152 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



From among them the following were selected. They 

 show how the electrical conductivity of the internodes of 

 the stem of Begonia semper flor ens increases from below 

 upwards. In the freezing-points obtained there is no 

 such regularity. For all these the whole of the stem tissues 

 were pressed. 



TABLE XXXV. 

 Begonia semper ftor ens. 



The conductivity determinations were carried out at 

 28, and are written thus: C 28 x 10* = x. 



The freezing-points of the leaves of ninety trees and 

 shrubs were determined by Ohlweiler (1912) by means of 

 the thermo-electric method. In nineteen other cases no 

 sap could be pressed from the leaves. Dixon and Atkins 

 (1910) also found this difficulty in certain cases. Treat- 

 ment with liquid air was found by them to make it possible 

 to obtain sap in many cases in which it was previously 

 impossible. 



Ohlweiler's numerous measurements show that pressures 

 of from 10 to 24 atmospheres exist in leaves of a large 

 number of species. Treatment of the leaves with liquid 

 air would certainly have in many cases largely raised 

 the results. The material for these experiments was col- 

 lected in the Missouri Botanic Gardens in full sunlight, 



