240 SENESCENCE AND REJUVENESCENCE 



earlier stages of development, when they are physiologically 

 young, than in later stages. Under given external conditions 

 the rate of oxidation in buds, for example, is higher than in 

 fully developed stems and leaves, and in germinating seeds it is 

 higher than in later stages of development. Evidently in plants, 

 as in animals, a decrease in rate of oxidation, a real metabolic 

 senescence, occurs and is accompanied by a decreasing rate of 

 growth and by progressive differentiation to a greater or less ex- 

 tent.' The case of the flower, which shows a very high rate of 

 respiration is considered in chap. xiv. 



The metabolic changes of age proceed much more rapidly in 

 some parts of the plant than in others. The leaf and the stem 

 undergo differentiation and grow old, at least in large part, while 

 the growing tip and other meristematic tissues seem to remain 

 young indefinitely or to undergo senescence relatively slowly. 



There can be no doubt that the behavior of the plant and its 

 parts in relation to senescence depends upon the relation between 

 individuation and reproduction. In general, the higher the degree 

 of individuation, or of physiological integration, the more definite 

 and continuous the process of senescence, because reproduction 

 is less frequent. In Part II it was shown for various animal species 

 that the reconstitution of a new individual from a part of a pre- 

 existing individual is associated with some degree of rejuvenescence. 

 In the case of the plant similar changes undoubtedly occur when the 

 part concerned in the reconstitution is a differentiated part, as it 

 often is, but the cells chiefly concerned in reproduction in the higher 

 plants are commonly regarded as undifferentiated or embryonic, 

 i.e., as physiologically young. In general the degree of rejuvenes- 

 cence associated with the reconstitution of a part into a new whole 

 depends upon the degree of individuation. In certain of the algae 

 and fungi the degree of individuation is so slight it is diflicult to 

 determine whether the plant is anything more than a cell or an 

 aggregate of cells. In such plants as these the formation of a new 

 individual from any part of the old doubtless involves Httle 

 change beyond nuclear or cell division, and therefore but httle 



'For references to literature concerning respiration in plants see Pfeffer, '97, 

 pp. 523-31; Nicolas, '09. See also Nicolas, '10. 



