H 



BIOLOGICAL LECTURES. 



gametophyte. While the latter in the lower ferns is a com- 

 paratively large liverwort-like plant, sometimes living for sev- 

 eral years, we find in the higher forms that it becomes much 

 smaller and lives but a short time, and it is not uncommon to 

 find that separate male and female plants are produced ; but 

 there is no apparent difference in the spores from which they 

 arise. 



In the heterosporous Pteridophytes, however, some of the 

 sporangia produce a small number of spores, sometimes but 

 one. These " macrospores " are many times larger than the 

 much more numerous "microspores" developed in the remain- 

 ing sporangia. From the large spore is produced the female 

 gametophyte, from the small one the male. These are both 

 much reduced, and their whole development may take place 

 within twenty-four hours. The development of the sporo- 

 phyte from the fertilized ^%g is in such cases very rapid, and 

 it soon becomes independent, and all trace of the gametophyte 

 disappears. In most of these heterosporous forms the gameto- 

 phyte never becomes self-supporting, depending for its growth 

 upon the materials already accumulated in the ripe spore. Such 

 gametophytes develop no chlorophyll, and scarcely exceed in 

 bulk the spore from which they arise, and within which they 

 are contained, there often being only sufficient splitting of the 

 spore coat to expose the reproductive organs. 



In the club mosses of the genus Selaginella, heterospory is 

 carried still further. Before the spores are discharged from the 

 sporangium they have already undergone the first steps in ger- 

 mination, and by the time the spore is ripe the gametophyte 

 has begun to develop within it. In this case the growing 

 gametophyte does not depend entirely upon the food stored in 

 the spore, but draws its nourishment to some extent directly 

 from the sporophyte through the wall of the sporangium, which 

 thus serves not only to protect the spores but also, to some 

 extent, as a medium for the transfer of food to the develop- 

 ing gametophyte. In short, the relation of gametophyte and 

 sporophyte is the reverse of what obtains in the mosses, where 

 it is the sporophyte which is permanently dependent on the 

 gametophyte. 



