230 



PLANT LIFE. 



irregularly thickened (see fig. 401). The strains caused by 

 the unequal absorption and loss of water burst the sporangium 



at some definite point. This 

 line of dehiscence is often 

 between a pair of large sad- 

 dle-shaped cells (fig. 401). 



324. Sporophylls. In 

 many of the ferns the leaves 

 which produce sporangia are 

 not different from the foliage 



FIG. 232. 



FIG. 233. 



FIG. 232. Diagram of a longitudinal and transverse section of the very young capsule 

 of a true moss (Bryiuii). The transverse section is taken along the line A B. a, the 

 mother cells of the spores ; c, the columella ; is, intercellular space. The constriction 

 at the top marks the limit of the lid. The part below the sporangium is the neck, with 

 nutritive tissues. Original. 



FIG. 233. A leaflet of a fern (Aspidiu-m) seen from the back. Eight sori are shown, 

 each covered by its own indusium, z. Magnified 2 diam. After Sachs. 



leaves. In others, certain leaves are so specialized for 

 bearing the sporangia that they lose their nutritive function 

 in part or entirely. To such specialized leaves the name 

 sporophyll is applied. 



325. Horsetails. In the horsetails the sporangia have the 

 form of sacs, varying in number from six to twelve. They 

 arise upon the lower face of a shield-shaped sporophyll (figs. 

 235, 236). These sporophylls are aggregated in a close 

 cluster at the upper end of the axis, constituting what 



