Relation of Plasmolysis to Shrinkage of Plant Tissue. 40 
Hence we think our remarks involving the use of the word 
“ elastic ” were perfectly clear, and we regard Thoday’s statement 
that we “ have confused elasticity with extensibility ” as incorrect 
and unwarranted. 
Summary. 
1. The swelling and shrinkage of beetroot in distilled water 
and aqueous solutions of sodium chloride of various concentration 
was measured by the change of weight method previously described 
by us. In solution of sodium chloride of concentration 0.40 N the 
tissue examined underwent almost no change in weight. 
2. Sections of the same roots were mounted in aqueous solutions 
of sodium chloride and examined microscopically. The solution 
which was just not strong enough to produce the first signs of 
plasmolysis was one of concentration 0.40 N. For reasons given 
in the paper this value must of necesssity be an approximation. 
3. It can be thus said with truth that in the case of the 
particular tissues examined the solution of sodium chloride which 
produced neither loss or gain in weight of beet root was approxi¬ 
mately isotonic with the cell sap on the basis of the osmotic 
hypothesis. If it be assumed that potato has similar properties to 
beet root in regard to water relation, then it may also be said in 
regard to potato that a solution which produces neither gain or loss 
in weight is approximately isotonic with the cell sap of the tissue. 
4. But it is obvious that this is only a crude approximation, and 
no more than this is now or has been claimed for it. In the case of 
cells with a high degree of turgor the cell will remain in equilibrium 
as regards water exchange with a solution of lower concentration 
than the cell sap, or even with distilled water, as we have explained 
in a recent paper. (Ann. of Bot., XXXI, 415-434,1917). 
5. In view of our having deliberately explained how on the 
osmotic theory of the cell, a tissue can remain in equilibrium with 
distilled water, Thoday’s statement that we “ have not succeeded 
in freeing our minds from the misconception that a solution in which 
a turgescent tissue shows no gain or loss of water is isotonic with 
the cell-sap,” is as surprising as it is misleading. 
6. Thoday’s statement that we “ have confused elasticity with 
extensibility” is also incorrect. 
University College, 
London, 
August , IQ 18. 
