VEGETAJ^IVE REPRODUCTION. 



may be called, properly enough, a flower.* The wall of the 

 sporangium when young is formed by three layers of cells, 

 but consists at maturity of one layer only, which, having its 

 cell-walls thickened in an irregular manner (fig. 238), tears 

 open the sporangium, usually along a vertical line. The wall 

 of the spore consists of three layers, the outer one splitting 

 into narrow strips and remaining lightly attached to the spore 

 at one point (fig. 239). To these parts of the cell-wall the 

 name elaters has also been given. ( Compare ^f 321.) Their 



FIG. 234. Vertical section through the leaflet shown in fig. 233, passing through the cen- 

 ter of a sorus. e, ventral epidermis; e', dorsal epidermis; between them the meso- 

 phyll, showing 3 veins cut across ; over the central one is a cushion of tissue from 

 whose surface arise the stalked sporangia s, s. i, z, the indusium. Magnified about 30 

 diam. After Sachs. 



purpose seems to be to entangle the spores so that they may 

 not be too sparingly distributed. 



326. Club-mosses. --In club-mosses the sporangia are sac- 

 like outgrowths upon the upper surface of the leaf near its 

 base, or occasionally of the axis itself just above the leaf. 

 Sometimes the leaves bearing them are the ordinary foliage 

 leaves ; in other species they are specialized and crowded 

 into a terminal cluster or spike (fig. 240). 



327. Differentiation of spores. Among the higher fern- 

 worts the spores are of two sizes : large ones, known as mega- 



* This term is not generally applied to these sporophylls. But see defi- 

 nition of a flower, ][ 329, and compare fig. 237 of a " flower" of Zatnia. 



