8o VASCULAR CRYPTOGAMS 



leaf, which open only by a narrow slit. Although the sori usually 

 originate on the veins of a leaf, and only on the under side of the 

 lamina, this is not always the case. In Olfersia they spring from both 

 surfaces of the lamina by the side of the midrib ; and in this and other 

 species of Acrosticheae from the mesophyll as well as the veins. In the 

 Hymenophyllaceae they are enclosed in a cup-like indusium, and are 

 attached to the apex of the veins, which projects beyond the margin of 

 the leaf. The spot on the fertile vein which bears a sorus is known as 

 the placenta or receptacle. In the Polypodiaceae, and probably also in 

 the Cyatheaceae, each sporange originates from a single epidermal cell, 

 which swells up considerably, and is cut off from the rest of the leaf 

 by a septum. This mother-cell of the sporange subsequently divides 

 by another septum into a basal cell which develops into the pedicel, 

 and an apical cell which becomes the capsule. The pedicel usually 

 consists ultimately of three rows of cells produced by longitudinal and 

 transverse divisions. The nearly hemispherical mother-cell of the 

 capsule first divides, by four successive oblique walls, into four parietal 

 and a nearly cubical central cell, the archespore. In the parietal cells 

 further divisions follow at right angles to the surface ; while from the 

 archespore are formed four tabular segments parallel to the parietal 

 cells, which again divide by walls vertical to the surface into one or two 

 layers of tapetal or mantle-cells, constituting together the tapete. The 

 row of cells in the wall of the sporange which constitute the annulus are 

 the result of divisions at right angles to the surface of the sporange ; 

 their outer walls bulge out and project above the surface. The tapetal 

 cells ultimately disappear, and the whole of the space within the wall of 

 the capsule is occupied by a fluid, in which float the mother-cells of the 

 spores formed by successive bipartitions of the archespore, and nor- 

 mally sixteen in number in the Polypodiaceae and Schizaeacese. In these 

 families the formation of the spores takes place in the following way. 

 Each of the sixteen spore-mother-cells divides into four by two succes- 

 sive bipartitions of the protoplasmic contents, preceded by correspond- 

 ing divisions of the nuclei. Sixty-four spores are thus normally formed 

 in each sporange. They then invest themselves with a cell-wall, which 

 is usually double, consisting of an inner coat, or endospore, generally 

 but not always composed of cellulose, and sometimes itself consisting of 

 two layers, and an outer brown cuticularised exospore, provided with 

 ridges, papillae, warts, or other elevations. Leitgeb states that in 

 Osmunda and some other genera there is no true endospore ; while in 

 Onoclea (L.), according to Campbell, there are three coats. The spores 

 of the Osmundaceae and Hymenophyllaceae contain chlorophyll. 

 Another mode of spore-formation, more analogous to what takes place 



