PTERIDOPIIYTES 151 



81. Heterospory. This phenomenon appears first among 

 Pteridophytes, but it is not characteristic of them, being en- 

 tirely absent from the true Ferns, which far outnumber all 

 other Pteridophytes. Its chief interest lies in the fact that 

 it is universal among the Spermatophytes, and that it rep- 

 resents the change which leads to the appearance of that 

 high group. It is impossible to understand the greatest 

 group of plants, therefore, without knowing something 

 about heterospory. As it begins in simple fashion among 

 Pteridophytes, and is probably the greatest contribution 

 they have made to the evolution of the plant kingdom, 

 unless it be the leafy sporophyte, it is best explained here. 



In the ordinary Ferns all the spores in the sporangia 

 are alike, and when they germinate each spore produces a 

 prothallium upon which both antheridia and archegonia 

 appear. It has been remarked, however, that some pro- 

 thallia are dioecious that is, some bear only antheridia 

 and others bear only archegonia. In this case it is evident 

 that the spores in the sporangium, although they may ap- 

 pear alike, produce diiferent kinds of prothallia, which 

 may be called male and female, as each is distinguished by 

 the sex organ which it produces. As archegonia are only 

 produced by well-nourished prothallia, it seems fair to sup- 

 pose that the larger spores will produce female prothallia, 

 and the smaller ones male prothallia. 



This condition of things seems to have developed finally 

 into a permanent and decided difference in the size of the 

 spores, some being quite small and others relatively large, 

 the small ones producing male gametophytes (prothallia 

 with antheridia), and the large ones female gametophytes 

 (prothallia with archegonia). When asexual spores differ 

 thus permanently in size, and give rise to gametophytes of 

 different sexes, we have the condition called heterospory 

 ("spores different"), and such plants are called heterospo- 

 rous (Fig. 139). In contrast with heterosporous plants, those 

 in which the asexual spores appear alike are called homos- 



