AGE CYCLE IN PLANTS AND LOWER ANIMALS 243 



of the conditions determining cell division and of the conditions 

 in different parts of the plant is very incomplete. As regards the 

 plant, we can only say that in certain regions progressive and 

 regressive changes balance each other more or less completely, 

 and consequently these regions remain undifferentiated and young 

 or undergo differentiation and senescence very slowly, while in 

 other regions progressive changes are more nearly or quite con- 

 tinuous. The fact that differentiated cells may become embryonic 

 or embryonic cells may differentiate when physiological conditions 

 change, shows that these differences in behavior depend, not upon 

 a fundamental difference in constitution of the cells, but upon the 

 conditions to which they are subjected in the regions of the plant. 

 The solution of the problem undoubtedly lies in the physiological 

 make-up of the plant individual as a whole and the character of 

 its metabolism. 



But even in the so-called embryonic tissue of the higher plants 

 the cells are not absolutely alike. Some degree of individuation 

 exists, for the activities of this tissue are orderly, and a relation 

 of dominance and subordination exists in it. Many facts indicate 

 the existence of an axial gradient in rate of metaboKsm, the region 

 of the vegetative tip possessing the highest rate and dominating 

 other parts. Since this is the case, the formation of new vegetative 

 tips, i.e., of buds — a characteristic feature of the vegetative Hfe 

 of most of the higher plants — must involve a change of some degree 

 and kind in the embryonic tissue. This change is probably pri- 

 marily an increase in rate of metabolism in the part concerned, but 

 such an increase in rate is essentially a rejuvenescence in some 

 degree. The changes involved in these vegetative reproductions 

 are undoubtedly shght in many cases, but nevertheless they 

 constitute a factor in the maintenance of the embryonic condition. 

 Each new bud formed from a part of a pre-existing plant individual 

 involves to some extent a reconstitutional process, even though it 

 may be merely an increase in rate and the estabhshment of a new 

 axial gradient. Vegetative reproduction is then another factor 

 concerned in retarding the progressive course of senescence and 

 differentiation in the plant tissues chiefly concerned. Observation 

 confirms this conclusion, for we find that the plants or the phytoids 



