Permeability 129 
The latter was used in a concentration of o-ooii N and changes in 
its value with time were determined electrometrically by means of 
the hydrogen electrode. The absorption of the acid was followed in 
this way at temperatures of o°, io°, 20° and 30° C. The relation be¬ 
tween time and absorption found in these experiments is represented 
graphically in Fig. 15. The relation is almost a logarithmic one, so that 
the equation representing the rate of absorption is approximately 
dx , , . , 
dt =k( A -x), 
dx 
where is the rate of absorption at any time when x is the diminution 
in the concentration of acid in the external solution and A represents 
the initial concentration. In this equation k is determined by the 
temperature and in Table XLIV are shown values of k calculated 
for the different temperatures employed. From these results it 
appears that the rate of absorption of hydrogen ions by potato tuber 
is increased about 2-2 times for every rise of temperature of io° C. 
within the temperature range o° C. to 30° C. 
Table XLIV 
Influence of Temperature on the Rate of Absorption of Hydrogen Ions 
by Potato Tuber. (Data from Stiles and Jprgensen) 
Temperature in 
centigrade 
degrees 
“k” 
0 
0-036 
10 
0-081 
20 
0-174 
30 
0-380 
This temperature coefficient (Q 10 ) is that characteristic of chemical 
reactions rather than of physical reactions such as diffusion or 
adsorption. But having regard to the great variations in the value 
of the temperature coefficient in different cases of the absorption 
of water by plant tissue, it would clearly be unwise to draw far- 
reaching conclusions with regard to the nature of the process of 
absorption of hydrogen ions from the temperature coefficient alone. 
But from the magnitude of the absorption it seems clear that some¬ 
thing more than simple diffusion alone is required to account for the 
process. Thus, after three hours in the acid at 30° C. the potato tissue 
had absorbed so much acid that the absorption ratio was 39 and 
there was no indication that equilibrium had been reached. 
Phyt. xxii. 3, 
9 
