148 BOOKS AND CURRENT LITERATURE 



or genera] metabolism, rather than by. the van't Hof formula. Above 

 the higher temperature named the plotted rate is a series of broken 

 lines. Leitch was unable to find any effect from simple changes in 

 temperature; his growing material appeared to assume the rate per- 

 taining to any new intensity, without shock or stimulatory effects. It 

 is to be supposed that the diverse conclusions on this matter are prob- 

 ably referable to unlike conditions of humidity or food supply, rather 

 than to auxetic processes. Four cardinal points are distinguished, the 

 Minimum, at which growth may take place, 2°C. in Pisum; the Maxi- 

 mum, the highest, 44.5°C. in Pisum, the Optimum, the point at which 

 the highest constant rate occurs, betwen 28 and 30°C. and the Maximum 

 rate point, at which the process may attain its highest intensity, but is 

 not maintained. In Pisum this occurs at about 30.3°C. and begins to 

 decrease after ten minutes of exposure. It is obvious that this concep- 

 tion may express some of the facts upon which conclusions as to stimu- 

 latory effects might be based. It is notable that Leitch was unable to 

 detect any effect of light on the growth of the roots although but httle 

 attention was given to this phase of the subject. — D. T. M. 



