242 SENESCENCE AND REJUVENESCENCE 



embryonic character are more or less definitely localized as vege- 

 tative tips and certain other regions, but in these forms vegetative 

 reproduction often occurs from other more highly differentiated 

 parts of the plant as well as from these regions. In the seed plants, 

 however, these embryonic or meristematic tissues, as they are 

 called, are still more definitely locahzed, and in the highest forms 

 other regions of the plant body usually take but little part in 

 vegetative reproduction, at least as long as the meristematic 

 tissues are present and active. 



The continued existence of this embryonic tissue in plants at 

 the same time that differentiation and senescence are occurring in 

 other parts raises at once the question why different parts of the 

 plant behave differently in these respects. While it is impossible 

 to give a complete answer to this question, certain facts indicate 

 very clearly the direction in which an answer is to be sought. 



In the first place, cell division in the plant occurs chiefly in the 

 embryonic cells and the earlier generations of their descendants. 

 The susceptibihty experiments on the infusoria recorded in Part II 

 (pp. 141-42) indicate that in those forms cell division is accompa- 

 nied by some degree of rejuvenescence and in all cases cell division 

 is a reproductive process, and as such involves more or less rejuve- 

 nescence. Cells which divide rapidly do not undergo any great 

 degree of differentiation, and cells which resume division after 

 udergoing differentiation first undergo a greater or less degree of 

 dedifferentiation. In short, continued nuclear and cell division 

 is undoubtedly an important factor in maintaining cells in the 

 embryonic condition and continued metabolism in the presence of 

 nutrition and without cell division is a factor in senescence. But 

 cell division alone is by no means always adequate to maintain cells 

 in the embryonic condition. In embryonic development many 

 cells apparently grow old in spite of division, and sooner or later 

 division becomes impossible or possible only under altered condi- 

 tions. 



If frequent cell division is a factor in maintaining certain plant 

 tissues in the embryonic condition, we must inquire why cell 

 division is more frequent in certain regions than in others. This 

 question we are unable to answer at present, since our knowledge 



