12 
Walter Stiles 
in the development of the plant, and have only a passing interest 
from the point of view of permeability. 
The chemical analysis of the whole plant can, of course, also be 
employed as a test of the permeability of the cells to substances other 
than salts. Thus Bourget (1899) showed that members of the Liliaceae 
and Chenopodiaceae were able to absorb considerable quantities of 
iodine through their roots, while the cells of the roots of potato were 
apparently impermeable to this element. Similarly, it is well known 
that some marine algae absorb iodine from sea water to such an extent 
that they have been used as a commercial source of that element. 
By direct analysis Stoklasa, Sebor, Tymich and Cwacha (1922) have 
shown that aluminium ions are absorbed by the hydrophytes Gale- 
opsis versicolor and Caltha palustris and the mesophytes Dactylis 
glomerata and Festuca pratensis, but scarcely by the xerophytes 
Sesleria ccerulea and Anthyllis Vulneraria. 
Brooks (1917 a) raises the objection to this method that the 
quantity of substance found by analysis of tissue extracts includes 
any of the substance held in the cell walls and intercellular spaces. 
It is clear that the method cannot be used as a method for the quanti¬ 
tative measurement of permeability of the cell membranes, as other 
factors, namely, adsorption and chemical action, may influence the 
quantity and rate of absorption of the substance. When expressed 
sap is analysed there is the additional objection that it is difficult to 
obtain a reliable sample of sap, although this difficulty can apparently 
be overcome by suitable methods of extraction (cf. Chapter IX). 
4. Determination of Permeability by Visible 
Changes in the External Medium 
Visible changes in the external solution can be used in some cases 
-as a test of the permeability of the cell membranes to substances 
contained in the cell. Thus when the cell sap contains a pigment the 
diffusion of this out of the cell can be observed by the coloration of 
the external liquid as soon as sufficient of the pigment has exuded, 
and the quantity that has passed out from the cells or tissues em¬ 
ployed can be estimated colorimetrically (cf. Stiles and Jorgensen, 
1917 a). If the external solution should contain an indicator, the 
diffusion out of acids or alkalies can similarly be observed, while if 
any of the constituents of the cell sap should give a precipitate or a 
colour reaction with a substance present in the external liquid, the 
exosmosis of this constituent can be similarly observed. The disad- 
