PTERIDOPHYTA 



285 



appendages, Glochidia (Fig. 250). The gloclridia attach the massulse to the 

 episporic outgrowths of the macrospore, and thus facilitate fertilization, as the 

 germinating microspores are thus kept near the macrospore. 



The wall of the indusium (sporocarp) is composed of two layers of cells, 

 which in Salvinia are partly separated by air-spaces. The absence of an annulus 

 in the sporangia is to be explained by the aquatic nature of these plants. 



In Azolla the infection of the plant by the Anabsena always asso- 

 ciated with it, occurs while the macrosporangium, is developing. 

 The Anabaena filaments enter the young sporocarp and remain dor- 



FIG. 250. Azolla filiculoides. A, massula with enclosed microspores, sp ; gl, glo- 

 chidia (X250). B-D, development of male gametophyte (X 560) ; o, opercular 

 cell. E, two cross-sections of an antheridium (X 750). 



mant until the germination of the macrospore begins ; and by the 

 time the young sporophyte emerges from the gametophyte, the 

 Anabaena is in condition to infect it. 



The Gametophyte 



The ripe sporocarps, with the enclosed sporangia, fall away from the sporo- 

 phyte, and after a period of rest germinate. The spores are set free by the 

 decay of the wall of the sporangium, and in Azolla the massulse separate and 

 soon attach themselves to the base of the macrospores. From each microspore 

 a rudimentary prothallium is developed, consisting of a large basal cell, from 

 which a smaller rhizoidal cell is later cut off, and a terminal cell, from which the 



