HYDBATION AND GROWTH. 



BY D. T. MAcDouoAL. 



I. GROWTH AND COLLOIDAL REACTIONS. 



Growth consists in increases in volume of masses of living matter, 

 usually but not invariably accompanied by accretions of material 

 other than water to the colloids of the protoplasm. Auxetic changes 

 in members, organs, or cells of the larger plants may be readily deter- 

 mined by external measurements, and the greater part of the available 

 information concerning the subject has been obtained in this manner. 

 Many generalizations, however, rest upon data secured by taking the 

 gross weight of organisms; in other cases the dry weight is used as a 

 criterion, a method which obviously may be used only in securing end 

 or total results. A count of the number of individuals may afford a 

 reliable basis for the estimation of the rate of growth and multiplica- 

 tion of unicellular organisms such as bacteria, in which the limits of 

 enlargement of the individual are quickly reached. Much of the value 

 of the results presented in the present volume is to be attributed to 

 methods by which the varying dimensions of organs and of individual 

 plants were followed not only through the entire period of develop- 

 ment, but alterations accompanying maturity were measured with 

 some precision. The information thus secured made it possible to 

 interpret the effects of the ever-changing daily complex of environic fac- 

 tors and to evaluate to some extent the effects of previous experience 

 upon the behavior of a growing organ at any stage of its development. 



Thus, for example, the action of a cell-mass at any given tempera- 

 ture is influenced not only by the degree of the temperature and other 

 environic conditions at that time, but also by the previous experience 

 with these factors, particularly temperature. This ' ' memory " of ante- 

 cedent impressions is not psychological in any sense, but rests upon 

 definite properties of colloids which are known or are measurable. 



The diversity of constitution and consistency and variation in col- 

 loidal condition of living matter is so great as to evade exact or detailed 

 description. But the general composition of protoplasm, the character 

 of its activities, the mode or manner of changes in its colloidal states, 

 and a measure of the factors affecting its activities may be compre- 

 hended without leaning upon vitalistic conceptions or resorting to 

 mysticism in any form. The fundamental and ultimate structure or 

 architecture of protoplasm is a result of the force of surface tension and 

 is a gel in which the solid material occurs in two main states or phases 



