Measurement of Rale of Shrinkage of Turgid Tissues. I. 295 
tion. for the difference in concentration between the cell-sap and the 
outer solution is then at its greatest and, other things being equal, must 
give the fastest rate of plasmolysis in the first phase of an experiment. 
For these reasons it is hardly possible to estimate closely the effect of 
temperature on the course of plasmolysis produced by means of markedly 
hypertonic solutions, and in all the critical experiments on temperature 
effects in this research subtonic solutions alone were employed. 
Section VI. Rate of Plasmolytic Shrinkage in a Subtonic 
Solution. 
It was necessary to make trial experiments in order to ascertain which 
strength of subtonic solution would be most suitable for the experiments at 
different temperatures. The general considerations were that the solution 
should be weak enough to give a definite end-point in not more than two or 
three hours at ordinary temperatures, and not too weak to give a contraction 
of less than about 20 divisions on the scale. In practice, it was found that 
solutions which gave contractions varying from 30 to 50 divisions on the 
scale gave a definite ending in about two hours at ordinary temperatures, 
and also gave a curve of nearly logarithmic form. It can be shown, on 
theoretical grounds, that this is the form of curve which would be expected 
with dilute solutions apart from any secondary disturbing causes. It was 
found that the best results were given with solutions of 0-18 grm. M. sugar for 
the onion and 0-3 grm. M. for the dandelion ; these two cases will be 
considered separately. 
A. Leaf of Onion. 
The mean osmotic pressure of the cell-sap of onion leaves was found to 
vary with the age of the leaves, those which had only just finished their 
growth being isotonic with a sugar solution of 0-20 or 0-2 1 grm. M., and 
older ones with a solution of 0-23 grm. M. (i. e. four to five atmospheres 
pressure). The original solutions tried were 0-25, 0*20, 0-18, and 0*15 grm. M., 
but the last was at once rejected on account of the small contractions given 
with some leaves. 
Even with great care in the choice of material and in repeating the 
same experimental conditions, there was a certain amount of variation 
in the rate of plasmolysis at the same temperature. At medium tempera- 
tures, therefore, it was usual to perform several experiments under the same 
conditions, and to select the ones which gave the curve of approximately 
logarithmic form (e. g. A, in Fig. 13). 
It was found that at temperatures above that of the laboratory there 
was a short period of expansion of the tissues when first the material 
was fixed in place. At temperatures not higher than 30° C. this was 
