54 
Walter Stiles 
potassium chloride may be present in considerable quantity, while 
in many cases organic acids are the principal substances present 
(De Vries, 1879, 1883; Kraus, 1886). 
At the junction of the vacuole and the cytoplasm we have again 
a surface separating two immiscible phases with consequently pro¬ 
perties differing from those of either phase, and it has become usual 
to regard that part of the cytoplasm directly surrounding the vacuole 
as a distinct layer spoken of as the internal plasmatic membrane 
(Pfeffer, 1900) or the vacuole wall (De Vries, 1884). Price, by ultra- 
microscopic observation, has obtained indications of a definite layer 
in this location similar to that bounding the cytoplasm externally. 
To this question we shall have to return later. 
Yet another complication is present in the case of plant cells, 
namely, the cell wall. Although this is absent in a few cases, nearly 
every plant cell is bounded by a firm envelope which thus separates 
the individual protoplasts from one another. The cell wall differs 
widely in morphological construction and chemical composition 
among different species and in different parts of the same plant, but 
with the exception of the Fungi one of the main constituents of the 
cell wall is cellulose, a carbohydrate, or more probably a group of 
carbohydrates, of high molecular weight. A number of other sub¬ 
stances are present, these varying in different species and in different 
tissues of the same species. In the Fungi, including the Bacteria, the 
principal constituent of the cell wall is chitin. The cell wall of the 
young plant is always thin, but after the cell has reached its ultimate 
size, various morphological and chemical changes take place in the cell 
wall, for a description of which reference may be made to standard 
botanical text-books (for example, Sachs, 1875; Strasburger, 1903) 
and especially to the work of Mangin (1893). It should be mentioned 
here that thickening of the cell wall may consist entirely of cellulose 
and allied substances, or the walls may be modified by the processes 
Qf lignification, suberisation or cutinisation, in which various sub¬ 
stances grouped under the names lignin, suberin and cutin (see 
Priestley, 1921) are deposited in the cell wall and profoundly alter 
its properties. It should also be observed that when cell walls be¬ 
come thickened the thickening is rarely uniform. Scattered over 
the cell walls places occur at which thickening does not take place 
so that thin spots known as pits are dotted over the surface. The 
pits on the two sides of a wall separating two cells are always 
opposite one another, so that at the pit there is only the original 
primary wall separating the two protoplasts. Chiefly through the 
