18 VEGETABLE PHYSIOLOGY 



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. 

 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. 



The first indication of this differentiation in the vegeta- 

 tive body of the plant is a change in the character of the 

 exterior, which has for its object the 

 protection of the plant from external 

 injurious influences. This can be 

 seen even among the seaweeds, simple 

 as is generally the structure of mem- 

 bers of this group. Fucus and its 

 allies, which form part of the class 



FIG. 19 PIECE OF THE 



THALLUS OF Peivetia, of the brown Algae, have their external 



SHOWING CHARACTER OF -, , , -,, -, , 



THE EXTERNAL AND SUB- cells much smaller, more closely put 



CELLS, x 300. together> and generally much denser 



than the rest of their tissue (fig. 19). In the group 



of the Mosses certain arrangements of this kind can 



