290 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



Gelatine alone has been found to furnish valuable analogies in the 

 study of the action of animal tissues. It is not adequate for the 

 vegetable protoplast however. Mixtures consisting largely of the 

 amorphous condensed carbohydrates such as agar to which is added 

 a small proportion of albumen or amino-acid are found to respond 

 to the action of acids, alkalies and salts in a manner similar to that 

 of the plant. 



Some new conceptions of the general nature of respiration and 

 its correlation with growth have been made possible. The origin 

 and fate of the sugars, particularly the pentosans, have been made 

 the object of extended experimentation, and the results obtained are 

 not the least important of those presented herewith. Most of the 

 attempts which have been made to ascertain the essential nature of 

 growth have been made on the assumption that it is a single, simple 

 or unified process. Thus for example, much attention has been 

 concentrated upon fixing the lower and upper limits of growth with 

 regard to temperature, and recently much has been written concern- 

 ing the temperature coefficient. A number of authors concur in the 

 assertion that within a certain range, generally between 15° C. and 

 30° C, the rate of acceleration is one which follows the van't Hof 

 law of doubling or tripling for every rise of 10° C, it being agreed 

 that no such conformity is shown in the extreme upper and lower 

 ranges of temperature. This partial or accidental agreement of 

 smoothed curves of growth with those depicting the course of simple 

 reactions has diverted attention more than once from the funda- 

 mental fact that growth depends primarily on respiration, imbibition 

 and osmosis. Respiration is essentially a complicated swirl of sugar 

 disintegration processes which may be influenced in any one of its 

 parts by the oxidation potential, by the dearth of material or over- 

 accumulation of products in any part of the complex. The con- 

 centration of the various reaction products may exert their own 

 direct effect on imbibition and consequent enlargement. In addi- 

 tion to, and partly dependent upon the imbibition phenomena, 

 elongation may be modified by such factors as water-loss. Thus 

 for instance, growth upon a rising temperature may reach a point 

 where, as a result of temperature, the water-loss would temporarily 

 be greater than the supply, with the result that a cessation, slacken- 



