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 ah 1 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 oufc 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 



