INTRODUCTION TO CRYPTOGAMIC BOTANY, 553 



different. We can, indeed, point rather to analogies than 

 affinities. Where the limb of the leaf is expanded and firm, 

 there is a reticulate venation like that of Ophioglossu7)i. 

 Azolla resembles Jungermannio} in habit and the antheridia, 

 or perhaps more properly the receptacles of the antheridia, as 

 those of Salvinia also, have the same external form. In the 

 perfection of their spiral vessels, which are sometimes capable 

 of being unrolled, they resemble Equisetacece. Azolla is 

 extra European, and is found in hot countries and the tempe- 

 rate regions of the south, and also in North America. The 

 other genera all occur in Europe, but are not confined to it, 

 or in general to temperate regions. 



633. As the structure of these plants is so peculiar I must 

 give a few details of each genus separately. Pilularia (Fig. 

 119) has a filiform creeping rhizoma, which from space to space 

 gives off erect and, at first, circinate filiform leaves or foot- 

 stalks, whichever may be their proper name. From the same 

 thread-like rhizoma, short processes are given off, each of 

 which produces a pillshaped receptacle, embraced by the stalk, 

 and evidently formed either by the tip of the metamorphosed 

 footstalk, or from the limb of the leaf, which is not developed 

 in other cases. This receptacle (Fig. 120) is divided by septa 

 into two or four cells, each of which has a sort of placenta, to 

 which the obovate antheridia and sporangia are attached. 

 Each sporangium contains only a single spore. This has a 

 firm outer coat, which tapers to a point, leaving a little cavity 

 at the top of the nucleus. This cavity, according to Hofmeister, 

 is gradually filled up with cellular tissue, constituting a conical 

 prothallus confluent with the nucleus. A single archegonium is 

 formed in the centre, the orifice of which corresponds with the 

 apex of the prothallus. The antheridia contain a large 

 number of grains, from which the spermatozoids are ultimately 

 developed. The spermatozoids are long, spiral, and very deli- 

 cate. The embryo formed in the archegonium germinates after 

 the fashion of a monocotyledon, sending off" a frond in one direc- 

 tion, and a root opposite to it. Pilularia occurs in the north 

 of Africa and Tasmania, as well as in Europe. The species, 

 which are very few in numbei", are mere botanical curiosities. 



