420 Stiles and j0rgensen. — Studies in Permeability. V. 
The behaviour of the two tissues with tap-water is different. With 
carrot there is the same rapid initial rate of swelling, but the maximum 
value of this is less and is reached somewhat sooner, after which there is 
a slight, though distinct, gradual shrinkage of the tissue for the next eleven 
days. Having regard to the course of events observed with potato to be 
immediately described, there is little doubt that this shrinkage would have 
continued until death of the tissue finally resulted. 
The line in Fig. i marked Pt illustrates the course of swelling of 
potato in running tap-water. The tissue swells at first in the same way as 
in distilled water. As with carrot a lower maximum swelling is reached 
than with distilled water, and in this case after about 2-5 days, and then 
immediately a shrinkage of the tissues sets in, which was continued very 
uniformly until the end of the experiment. Thus, after 17 days in running 
tap-water, the potato tissue had shrunk to its original weight. 
De Vries and Pfeffer explained the plasmolysis 
of plant tissue in more concentrated solutions of various 
substances on the theory that the limiting layer of the 
protoplasm of the cell acts as a semi-permeable 
membrane, allowing the passage of water, but not 
permitting the passage of the particular solutes. On 
this view such a swelling as we have recorded for 
potato and carrot would have to take place, if it is 
assumed that the various crystalloidal substances of 
the cell cannot pass through the membrane, or can 
pass only with great difficulty and slowness. If the 
cell wall were absolutely elastic the intake of water 
should continue indefinitely, as equilibrium could only 
be reached when there was the same osmotic concentration inside and out. 
On the other hand, if the cell wall were perfectly inelastic and quite incapable 
of stretching, water would pass in as before on account of the difference in 
osmotic concentration on the two sides of the cell boundary. This will 
result in an increase in pressure inside the cell which tends to force water 
out of the cell. Under these conditions water will*pass in by osmosis 
until the pressure forcing water into the cell by osmosis is equal to the 
pressure forcing water out of the cell owing to its compression inside. In 
this condition the cell will be in equilibrium with the external medium in 
regard to the exchange of water. 
In reality the condition of affairs is intermediate between these two 
extremes. Although the rigidity of the cell wall prevents indefinite 
increase in volume of the protoplast, yet the wall is capable of a certain 
amount of stretching so that a certain increase in volume takes place. 
Hence the actual course of events may be considered as follows: Water 
passes into the cell in consequence of the difference in osmotic pressure on 
Time in hours. 
Fig. 2. Swelling of 
potato and carrot in dis- 
tilled water. C D , carrot 
in distilled water; P D , 
potato in distilled water. 
