CELL WALL. 
37 
cells, but by the division of these to form new cells. 
Cells that have the property to divide and form new 
cells are known as meristematic cells and constitute 
the meristem. The new and dividing walls resulting 
from the division of the cells consist of a number of 
substances: the first layer formed is apparently differ- 
ent from the subsequent layers and is known as the 
“ middle plate,” middle lamella or “ intercellular sub- 
stance.” This layer is soluble in, or readily attacked 
by, solutions of the alkalies or solutions containing 
free chlorine. It is insoluble in sulphuric acid, and 
readily stained by the aniline dyes. While usually 
more or less permanent, this middle plate may be 
finally absorbed, as in the secreting hairs of kamala, or 
it may be changed into mucilage, as in chondrus, or 
transformed into pectin compounds, as in fleshy roots 
and fruits. 
To this middle plate is added on either side by the 
newly formed cells a layer of substance closely resem- 
bling cellulose, this constituting the primary membrane 
or “ primary lamella.” 
Still other layers may be added, consisting of one or 
more of the following substances : cellulose, or some 
modification of it ; wax, silica or calcium oxalate, 
these layers constituting what may be termed the 
secondary lamella. 
II. KINDS OF WALLS. 
It is thus seen that cellulose in its various modifica- 
tions constitutes the greater proportion of the cell wall. 
The cellulose making up the cotton fiber may be said 
to be the typical cellulose, and is known as “ cotton 
cellulose.” It is soluble in copper ammonium sulphate 
solution ; is colored blue with chlor-zinc-iodide solution 
or iodine and sulphuric acid, and is stained by acid 
