396 MOSSES AND FERNS chap. 



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 sixteen 

 spore mother cells. ^ Shortly after the divisions are completed 

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

 dissolved, but for a time the sporogenous cells remain together. 

 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 Azolla^ 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 epispore 

 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 sumniit extend numerous fine filaments like those 

 growing from the rest of the epispore. The name " swimming 

 apparatus," vs'hich has been applied to this apical mass, is a 

 misnomer, as the ripe sporangium sinks promptly when freed 

 from the plant. 



The indusium rapidly grows above the young macrospor- 

 angium, or group of microsporangia, and its walls, which become 

 double, converge at the top and finally become completely 

 closed. In the former, before this happens, filaments of Anabcena 

 creep in and enter the resting condition. Thus they remain 

 until growth is resumed with the germination of the spore, 

 when the embryo is infected. The upper cells of the indusium 

 become very dark-coloured and hard, and remain after the lower 



^ Heinricher (2), however, states that there are but eight, as in Azolla. 



