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 Equisetinese 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 Isoetes 

 (Fig. 39, A), where heterospory is very pronounced, is 

 related, although remotely, to the homosporous Euspo- 

 rangiatae. It certainly seems to be nearer to the ferns 

 than to the club-mosses with which it is usually asso- 

 ciated. 



In Isoetes 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 is an 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 



