ROOTS AND THEIR FUNCTIONS 79 
form, are generally called parenchymatous 
cells (Fig. 11, WP). The wood parenchyma, 
which largely serves the purpose of storage 
(or occasionally as an excretory tissue) is very 
often lignified. There are also other cells 
which are specialised for mechanical purposes. 
They are of various forms and sizes, and are 
grouped into more or less definite tissue 
systems. To the consideration of the latter 
we shall return when we come to consider the 
architecture and mechanics of the plant. For 
the present, however, we are only concerned 
with those tissues of the wood that are de- 
tailed for the service of translocation of water. 
The vessels and tracheids form a continuous 
communicating system in the plant, and when 
water enters this system it can readily be 
transmitted from any one point to any other, 
the direction of flow being determined by 
purely physical conditions of pressure. 
We can now endeavour to trace the passage 
of water from the soil into the water-conduct- 
ing tissue of the plants, and thence into the 
leaves, to which most of the water that is 
absorbed ultimately finds its way. The root- 
hair is in close contact with the particles of 
soil, and it not only absorbs water from it, 
but it exerts a disintegrating influence on it 
owing to the excretion of carbonic acid from 
the living cell. 
The absorption of water (which contains 
very small quantities of salts in solution) by 
the root-hairs is an active process, and it has to 
