Permeability 53 
0*02 M solution of calcium chloride than from solutions of sodium 
and potassium chlorides of the same molecular concentration. Mem¬ 
branes from which the lipoid substance had been removed took up 
more water than similar membranes containing fatty constituents. 
Parchment membranes did not resemble cell wall membranes with 
regard to the influence of the composition of the surrounding solution 
on their uptake of water. 
It is clear from these considerations that the cell wall cannot be 
dismissed as a negligible factor in cell permeability. 
We have so far considered the cell wall of young active cells in 
which thickening has not taken place, or in which the thickening 
consists of “cellulose.’’ But in many cases the cellulose walls become 
modified so that their permeability properties undergo much altera¬ 
tion. In some cases the change results in the wall becoming muci¬ 
laginous. It appears that in some species at any rate, the mucilage 
results from the conversion of polysaccharides into pentosans (con¬ 
densation products of pentose sugars) which have great water¬ 
absorbing capacity (MacDougal, Richards and Spoehr, 1919). Other 
cell walls become lignified, suberised or cutinised (cuticularised). 
These changes are due to the deposition in the cell wall of substances, 
or rather mixtures of substances, known as lignin, suberin and cutin 
respectively. Among the substances which compose lignin a number 
have been isolated which are claimed to be characteristic of lignified 
walls; among these are xylan, lignic acids, and hadromal, an aromatic 
aldehyde (Czapek, 1899). Particularly important from the point of 
view of the student of permeability is the fact that in lignified walls 
the “ cellulose ” and lignin appear to be present in an intimate mixture 
(Robinson, 1920). This may partly account for the fact that water 
passes readily through lignified walls, but not through suberised walls. 
Suberin is the name given to the group of substances to which 
the characteristic properties of cork are due. Various substances 
have been isolated from suberin, among these being the so-called 
suberogenic acids (Gilson, 1890). Some of these substances have 
been obtained pure and empirical formulse obtained for them, e.g. 
phellonic acid, C 22 H 43 03, and phloionic acid, C n H 21 0 4 (after drying 
for weeks, C 20 H 40 O 7 ), while a third substance, suberinic acid, appears 
to possess the formula C 17 H 30 O 3 . There appears to be a divergence of 
opinion on the fatty nature of suberin, but Gilson decides that the 
substances composing it cannot be regarded as true fats, an opinion 
with which Priestley (1921) agrees. 
The significance of these suberogenic acids in regard to the im- 
