352 HETEROSPORY 



4. A tendency was developed to reduce the number of mega- 

 spores by sacrificing many of the cells which might be fertile so 

 that relatively few megaspores are formed, but these are very 

 large and richly supplied with food material, as illustrated by 

 Selaginella and the seed plants. This principle is clearly similar 

 to that by which plants have found it advantageous to produce 

 a limited number of large eggs well stocked with food, even at 

 the sacrifice of cells which may have been originally gametes, 

 such as the canal cells in the archegonium. 



5. The gametophytes degenerated, as self-supporting green 

 plants, to a condition in which they lost their chlorophyll and 

 became dependent upon food stored in the megaspores and mi- 

 crospores and even live somewhat parasitically upon the sporo- 

 phytes, as is illustrated in the early stages in the development of 

 the female gametophyte of Selaginella and in the gametophytes 

 (pollen tube and embryo sac) of the seed plants. 



There is another important advance in plant evolution which 

 is closely related to heterospory, but may be treated to better 

 advantage in the account of the origin of the seed habit 

 (Sec. 367). This advance arose in the seed plants when the 

 megaspore became retained within the megasporangium (a por- 

 tion of the ovule), so that the female gametophyte (embryo sac) 

 developed like a parasite upon the parent sporophyte, and the 

 male gametophyte (pollen tube) was required to grow down to the 

 female gametophyte somewhat parasitically through the tissues 

 of the ovule to bring about the fertilization of the egg cell. 



339. Sexual characteristics given to the megaspore and mi- 

 crospore by means of heterospory. The megaspore and micro- 

 spore are of course asexual spores because they are formed by 

 an asexual plant, the sporophyte. They are simply specialized 

 forms of the similar spores present in the liverworts, mosses, 

 the common ferns, horsetails, and lycopods, as shown by their 

 similar origin in tetrads at the end of the sporophyte generation. 



But when the microspore and megaspore became clearly dif- 

 ferentiated through heterospory from the earlier conditions of 



