CONDITIONS OF GAMETE FORMATION 367 



occurs by the isolation of leaves, branches, specialized gemmae, 

 etc., and in many cases from single cells of various regions of the 

 gametophyte. In many such species sex organs and gametes appear 

 only occasionally, or very rarely, and vegetative propagation of the 

 gametophyte may continue indefinitely. 



If the viewpoint developed in preceding chapters has any 

 foundation in fact, we must believe that in every one of these 

 vegetative reproductions a new individuation and some degree of 

 reconstitutional change in the cells involved occur. And again, if 

 this is the case, the new individuals resulting from reproduction 

 are, at least to a slight degree, younger physiologically than the 

 individual of which they originally formed a part. The result of 

 continued vegetative reproduction, whether it is induced by external 

 factors or determined by a low degree of individuation in the 

 species, is then to prevent the gametophyte generation from attain- 

 ing physiological maturity; consequently the specializations and 

 morphological differentiations characteristic of maturity, viz., the 

 development of sex organs and gametes, do not take place, or take 

 place only rarely when individuals in consequence of external or 

 internal conditions happen to reach maturity. 



Various botanists have suggested that in such cases the vege- 

 tative reproduction is the consequence of the failure to produce 

 sex organs and gametes, but the facts point to the opposite con- 

 clusion that the continued vegetative reproduction with the 

 accompanying reconstitution simply prevents the individual from 

 attaining maturity. Whether in any case the capacity for gamete 

 formation has been lost or is disappearing, can be determined only 

 after the most extensive and intensive research. But the low degree 

 of individuation accounts without difficulty for the preponderance 

 of vegetative reproduction, and there is no reason for believing that 

 a loss in the capacity for gamete formation has occurred. Failure to 

 produce gametes in such cases probably means only that the indi- 

 vidual never attains the physiological condition of which that par- 

 ticular process is a feature. 



The occurrence of apogamy in the ferns 1 indicates, as already 

 pointed out (p. 322), that there is no segregation of germ plasm 



1 Farlow, '74; de Bary, '78; Heim, '96; Farmer and Digby, '07; Woronin, '08; 

 Winkler, '08. 



