4i8 MOSSES AND FERNS CHAP. 



closer, and of the two genera Pilularia is evidently the nearer 

 to the latter. The development of both gametophyte and 

 sporophyte in the two corresponds very closely. 



The sporangia are borne in "sporocarps," which are mor- 

 phologically very different from those of the Salviniacese, be- 

 ing metamorphosed leaf segments enclosing several sori, and 

 not single sori enclosed simply in an indusium. The spores 

 germinate with extraordinary rapidity, especially in Marsilia, 

 and in M. ZEgyptiaca the writer has found a two-celled embryo 

 developed within thirteen hours from the time the ungermi- 

 nated spores were placed in water. 



The sporocarp of Marsilia is a bean-shaped body, which is 

 attached to the petiole of the leaf by a more or less prominent 

 pedicel. It is very hard, and unless opened artificially may 

 remain a long time unchanged, if placed in water ; but if a little 

 of the hard shell is cut away, the swelling of the interior muci- 

 laginous tissue quickly forces apart the two halves of the fruit. 

 As more water is absorbed, this gelatinous inner tissue con- 

 tinues to expand and forms a long worm-shaped body (Fig. 

 243, D), to which are attached a number of sori, each sur- 

 rounded by a sac-shaped indusium in which the sporangia are 

 closely packed. Macrosporangia and microsporangia occur in 

 the same sorus. The former contain a single large oval white 

 spore, the latter much more numerous small globular ones. 

 The indusium remains intact for several hours, if not injured, 

 but finally, with the sporangium wall, is completely dissolved, 

 and the spores are set free. 



The Microspores and Male Prothallium 



The microspores of M. vestita (Fig. 244) are globular cells 

 about .075 mm. in diameter. The outer wall is colourless and 

 sufficiently transparent to allow the contents to be dimly seen. 

 Lying close to the wall are numerous distinct starch granules, 

 and in the centre the nucleus is vaguely discernible. Sections 

 through the ungerminated spore show that the wall is thick, 

 with an inner cellulose endospore, outside of which are the 

 exospore and the epispore or perinium, composed of closely- 

 set prismatic rods. The central nucleus is large and distinct, 

 with usually one or two nucleoli. 



The first division takes place at ordinary temperatures, 



