36 



\J 



FIGS, i, z.Planaria 

 dorotocephala: Fig. i, a 

 well-fed animal 25 mm. in 



length; Fig. 2, an animal 

 reduced by starvation from 



25 to 4 mm. 



SENESCENCE AND REJUVENESCENCE 



Logically our definition of growth might well 

 include both positive and negative growth, or 

 production and reduction, but since the word 

 growth has come to be so generally associated 

 with an increase in substance it is perhaps 

 inadvisable to attempt to change its meaning. 

 We may then retain the word growth for posi- 

 tive growth or production, and use the term 

 reduction for negative growth. But in so doing 

 we must not forget that both these processes 

 are in the broad sense, though not necessarily in 

 the chemical sense, reversible, and that any 

 adequate conception of the relation between 

 the substratum and the dynamic processes in 

 the organism must be based, not on growth 

 alone, but upon both growth and reduction. 

 In other words, the activity of the organism 

 may either increase or decrease the amount of 

 its substance according to conditions. 



The question has often been raised whether 

 the increase in the water-content of the organism 

 is to be regarded as growth, or only the increase 

 in the structural substance. Some definitions 

 of growth have taken the one view, some the 

 other, but if water is included among the sub- 

 stances concerned in growth we have then 

 to determine whether increase in water- 

 content is in all cases to be regarded as 

 growth, or whether we shall make a dis- 

 tinction between growth and passive dis- 

 tension due to external factors. Here 

 again views differ. As a matter of fact, 

 various investigators have shown that the 

 imbibition of water is a very characteristic 

 feature during at least certain stages of 



, , ,, , 



what WC are accustomed to call growth: 



on the other hand, loss of water is a 



