232 SENESCENCE AND REJUVENESCENCE 



many cases are known where conditions which decrease metabolism 

 bring about budding or fission. A comparison of these two methods 

 of physiological isolation makes it evident that the same result, 

 viz., the physiological isolation of parts and their development into 

 new individuals, may be attained by subjecting the organism to 

 conditions which act in very different ways, producing in the one 

 case an increase in rate of metabolism, growth, and increase in size, 

 in the other a decrease in rate of metabolism (Child, '10) . It is pos- 

 sible that both of these factors are concerned in many cases of bud- 

 ding and fission, that is, if an organism has attained a size at which 

 some part is approaching physiological isolation, a slight physiologi- 

 cal depression may bring about a sufficient isolation to initiate 

 dedifferentiation and reproduction. 



Thirdly, physiological isolation of a part may conceivably result 

 from a decrease in the conductivity of the path over which the 

 correlative factors from the dominant region are transmitted. 

 In many organisms the conductivity of the paths apparently 

 increases as the morphological differentiation of conducting struc- 

 tures proceeds during development, so that in spite of a decrease in 

 rate of general metabolism the general physiological limits of the indi- 

 vidual are extended and physiological isolation of parts is delayed 

 or prevented. In many of the flowering plants, for example, new 

 growing tips arise and pass through the early stages of their devel- 

 opment at very short distances from each other and from the axial 

 growing tip (Fig. 101), but in later stages, when the conducting 

 structures are fully developed, the dominance of the growing tip 

 extends over a much greater distance. In the flatworms likewise 

 the length which the individual attains before formation of a new 

 zooid at the posterior end increases up to a certain point with 

 advancing development (Child, 'nc), while any considerable 

 changes in conductivity in the opposite direction may bring about 

 reproduction in many cases. 



And, finally, it is possible that physiological isolation of a part 

 may result from the direct action of external factors upon it, 

 increasing its rate of metabolism, or otherwise altering it, so that it 

 is less receptive, or no longer subordinate to the correlative factors, 

 and so becomes independent in spite of them. In various plants 



