32 2 SENESCENCE AND REJUVENESCENCE 



The whole process of development of the gametes in the plants 

 bears all the marks of a highly speciahzed process, far removed from 

 anything which occurs in unspecialized embryonic cells, and no- 

 where do we find a separation of the gametic from the somatic 

 material before the later or final stages of the developmental 

 process. 



The occurrence among mosses, ferns, and seed plants of what is 

 known as apogamy, i.e., the formation of a sporophyte without 

 fertilization from a vegetative cell of the gametophyte instead of 

 from the egg,' is of interest in this connection. In apogamous 

 ferns the embryo apparently may rise from any vegetative cell of 

 the prothallium, which is the gametophyte, and in seed plants it 

 may arise either from the synergids or the antipodals of the embryo 

 sac, or from both. In some cases also among angiosperms sporo- 

 phytes may arise from cells of the nucellus or of the integument 

 adjacent to the embryo sac. These cells are not even parts of the 

 gametophyte, but belong to the sporophyte generation, yet in the 

 region of the embryo sac they may produce embryos and sporo- 

 phytes as does the egg. In such cases the gametophyte generation 

 is omitted from the life history. 



All of these cases of non-sexual development from vegetative or 

 "somatic" cells of the sporophyte — the generation which usually 

 develops from the fertilized egg — vindicate that the capacities of the 

 egg are not fundamentally different from those of other ceUs of the 

 gametophyte and of some cells of the sporophyte. It is of course 

 easy to assume with the Weismannians that, in spite of their 

 visible differentiations, all such cells contain an undifferentiated 

 germ plasm, but, so far as scientific analysis is concerned, this 

 assumption is equivalent to begging the whole question. A simpler 

 view and one much more nearly in accord with the facts of observa- 

 tion and experiment is that which is held by most botanists, viz., 

 that many, or in some plants all, specialized or differentiated cells 

 may under proper conditions lose their specialization and become 

 embryonic and so give rise to new individuals.'' 



' See Winkler, '08, for a general survey and bibliography of the subject. 

 ' In the usual course of development all the cells of the gametophyte have the 

 reduced or haploid number of chromosomes like the animal egg after maturation, 



