106 



FOUNDATIONS OF BIOLOGY 



kind of spore is formed. Among the higher Ferns, however, 

 spores of two kinds occur which, since they differ greatly in 

 size, are called MICROSPORES and MEGASPORES. The produc- 

 tion of two kinds of spores is 

 known as HETEROSPORY and 

 leads to the differentiation of 

 the sporophylls into MICRO- 



SPOROPHYLLS and MEGASPO- 



ROPHYLLS. Moreover, the 

 microspores on germination 

 form gametophytes which 

 produce sperm, and therefore 

 are called MALE GAMETO- 

 PHYTES, while the megaspores 

 develop into gametophytes 

 bearing eggs, and accordingly 

 are known as FEMALE GAME- 

 TOPHYTES. Finally, in these 

 heterosporous forms, the 

 gametophytes are no longer 

 even small independent 

 plants, such as the prothallus 

 of the common Ferns, but 

 both male and female gameto- 

 phytes are so greatly reduced 

 that they practically remain 

 permanently in the parent 

 microspore and megaspore, 

 respectively, which supply 

 them with food. This, it will 

 be noted, is just the reverse of the condition which exists in 

 the Moss, where it is the sporophyte which is the dependent 

 generation. (Figs. 56, 57.) 



FIG. 50. Stages in the life history of 

 a higher Fern (Marsilia). A, micro- 

 spore, enclosing the male gametophyte 

 with two groups of sperm mother cells, 

 and prothallial cells (p); B, sperm; 

 C, megaspore, enclosing food material 

 (starch grains), and female gametophyte 

 comprising a single archegonium (with 

 egg) at one end of the spore; D, a week- 

 old embryo sporophyte, still attached to 

 the megaspore, with first leaf (I) and 

 root (r). (From Bergen and Davis.) 



