THE SALVINIACE^E. 171 



urn-like indusiu of the filmy ferns should completely 

 enclose the sporangia. In Salvinia the sporocarps are 

 borne on the submerged leaves, usually in clusters of 

 three ; in Azolla they are borne in pairs in the axils of 

 the aerial leaves. Some of the sporocarps in each cluster 

 of Salrinia always bear megasporangia. There are about 

 ten in each sporocarp, and each megasporangium bears a 

 single large ovoid megaspore. The 

 microsporangia are more numerous, 

 globose, and contain a large number 

 of microspores. In Azolla the pairs 

 of sporocarps are usually of two 



Cross-section of Salvinia 



sizes, the smaller bearing a single sporocarps. (Enlarged.) 

 megaspore, and the larger producing several microspo- 

 rangia and numerous microspores. The mature sporo- 

 carps drop from the plant and promptly sink to the 

 bottom of the water, where they continue to enclose the 

 spores for some time. The latter are finally liberated by 

 the decay of the walls surrounding them. The micro- 

 spores of Azolla are usually held together by 

 a solid sort of protoplasm which produces 

 hooked processes from the outer surface. 

 The sporocarp appears to be an outgrowth 

 of the leaf that, beginning with a ring sur- 

 rounding the sporangia, slowly increases 

 until it has completely enclosed them. Before this has 

 happened, however, certain algre take up their abode in 

 the sporocarps, and when the latter mature they go into 

 a resting condition, forming little colourless spheres. 

 These spheres are seldom absent from the ripe sporo- 

 carps. 



The genus Salinuia was named for Salvini, a Floren- 

 tine professor. There are about a dozen species known 



