THE MECHANISM AND CONDITIONS OF GROWTH 



D. T. MacDougal 



Carnegie Institution of Washington 

 (with plate i) 



One of the most important characteristics of the activities of 

 living matter is that external substances pass into it more or less 

 constantly, thus adding to its bulk, and the introduced material 

 is ultimately partly burned and its energy rendered available, 

 while some of it is converted into constituents of the colloidal 

 protoplast and its envelope, increasing their mass, the accretion 

 being followed by changes in arrangement and structure which 

 find external expression in alterations in form and size. Such 

 increases are accompanied and made possible mechanically by 

 differentiations into tissues or specialized tracts. 



The general external features of growth are of the most obvious 

 kind. When, however, we set ourselves the task of determining 

 the conditions under which it proceeds, of analyzing the contribu- 

 tory factors, and of measuring their relative influence on its rate 

 and course, technical difficulties of a very refractory kind are 

 encountered. The elements and their compounds necessary for 

 growth are in the main known to us, and also the fact that water 

 is of an importance in the process corresponding to its high pro- 

 portion in protoplasm. 



The purpose of the present paper is not to discuss these 

 features of the matter, but rather to present the results of some 

 experimentation which seems to have important bearing upon 

 the main problems of growth. In any picture we may draw 

 of the irreversible accretions to living matter and its accessory 

 and enveloping structures, attention may well be centered upon 

 the origination of the material to be used. The greater amount 

 of growth is accomplished in the higher plants by material which 

 has accumulated in storage tracts or is being formed by photo- 

 synthesis or otherwise, and this material must be diffused a 



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