1914] 



ECKERSOX— THERMOTROPISM OF ROOTS 



259 



molecular solution which produces slight plasmolysis. The 

 increase or decrease of permeability with change of temperature is 

 shown by the increase or decrease of these concentrations, the 



permeability of roots of Raph 



remaining 



from io° to 18 C; does not change from 18 to 24 C; increases 



Fig 



1. 



Raphanus sativus: curves showing the effect of temperature on the per- 

 meability of roots to potassium nitrate and glucose; ordinates indicate the percentage 

 wt.^mol. solutions producing slight plasmolysis; dots indicate temperatures at which 

 determinations of plasmolysis were made. 



Fig. 2. — Pisum sativum: the potassium nitrate and sucrose curves are identical 

 from 2 5 to 30 C. 



Irom 24 to 40 C, and then decreases. The turning point in 

 permeability to glucose is at 42 C. The drop in the sucrose curve 

 above 42 C. suggests a decreased osmotic pressure due to exos- 

 mosis. To test this, roots were put in distilled water at 45 C; 

 after 20 minutes the water gave a sugar test with Fehling's solu- 

 tion. Thus at 42 C. there is an increased permeability, in all 

 probability caused by coagulation of the protoplasm. 



