 fully -developed leaves scarcely a centimetre in length. The 

 other extreme is offered by the giant tree-ferns belonging to 

 the Cyatheaceae, e.g. Alsophila, CyatJiea, Cibotiuni. The leaves 

 are in most cases compound, and either firm and leathery in 

 texture, or in the delicate Hymenophyllaceae have the lamina 

 reduced to a single layer of cells, so that in texture it recalls a 

 moss leaf. With the single exception of the Salviniaceae the 

 leaves are always circinate in the bud. The surface of the 

 stem and leaves is frequently provided with various epidermal 

 outgrowths, scales, and hairs, which show a strong contrast to 

 the mostly glabrous Eusporangiatae. The vascular bundles 

 are, both in the stem and petioles, of the concentric type with 

 a very distinct endodermis, and in the older parts of both 

 stems and leaves parts of the ground tissue are often changed 

 into thick-walled and dark-coloured sclerenchyma. In the finer 

 veins of the leaf the vascular bundles are reduced in structure 

 and more or less perfectly collateral. 



The sporangia are extremely uniform in structure through- 

 out the group. They can be traced back to a single epidermal 

 cell, in most cases developed from the lower side of the un- 

 modified sporophylls, as in the Marattiaceae. They are always 

 more or less distinctly stalked, and grow for a time from a 

 pyramidal apical cell, whose growth is stopped by the formation 

 of a periclinal wall (Fig. 167). The central tetrahedral cell 

 has first a layer of tapetal cells cut off from it, and the central 

 cell then forms the archesporium. No sterile cells are formed 

 in the archesporium, but all the cells (except in the macro- 

 sporangium of the Hydropterides) develop perfect spores. The 

 ripe sporangium is provided, except in the Hydropterides, with 

 an annulus or ring of thickened cells, which assists in its 

 dehiscence, and forms the most characteristic structure of the 

 ripe sporangium. 



N on- Sexual Reproduction 



In a few of the Ferns special non-sexual reproductive bodies, 

 buds of different kinds, occur upon the prothallium, which thus 



