50 
Walter Stiles 
Structure and Composition of the Cell Wall 
Cell walls exhibit a great range in composition' and structure 
according to the age of the cell and the tissue of which the cell forms 
a part. The cell wall which arises between the daughter cells after 
the division of a meristematic cell is always at first thin and ap¬ 
parently homogeneous in structure, and remains thin so long as the 
cells it separates are meristematic. But as conversion of the cells 
into part of a permanent tissue takes place, the cell wall increases in 
thickness, the material added in this process being apparently de¬ 
posited in at least two different ways, known as growth by apposition 
and growth by intussusception. In the former fresh particles are 
added on the surface of the existing wall. Where this method takes 
the form of the deposition of entire discernible layers or lamellae it is 
sometimes distinguished as growth by superposition. In growth by 
intussusception the new particles are deposited within the existing 
cell wall, so that they form an intimate mixture with the pre-existing 
particles. The method of thickening the cell wall may play an im¬ 
portant part in determining its permeability. 
Stratifica!ion of the cell wall is stated to be almost universal 
(Haberlandt, 1914). It is clear that this appearance must be due to 
differences in composition of adjacent layers, the differences being 
either physical, such as differences in water content (Nageli, 1866) 
or in chemical composition. 
In all cases it appears possible to distinguish between a layer 
forming the middle of the cell wall, the middle lamella, and layers on 
each side of this. It has been stated that the middle lamella in soft 
tissues is composed of a calcium compound of pectin, a name given 
to a group of substances which are nearly related to pentosans, that 
is, condensation products of pentose sugars. A substance or group 
of substances described as pectin or pectinogen (Schry ver and Haynes, 
1916) can be isolated from the walls of plants, and this substance 
can be converted by a clotting enzyme pectase into an acid, pectic 
acid, with splitting off of methyl alcohol and acetone (cf. Tutin, 
1921). This acid forms soluble salts with the alkali metals, but an 
insoluble salt with calcium, and it is this salt which it has been 
supposed forms the middle lamella of parenchymatous cell walls. 
However, if plant tissue is treated so that all the pectin compounds, 
including the calcium pectate, are converted into pectic acid, and is 
then treated with a solution of ammonium oxalate or dilute sodium 
or ammonium hydroxide, the pectic acid dissolves, so that cells 
