PTEROPSIDA 171 



represents an evolutionary offshoot from an ancient schizae- 

 aceous stock. Arguments for this are based on the leaf form, 

 the type of hairs, the form of the sorus and the vestigial 

 annulus round the apex of the sporangium in Pilularia,^^ 

 but the evidence is not very convincing and, in the absence 

 of early fossil representatives, the group must be regarded in 

 the meantime as an isolated one. 



Salviniales 



Salviniaceae Salvinia 

 Azollaceae Azolla 



Whereas most of the members of the Marsileales are rooted 

 in the soil, either in or near water, all the members of the 

 Salviniales are actually floating. Azolla has pendulous roots, 

 but Salvinia is completely without them. Like the Marsile- 

 ales, their sporangia are borne in sporocarps. However, the 

 morphological nature of the sporocarps is quite different, 

 for each sporocarp represents a single sorus whose indusium 

 forms the sporocarp wall. 



The only member of the group to be represented in the 

 British flora is Azolla filiculoides, described as recently 

 naturahzed from N. America. However, it was a native British 

 plant in Interglacial times.^^ It has an abundantly branching 

 rhizome, with a minute medullated protostele, and with 

 crowded overlapping leaves about i mm long (Fig. 27A). 

 These have two lobes, within the upper of which is a cavity 

 containing the blue-green alga Anabaena azollae. 



Sporocarps arise on the first leaf of a lateral branch and 

 are usually of two kinds— large ones containing many 

 microsporangia and small ones containing a single mega- 

 sporangium, although sporocarps with both types of spor- 

 angium are sometimes present. The early stages of develop- 

 ment are similar in both types of sporocarp, for there is an 

 elongated receptacle on which numerous sporangial initials 

 arise. However, during development, the microsporangial 



