20 VASCULAR CRYPTOGAMS 



capsule borne on a stalk, of small size and simple structure. Its morpho- 

 logical value varies greatly, and will be referred to more particularly 

 under the separate families. In the majority of Filices the sporanges are 

 trichomic structures, and are collected into groups or sori, which are 

 always located in connection with a ' vascular ' bundle on the under side 

 or margin of the leaf. In the Marattiacese they spring from a hypo- 

 dermal mass of tissue. In the Ophioglossaceae a segment of a leaf is 

 transformed into sporanges. In Selaginella and Lycopodium they 

 arise from the growing point of the stem above the axil of a leaf In 

 Psilotum they are sunk in the extremity of branchlets of a peculiar form. 

 In the Salviniaceae they are enclosed in receptacles or sporocarps, which 

 are themselves modifications of divisions of the leaf. The mode of 

 formation of the spores closely corresponds to that of the pollen-grains in 

 Flowering Plants. The spore-forming tissue can always be traced back to 

 a single cell or a row or layer of cells, the archespore, which may be dis- 

 tinguished at a very early period from the remaining cellular tissue by 

 the nature of its contents. From this proceeds the sporogenous tissue, 

 which afterwards becomes the mother-cells of the spores by perfectly 

 regular divisions, the details of which differ in the different families. 

 This is surrounded by one or more layers of cells, the tapetal cells or 

 tapete, and the whole is enclosed in the wall of the sporange, itself 

 composed of one or more layers of cells. In the heterosporous 

 families the distinction between megaspores and microspores is manifested 

 only at a comparatively late period in their development. In the iso- 

 sporous families the spores are always strictly unicellular, very commonly 

 elliptical or reniform in shape. The coat always consists of two separ- 

 able layers — an outer cuticularised exospore, often elevated into warts or 

 other prominences ; and an inner endospore, composed of cellulose, which 

 bursts through the exospore on germination, producing the germ-^lament, 

 which develops by cell-division into the prothallium. In the mega- 

 spores of the heterosporous families these are further protected on the 

 outside by a third separable, greatly hardened layer, the epispore. The 

 mode in which the spores escape from the sporange differs in the dif- 

 ferent families. 



A purely vegetative mode of propagation by means of gemmse or 

 bulbils borne on the sporophyte occurs especially in Filices and 

 Equisetacese ; on the oophyte vegetative propagation is less common. 



The classification of Vascular Cryptogams is attended with consider- 

 able difficulty. None of the systems as yet produced have much claim 

 to be regarded as natural ; and, until some doubtful points are cleared 

 up connected with fossil forms which may be links between existing 

 families, the primary distinction into Heterosporous and Isosporous 



