220 



SENESCENCE AND REJUVENESCENCE 



and growth beyond this size results in the formation of a new indi- 

 vidual or individuals from some part of the old, that is, in some form 

 of reproduction. The repetitive development in 

 series of parts, such as node and internode, in 

 the stem of the plant, of segments in segmented 

 animals, and many other cases, are examples of 

 similar relations between parts. The organic 

 individual in fact exhibits a more or less definite 

 sequence of events in space as well as in time, 

 and it is impossible to doubt that a physiological 

 spatial factor of some sort is concerned. This 

 problem has been considered at some length in 

 an earher paper (Child, 'iia), and only brief 

 mention of some of the important points is 

 possible here. 



In the simpler cases of reproduction the 

 spatial factor in dominance is clearly evident in 

 the position of the part concerned in reproduc- 

 tion with respect to the original dominant region. 

 In Tubularia (Fig. 75, p. 211), for example, the 

 stem and stolon increase in length, and when 

 a certain length, varying with conditions which 

 affect rate of metabolism, is attained, the tip of 

 the stolon turns upward away from the sub- 

 stratum and gives rise to a hydranth, as in Fig. 

 94. This hydranth and its stem grow in turn; 

 a stolon arises from the base, and when a cer- 

 tain length 

 of stem plus 

 stolon is 

 reached, the 

 process of 

 reproduc- 



FlG. 94. — The primary form of agamic reproduction in Tubularia 



repeated. 



Evidently the stolon tip gives rise to a hydranth only when it has 



attained a certain distance from the original hydranth. The 



