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 



