THE VALIDITY OF THE MASS RELATION 55 



of root formation at the base of the stem in the dark. We shall 

 see later that the favorable influence of the leaf on root formation 

 in the stem also disappears when the leaf is deprived of Hght. 



These experiments indicate that the quantity of regeneration 

 in a defoliated piece of stem depends upon the mass of assimila- 

 tion products in the stem. This is rendered possible by the fact 

 that the cortex of the stem contains chlorophyll. 



6. Child's Hypothesis of Axial Gradients. — We may, in pass- 

 ing, discuss here a hypothesis of the polar character of regenera- 

 tion which Child has defended in a large number of publications, 

 namely that the polar character of regeneration is due to an axial 

 gradient of metabolism, where the rate of metabolism is supposed 

 to be a maximum at one pole of an organ or an animal and a 

 minimum at the opposite pole, dechning steadily with the dis- 

 tance of the piece from the pole.^ Since we have shown that both 

 roots and shoots grow in proportion to the mass of the piece of 

 stem there should be two opposite gradients of metaboKsm in 

 the stem, one increasing from apex to base to explain root forma- 

 tion, and the other from base to apex to explain shoot formation. 

 Child measures the relative metabohsm of a piece by the time 

 required to dissolve the piece in a solution of potassium cyanide. 

 The unit for the measurement of metabolism is the calory and 

 the calories produced by an animal or plant or by one of its seg- 

 ments are not measured by the time required to dissolve the 

 animal or plant in a solution of potassium cyanide. Furthermore, 

 regeneration is due to synthetic processes and the only approxi- 

 mate measure for this at present is the dry weight of the organs 

 produced. If the polar character of regeneration were due to a 

 synthetic gradient existing in the stem it would follow that the 

 rate of synthesis or growth of a bud should be the greater the 

 nearer the bud is to the apex of a plant. If we, therefore, cut a 

 long piece of stem into as many pieces as there are nodes, then 

 shoot formation should not start in the different pieces of stem 

 simultaneously but successively in the order in which the pieces 

 were originally arranged in the stem, beginning first in the most 

 apical and last in the most basal piece; and the rate of growth of 

 the bud of each piece should follow the same serial order. 

 Figures 36 and 38 give the results of exactly such experiments. 

 The pieces designated as 1 to 9 (Fig. 36) were cut out from one 

 stem and so were the pieces 1 to 10 (Fig. 38). The piece desig- 



1 Child, C. M.: "Senescence and Rejuvenescence," Chicago, 1915. 



