IIIj 



THE EFFECT OF TEMPERATURE 



113 



temperatures, yet it serves as a very satisfactory guide to the relations between 

 rate and temperature within the ordinary limits of healthy growth. Miss 

 Leitch holds that the curve is not a van't Hoff curve ; and this, in strict accuracy, 

 we need not dispute. But the phenomenon seems to me to be one into which 

 the van't Hoff ratio enters largely, though doubtless combined with other 

 factors which we cannot at present determine or eliminate. 



4° 



12° 16° 20° 

 Temperature 



24° 



'per 

 hour 



2-0 



1-8 



1-6 



28° 32' 



4 2 



2 p 



1-0 ^ 



•8 S 



O 



•6 

 •4 

 •2, 

 



Fig. 27. Relation of rate of growth to temperature in rootlets 

 Pea. (From Miss I. Leitch's data.) 



of 



While the above results conform fairly well to the law of the 

 temperature coefficient, it is evident that the imbibition of water 

 plays so large a part in the process of elongation of the root or 

 stem that the phenomenon is rather a physical than a chemical 

 one; and on this account, as Blackman has remarked, the data 

 commonly given for the rate of growth in plants are apt to be 



