474 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



is strikingly revealed by the fact that the curves of growth obtained in 

 Plants and even in the multiplication of Bacteria are essentially similar 

 in character to those obtained in animals. As a rule, however, the 

 growth of a plant or of a colony of bacteria displays evidence of only a 

 single growth-cycle. 



Each of these growth-cycles is approximately symmetrical about its 

 center, that is, on either side of the moment of most rapid growth; in 

 other words the second half of the S-curve reproduces in the reverse 

 order the characteristics of the first half. We have, then, in each 

 growth-cycle considered by itself, a chemical process which begins 

 relatively slowly, increases progressively in velocity until it is about 

 half completed, and then slows off to its termination. The inquiry 

 now immediately presents itself whether any chemical processes of 



Body-iueight. ^f^ Amount transformed 



FIG. 32 Comparison of the curve of growth of the white rat (constructed from data 

 collected by Donaldson) with chemical reaction curves. 



this general character are known to occur elsewhere than in the build- 

 ing up of tissue by a growing plant or animal? As a matter of fact, 

 chemical transformations of this character are abundant and they are 

 those in which one or more of the products catalyzes the further 

 progress of the reaction. We have already in preceding chapters had 

 occasion to dwell upon a number of chemical phenomena which occur 

 in living tissues and elsewhere which belong to this category; it will 

 merely be necessary therefore to refer in passing to the analogies 

 afforded by the hydrolysis of castor-oil in the seeds of Ricinus, the 

 hydrolysis of cane-sugar by boiling neutral water, the decomposition 

 of methyl acetate by water, the oxidation of metals and of a variety of 

 organic materials, and the chemical transformations which accompany 

 and underlie the performance of muscular work. In all of these various 



