Permeability 
245 
On the Values of the Temperature Coefficient of the 
Permeability of the Cell to Water 
Three points deserve special mention in regard to the temperature 
coefficient of the cell membranes to water. 
1. The permeability to water increases continuously from the 
lowest to the highest temperatures investigated. This means that, 
other factors being equal, water absorption or excretion increases 
continuously with rise of temperature. Above 30° C. a “ time factor ” 
(F. F. Blackman, 1905) appears, inasmuch as with prolonged im¬ 
mersion in water at such higher temperatures the initial absorption 
is followed by excretion of water; this has been observed in onion 
leaves, dandelion scapes, tubers of potato and artichoke, and roots 
of carrot and red beet. 
2. The temperature coefficients vary considerably for different 
tissues and over different temperature intervals in the case of the 
same tissue, the lowest observed being for carrot root and the highest 
for dandelion scapes. 
3. The temperature coefficients are higher, and often considerably 
higher, than those found for a physical process such as diffusion of 
solutes in water, which are generally between 1*2 and 1-3, and 
approach instead more closely those characteristic of chemical pro¬ 
cesses. Yet there is no ground for supposing that we have to do 
here with a ^chemical rather than a physical process. The results 
rather suggest the necessity for caution in drawing conclusions with 
regard to the nature of a process from the value of the temperature 
coefficient. 
Brown and Worley point out that an exponential increase in the 
value of a physical property is very rare, but that vapour pressure 
of water is an exception. The general equation 
v = ae ke 
gives both the relation between vapour pressure and temperature 
and that between water absorption by barley grains and temperature. 
Shull, however, does not agree that the rate of intake of water by 
seeds is determined by vapour pressure. He is no doubt correct in 
supposing that in the case of seeds the rate of water intake is deter¬ 
mined by a number of factors, among which chemical changes in the 
colloids of the seed coat may not be unimportant. Denny also, on 
account of the values of the temperature coefficient found by him, 
disagrees with Brown and Worley in regard to vapour pressure of 
water determining the rate of entrance of water into seeds. 
