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 
Fic, 19—Pircs or THE 
Tantus or Pelvetia, Of the brown Alge, have their external 
SHOWING CHARACTER OF 
THE ExTervan anp Sus. Cells much smaller, more closely put 
yacent Carns. x 800. 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 
