414 MOSSES AND FERNS chap. 



The development of the sporangium follows closely that of 

 the other Leptosporangiatae up to the final development of the 

 spores. The tapetum is composed of but a single layer of cells 

 in Asolla, but in Salvinia it usually becomes double (Juranyi 

 (i)). In both genera the wall remains single-layered, and no 

 trace of an annulus can be detected. 



In the macrosporangium of Azolla the archesporium pro- 

 duces eight sporogenous cells, the microsporangium sixteen. 

 In Salvinia, according to Juranyi, both sporangia contain six- 

 teen spore mother cells. ^ Shortly after the divisions are com- 

 pleted in the central cell and tapetum the cell walls of the latter 

 are dissolved, but for a time the sporogenous cells remain to- 

 gether. Finally, they become isolated and round off before the 

 final division into the young spores takes place. In the macro- 

 sporangium only one spore finally develops. This is at first, 

 in A-:oIIa, a thin-walled oval cell lying free in the enlarged cavity 

 of the sporangium. Examination shows it to be surrounded by 

 a thick layer of densely granular nucleated protoplasm derived 

 from the tapetum. As the spore grows the surrounding proto- 

 plasm and the abortive spores are used by it as it develops, and 

 through their agency the curious episporic appendages of the 

 ripe spore are deposited upon the outside. The spore itself is 

 perfectly globular and surrounded by a firm yellowish exospore, 

 which in section is almost perfectly homogeneous. The epi- 

 spore covering this shows over most of the spore a series of 

 thick cylindrical papillae, from the top of which numerous fine 

 thread-like filaments extend. In section the epispore shows two 

 distinct parts, a central spongy-looking mass and an outer more 

 homogeneous part covering all but the tops of the papillae. At 

 the top of the spore are three episporic masses, composed entirely 

 of the spongy substance and surrounding a central conical mass 

 from whose summit extend numerous fine filaments like those 

 growing from the rest of the epispore. The name '^swimming 

 apparatus," which has been applied to this apical mass, is a mis- 

 nomer, as the ripe sporangium sinks promptly when freed from 

 the plant. 



The indusium rapidly grows above the young macrospo- 

 rangium, or group of miscrosporangia, and its walls, which be- 

 come double, converge at the top and finally the opening is com- 



' Heinricher (2), however, states tliat in the macrospangium there are 

 but eight, as in Azolla, 



