18 VEGETABLE PHYSIOLOGY 
As soon as the cells of the plant begin to divide in 
three dimensions, so that a mass of protoplasts is formed, 
the progress of differentiation becomes marked. 
In such a mass the necessity of supplying water to all 
the constituent units involves particular difficulties which 
vary according to the environment of the plant under 
observation. Those which live in water need much less 
complex arrangements than those which are at home on 
land, as they can absorb water from the exterior by their 
general surface, and after absorption it can easily make its 
way from cell to cell. Those which derive their supply of 
water entirely from the soil, as is the case with nearly all 
terrestrial plants, need a specialised mechanism for trans- 
port of the water after it has been taken up. 
On the other hand, the supply of a suitable atmosphere 
to the interior of the plant for the service of its more 
deeply seated protoplasts is attended with more difficulty 
in the case of an aquatic than a terrestrial plant. 
In cell-masses, therefore, such as are found in all plants 
possessing more than microscopic dimensions, we meet 
with considerable differentiation of the plant-body, both 
in form and structure. The explanation of the details 
of such differentiation is to be found in the division of 
labour which the size and the mode of life of the particular 
plant demand. 
In all the higher plants the cell-mass or body of the 
plant can be seen to possess a subterranean portion—the 
root system—and a subaerial portion—the shoot, each of 
which has its own functions to discharge, and is exposed to 
particular dangers against which it needs protection. 
The first advantage secured for the plant by its root 
system is a firm anchorage to the soil. This is not secured 
without difficulty and even danger, for to become fixed 
in the soil the root must penetrate it, a process which it 
can only carry ou by the slow method of gradual growth. 
The composition of the soil offers certain difficulties to this 
penetration ; it may be too dense or too powdery, too dry 
