MACDOUGAL: mechanism and conditions of growth 21 



is not primarily determined by water-conditions as affected by 



light either directly, or indirectly by variations of the acidity. 



Irreversible changes in form and size are intimately and primarily 



dependent upon the amount and availability of the substances 



which for lack of a more definitive term may be designated as 



"building material," and upon its use by respiratory action, as is 



well evidenced by the fact that the process sustains an intimate 



relation to temperature, so that great variations in the rate are 



induced by temperature changes so small as 2 or 3 degrees F. 



The facts to be taken into account justify the assumption that the 



actual building material is derived from accumulated food in a 



hydrolized condition for example, the velocity of the chemical 



changes depending directly upon the temperature. 



This assumption harmonizes with the repeated observation 

 that growth at the higher temperatures proceeds at rates which 

 soon fall off. If growth took place at a rate governed by the 

 adduction of plastic material no such abrupt acceleration would 

 be possible, while as a matter of fact the rate rises suddenly after 

 a mode similar to the increase in reaction velocities. ^ 



Accelerations in rate of growth may be capable of interpretation 

 upon this basis but reaction velocity alone as governed by temper- 

 ature does not afford an adequate explanation of the slackening 

 and cessation of growth under favorable temperature conditions. 

 If reaction velocity may not be held to account for a decrease 

 m rate, attention would naturally be turned next to the size of 

 the supply of material and its availability for construction pur- 

 poses. 



The analyses of Richards, Spoehr, and Long show that owing to 

 the type of respiration prevalent in these plants, organic acids 

 accumulate in the cells during the night to a concentration of 

 N/20 to N/io, and their disintegration begins with the dawn and 

 continues throughout the day so that these substances are present 

 in minimum quantity by 4 P.M. The breaking down of these 

 acids includes a web of processes not capable of ready description, 

 but which does not yield substances capable of being used as 

 building material by the plant. 



On the other hand, these acids are the partially oxidized waste 



1 For recent measurements, see Lehenbauer, P. A. Growth of maize seedlings in 

 relation to temperature. Physiological Researches i: 247-288. 1914. 



