PTERIDOPHYTA 149 
reduced to a few vegetative cells and a single anthe- 
ridium. In all of these homosporous types, however, 
there is nothing in the appearance of the spore to 
indicate whether the resulting gametophyte is to be 
male or female, and indeed this is sometimes, to a 
certain extent at least, a matter of nutrition. 
In each of the principal groups of Pteridophytes, how- 
ever, we find at least one genus which develops two 
very distinct forms of spores, i.e. is heterosporous. In 
all but the Equisetinez there are existing examples of 
heterosporous genera, but in the latter class the single 
living genus is homosporous, although some of its fossil 
relatives are known to have been heterosporous. 
Among the eusporangiate ferns it is an open question 
whether there are any undoubted cases of heterospory, 
although it is probable that the peculiar genus Isoétes 
(Fig. 89, A), where heterospory is very pronounced, is 
related, although remotely, to the homosporous Euspo- 
rangiate. It certainly seems to be nearer to the ferns 
than to the club-mosses with which it is usually asso- 
ciated. 
In Isoétes the sporangia (Fig. 39, B), which are very 
large, and borne singly at the bases of the closely crowded 
rush-like leaves, are alike in structure and external form, 
but there isan enormous difference in the size of the ma- 
crospores and microspores. The male plant produced 
from the microspore (Fig. 39, C) is exceedingly rudimen- 
tary, consisting of a single minute vegetative cell and an 
antheridium which produces but four spermatozoids, and 
is the most reduced known among the Pteridophytes, 
and approximates nearest the condition found in the 
flowering plants. The macrospore is very large and 
