Permeability 243 
more than twice as great in carrot as in potato. This is probably 
due in part to the higher suction pressure observed in the carrot 
tissue used (cf. Chapter IX), but as there is no evidence that the 
tissues were in the same absolute stage of swelling, too much stress 
must not be laid on the difference between the two tissues. 
In the second place a phenomenon noted by Miss Delf in onion 
and dandelion appears also in carrot and potato tissue, namely, the 
contraction and loss in weight of the tissues at temperatures above 
30° C., which probably corresponds to a rapid exosmosis of solutes 
from the cell. This breaking down of the semi-permeability of the 
protoplasm requires some time to become obvious, so that, as the 
curves for 40° C. show in both potato and carrot, there is first an 
initial period of rapid absorption of water followed by exudation. 
This phenomenon has also been observed in other storage tissues, 
such as red beetroot and artichoke tubers (Helianthus tuberosus). 
Confining attention only to the curves where this complication 
is not present, and using the same notation as in the preceding 
section, we have the relation 
dw 
dt 
= t > (P-T)A, 
where it is assumed as before that the effect of temperature on 
imbibitional swelling is negligible. If it is also assumed as before 
that P — T is approximately constant over the range of temperature 
in question, the temperature coefficients given above may be 
accepted as those of the permeability of the cells of the tissues to 
water. 
Seeds. The influence of temperature on the absorption of water 
by barley grains was investigated by A. J. Brown and Worley (1912) 
by means of the change in weight method. Known weights of 
grains were immersed in water kept at temperatures of 3-8° C., 
2i-i° C. and 34-6° C., and the increase in weight determined after 
various intervals of time. From the absorption-time curves so ob¬ 
tained the rate of water intake at different stages of absorption was 
measured for the three temperatures employed. The values obtained 
indicated that the rate of water absorption at 21-1° is about 3-40 
times the rate of absorption at 3-8°, while the rate at 34-6° is about 
2-44 times that at 21-1°. These numbers give a mean value for the 
temperature coefficient (<2i 0 ) of about 2 (actually 1-99), a value char¬ 
acteristic of a number of chemical reactions. Somewhat lower values 
were obtained by Shull (1920) in a series of carefully conducted 
