PTERIDOPHYTA 151 
gonia in Isoétes 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 Isoétes as 
related to the ferns rather than to the club-mosses. 
Among the Leptosporangiate heterospory has devel- 
oped quite independently in at least two places. The 
two families, Marsiliaceze and Salviniaceew, 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- 
acez show certain resemblances to the filmy ferns, while 
the Marsiliacee are more like the Polypodiacee. 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 Salviniaces, especially Salvinia, the female 
gametophyte is much larger than in the Marsiliacee, 
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 Marsiliacez, 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- 
