Stiles and J ergensen. — Studies in Permeability. V. 421 
the two sides of the plasma membrane. This produces an increase in 
pressure inside the cell and in consequence the cell wall stretches. As the 
water passes in, the pressure inside the cell increases, and continued stretch- 
ing of the cell wall takes place until ultimately the pressure of the water 
inside is equal to the osmotic pressure forcing water into the cell. 
That such high turgor pressures can exist in the cell as are indicated 
by these considerations has been generally admitted by physiologists, and 
they are indicated also by the very turgid condition of discs of tissues so 
swollen in distilled water. 
We have considered the swelling of plant tissue in water in terms 
of the original theory of de Vries and Pfeffer. However, we may perhaps 
indicate that we use this theory only as a working hypothesis which explains 
the facts ; we do not wish to urge that it is the only hypothesis which will 
explain the facts or that it is either correct or complete. 
The depression of swelling observed when tap-water is used instead of 
distilled water may be explained on this hypothesis by the supposition that 
certain salts in the tap-water are absorbed and bring about an increase in 
permeability of the cell. The subsequent diffusion out of dissolved 
substances from the cell as a result of this lowers the osmotic pressure of 
the cell, and so water passes out into the external liquid. 
(b) The Influence of Temperature on the Szvelling of Potato 
and Carrot in Water . 
Our earlier experiments on the influence of temperature on the swell- 
ing of plant tissue were made on a number 
of different days. The results obtained 
showed no correlation between the swelling 
and the temperature, as the curves exhibited 
in Fig. 3 indicate. 
Such a result can only be due to varia- 
tions in the samples used in the experiments 
conducted on different days. The sampling 
error is indeed reduced as regards the com- 
parison of results obtained from experiments 
started at the same time, by the thorough 
mixing of the discs as already described ; but the error is not eliminated 
as regards experiments done at different times, and necessarily on different 
samples of potatoes or carrots. 
In order therefore to avoid this difficulty the measurements of swelling 
at different temperatures were made at the same time on discs all taken 
from the same mixed sample. The swelling at four temperatures was 
measured, the vessels containing the discs being kept during the experi- 
Ftg. 3. Swelling of different 
samples of carrot at different tem- 
peratures. 
