£2 ELEMENTS OF PLANT ANATOMY. 
thick-walled in distinction from the first class. Their walls are 
more or less uniformly thickened, and comparatively few thin 
places occur. 
It must not be inferred from this that the function of the 
cell can at once be determined by its shape and the nature of 
its wall. ‘These are only indications which help us to decide its 
leading or principal function. The division of cells into three 
classes as here made, does not actually exist in nature, for each 
class passes into the next one by such imperceptible differences 
that it is impossible to say where one ends and the other begins. 
For purposes of study, however, it is convenient to group into — 
classes, and this may be done in this instance by selecting the 
leading characteristics of the different cells. 
There are certain facts in connection with the surface 
erowth of newly formed cells, that lead to the opinion that the 
wall, at its very beginning, consists of two lamellae or plates, 
so closely connected that it is impossible even with the strongest 
lenses to see any indication of their line of contact. However 
this may be at first, as the cells grow older the double nature of 
the wall is seen by the separation at the corners by which the 
intercellular spaces are formed. 
In many cells where growth in thickness has succeeded 
that of surface growth, this lamellated structure becomes very 
evident. For example, a cross-section through the thick-walled 
cells of pine wood shows three distinct layers in the walls. In 
cells of many other woods the existence of layers may be made 
evident by the use of various chemical reagents. In cork cells 
five distinct layers may frequently thus be made to appear. In 
such cases, the middle lamella is said to be common to each , 
cell, while the other layers belong to the individual cell which 
they enclose. 
The process of maceration dissolves the middle lamella of 
such cells, showing that there is a chemical difference between 
it and the other layers. In living, active, and moderately thin- 
