PTERIDOPHYTA 151 



gonia in Isoetes are very much like those of the euspo- 

 rangiate ferns, and the spermatozoids are multiciliate 

 like those of the typical ferns, and it is largely for these 

 reasons that the writer is inclined to consider Isoetes as 

 related to the ferns rather than to the club-mosses. 



Among the Leptosporangiatse heterospory has devel- 

 oped quite independently in at least two places. The 

 two families, Marsiliacese and Salviniacese, usually asso- 

 ciated under the name of Hydropterides or water-ferns, 

 are obviously not closely related, and they show evidence 

 of having been derived independently from two widely 

 separated families of homosporous ferns. The Salvini- 

 acese show certain resemblances to the filmy ferns, while 

 the Marsiliacese are more like the Polypodiacese. Both 

 families agree in having the macrospores reduced to a 

 single one in each macrosporangium, through the 

 abortion not only of the other spore-tetrads, but also 

 of the three sister-spores of the macrospore. The latter 

 becomes very large, and its outer membranes much 

 modified (Fig. 39, E). 



In the Salviniacese, especially Salvinia, the female 

 gametophyte is much larger than in the Marsiliacese, 

 or indeed than in any other heterosporous Pteridophyte. 

 It has abundant chlorophyll and does not differ very 

 essentially from the green gametophyte of the homo- 

 sporous ferns. The male plant, too, is less reduced than 

 in other heterosporous forms. In the Marsiliacese, the 

 female gametophyte is reduced to little more than a 

 single archegonium (Fig. 39, E), and the male plant to 

 a single antheridium with one or two rudimentary 

 vegetative cells. 



In the genus Marsilia the development by the gameto- 



