ALTERNATION OF GENERATIONS 513 



We get in some of the former a succession of gametophytes, 

 which reproduce themselves many times in succession by gonidia 

 and do not develop sexual cells at all. After a number of genera- 

 tions have been thus produced asexually, a form arises which 

 bears gametes, and these after fusion produce again a gameto- 

 phyte which only develops gonidia. The gametophyte which 

 does not develop its gametes is called a potential one, in contra- 

 distinction to one which does, and which is known as an 

 aetual gametophyte. The alternation of actual and potential 

 gametophytes, where the resulting plant is in all cases similar 

 to its parent, is called homologous alternation in contradistinc- 

 tion to antithetical, explained above. In such an alternation 

 there is usually an absence of regularity, many potential game- 

 tophytes being developed in succession before an actual one 

 anses. 



The latter fact appears to indicate that in most plants con- 

 tinuous reproduction by asexual cells is attended with some 

 deterioration of the plant's constitution, which becomes again 

 invigorated by the occurrence of the sexual mode. 



The phenomenon of heterospory involves the production of 

 two gametophytes to one sporophyte, as each of the former pro- 

 duces its appropriate prothallus. Such plants show in their life 

 cycle, therefore, three forms, one sporophyte, and two gameto- 

 phytes, the latter occurring synchronously. 



It is noticeable, further, that as we pass through the several 

 divisions of the vegetable kingdom the predominant form of the 

 plant changes. In the Thallophyta it is always the gametophyte ; 

 the sporophyte is not universal there, and is never more than a 

 small structure, which nearly always remains attached to the 

 gametophyte. In the Bryophyta the two phases are more nearly 

 alike in degree of development ; the gametophyte is always the 

 vegetative body, whUe the sporophyte often shows the greater 

 histological differentiation. It is always parasitic upon the 

 gametophyte, and never attains a higher degree of morphological 

 value than a thaUus. In the Pteridophyta the sporophyte is the 

 predominant form, and takes on the vegetative functions. The 

 gametophyte shows contiauous retrogression, and in the highest 

 members of the group is almost entirely enclosed in the spore. 

 In the Phanerogams the reduction in size is stiU more marked ; 

 in the Angiosperms the female gametophyte consists of only a few 

 cells developed in the interior of the spore, while the male one is 

 a tubular outgrowth from the pollen grain. 



The gametophyte was doubtless the primitive form of the 



LL 2 



