SECT. I 



CRYPTOGAMS 



409 



In the structure of the sporangia and spores, and in the development 

 of the prothallia, the Hydropterideae differ in many respects from the 

 Filices. These differences may be best understood on reference to 

 Salviniit. natans as an example. The sporocarps contain either numerous 

 microsporangia or a smaller number of macrosporangia (Fig. 343, A, 

 ma, mi). In structure both forms of sporangia resemble the sporangia 

 of the leptosporangiate Ferns ; they are stalked, and have, when 





Fig. 343. — Salvinia natans. A, Three sporocarps in median, longitudinal section ; ma, macro- 

 sporocarp, mi, microsporocarp ( x S) ; B, a microsporangium ( x 55) ; C, portion of the con- 

 tents of a microsporangium, showing four microspores embedded in the frothy interstitial 

 substance ( x 250) ; D, a macrosporangium and macrospore in medium longitudinal section ( x 

 55) ; E, apex of a macrospore ; p, perinium ; c, exinium ; a, proteid grains ; n, nucleus ( x 240). 



mature, a thin wall of one cell-layer but no annulus (B, If). The 

 MICROSPORANGIA enclose a large number of microspores, which, as a 

 result of their development in tetrads from the mother-cells, are 

 disposed in groups of four (C), and embedded in a hardened frothy 

 mass filling the cavity of the sporangium. This frothy interstitial 

 substance is derived from the tapetal cells, which gradually lose their 

 individuality and wander in between the spore mother-cells. 



The microspores are not discharged, but while still enclosed within 



