ANATOMY OF THE CELL. 11 
walls remain nearly uniform in thickness, their growth being 
limited to two directions, length and breadth, and this is known 
as surface growth. When the cell has reached its normal shape 
and size, by this process, another kind of growth may ensue, 
which results in a thickening of the wall. These two processes 
are generally referred to as growth in surface and growth in 
thickness, and are so sharply distinct from each other in refer- 
ence to the time of their occurrence, that it is necessary to 
indicate which of the two is meant when speaking of the 
development of the wall. 
The new material which is used in this development is the 
product of the protoplasm of the growing cells. There are two 
theories respecting the manner by which this is added to the 
primary wall. It is supposed to consist of cellulose micellae 
which either make their way between those of the primary wall 
and so incorporate themselves with it, or are added to its 
surface, becoming a part of it without penetrating between its 
micellae. The first method is known as intussusception ; the 
second as apposition. 
As a rule, surface growth is completed before that of thick- 
ness begins. In certain cells destined to carry on the various 
life processes of the plant, this subsequent growth in thickness 
never takes place. It is customary to speak of such cells as 
thin-walled, even though they vary greatly among themselves 
in this respect. 
There is another class of cells whose principal function is 
that of the conduction of food and building material from place 
to place when it is necessary that these be carried considerable 
distances. Their walls are only partially thickened, so that 
liquid substances may easily pass through the thinner places, 
while at the same time the thickened portions furnish support 
and strength to the organ containing them. 
Finally a third class is represented by the cells whose chief 
function is that of support. These are usually referred to as 
