PTERIDOPHYTA 



287 



the otherwise rootless Salvinia, but it is probable that such is the case, as in other 

 respects the embryos of Salvinia and Azolla are much alike. The first leaf 

 (cotyledon) is heart-shaped in Salvinia, funnel-form in Azolla, where it encloses 

 the stem-apex and the younger leaves. The Anabsena filaments may be found 

 about the apex of the young shoot, and as soon as the leaves develop the charac- 

 teristic cavities, the Anabaena takes possession. 



Family 2. Marsiliaceae 



The Marsiliaceae also contain two genera, which are evidently 

 related to each other, and differ less, so far as the sporophyte is con- 



FIG. 252. Marsilia vestita. A, fruiting sporophyte (natural size). B, sporocarp 

 (X4). C, cross-section of sporocarp (X5). D, germination of sporocarp, the 

 sori, s, attached to a gelatinous ring (X 3). 



cerned, from the ordinary Ferns than do the Salviniaceae. The two 

 genera, Marsilia and Pilularia, are usually amphibious in habit, grow- 

 ing in the water during their early stages, but, at least in our species, 

 ripening their spores after the water has subsided. The California 

 species grow where they become completely dried up in summer, and 

 in Marsilia vestita there are found buds, which probably survive the 

 dry season, and thus make the plant perennial. The slender creep- 

 ing stem, and the position and coiled vernation of the young leaves, 



