186 The Phenomena of Morphogenesis 



advantage of this differentiation into sexual and nonsexual plants may 

 lie in the possibility of the extensive multiplication of the products of a 

 single sexual union. Among flowering plants, where mechanisms for 

 effecting fertilization are more efficient than in many lower ones, the 

 differentiation into two generations has almost disappeared. 



A gametophyte, coming from a spore produced by meiosis, typically 

 has the haploid number of chromosomes, and the sporophyte has the 

 diploid number. Many haploid plants are now known, however, which 

 are undoubtedly sporophytes, and diploid gametophytes may readily 

 be produced. Chromosome number is evidently not the cause of the 

 difference between the two generations, but it is difficult to see why a 

 haploid spore and a haploid egg (or diploids in each case) should pro- 



Fig. 8-1. Bryonia alba. Left, leaf from a shoot bearing male flowers; right, one from 

 a shoot bearing female flowers. ( From Umrath. ) 



duce two structures as unlike as the prothallus and the sporophyte of 

 a fern. The difference is probably attributable to the very different en- 

 vironments in which these two cells develop. 



Origin of Differences. In most cases the origin of an organ or part is 

 first evident as a group of meristematic cells which, by growing more 

 rapidly in certain dimensions than in others, produces a definite form. 

 For an analysis of such specific differentiation, however, it is necessary 

 to determine how such a developing organ originates and the successive 

 steps by which it becomes distinct from others. Sometimes this is 

 relatively easy. In leptosporangiate ferns, for example, the sporangium 

 can be shown to arise from a single cell of the epidermis. In eusporangiate 

 forms, like some of the ferns and all higher plants, the sporangium 



