IX FILICINEM LEPTOSPORANGIAT^ 307 



theacese, e. g., Alsophila, Cyathea, Cibotium. The leaves are 

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

 or in the delicate Hymenophyllace?e 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 Salviniacece 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 Eusporangiat^e. 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 liner 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 Marattiacese. They are always 

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

 pyramidal apical cell, wdiose growth is stopped by the formation 

 of a periclinal wall (Fig. 190). The central tetrahedral cell 

 has first a layer of tapetal cells cut ofT from it, and the inner 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. 



Non-Sexual Reproduction 



In a few of the Ferns special non-sexual reproductive 

 bodies, buds of different kinds, occur upon the prothallium, 

 which thus may have an unlimited growth. Such buds may 

 have the form of ordinary branches, or they are of a special 

 form. Buds of the latter class occur, sometimes in great num- 

 bers, in certain Hymenophyllaceae, where they are formed upon 

 the margin of the prothallium, to which they are attached by 

 short unicellular pedicels from which they readily become de- 



