EVOLUTION OF SEED-PLANTS 123 



spore-sac; in Azolla the spore-sac is enclosed in a 

 special coat of its own and closely simulates a 

 true seed or rather ovule. 



In the microspores, the prothallus is scarcely 

 developed; the spore has practically nothing 

 else to do than to form an antheridium and 

 emit the spermatozoids. On the female side, 

 provision has to be made for the nutrition of the 

 embryo, and here there is still a comparatively 

 bulky prothallus, though it tends to lose the 

 character of an independent plant, and to be- 

 come a mere bearer of archegonia and store- 

 house of food-materials; it never frees itself from 

 the megaspore in which it is formed (see fig. 18, 

 p. 164). There are some obvious advantages in 

 the differentiation of two kinds of spores. The 

 male spores are kept small for easy dispersal, and 

 can be provided in correspondingly large num- 

 bers; the prothallus, practically undeveloped in 

 the small spores, is treated on economical prin- 

 ciples and only produced where it is wanted, i. e. 

 in the megaspore, in connection with the egg- 

 cell from which the embryo is formed, 



Since the time of Hofmeister, who laid the 

 foundation of evolutionary botany, it has al- 

 ways been assumed that the condition with like 

 spores (homospory) preceded that with unlike 

 spores (heterospory). It is difficult to see how 

 the order could be reversed, though it must be 



