682 PART IV. THE PHYSIOLOGY OF PLANTS. 



as Monofcropa and Neottia). The root-hairs not only promote the 

 absorption of water, but also the absorption of salts from the soil, 

 coming, as they do, into very intimate relation with the minute 

 particles of the soil. They thus give the root a firmer hold on the 

 soil, and render it more serviceable as an organ of attachment. 



Roots are capable of absorbing certain substances in the soil 

 which are insoluble in water, or even in water holding carbon 

 dioxide in solution. This is due to the fact that the absorbent cells 

 (including root- hairs) contain acid sap, which saturates the walls, 

 and can thus act upon substances with which the cells may be in 

 contact. 



The tegumentary tissue of aerial roots is specially modified in 

 accordance with their external conditions. They have no root- 

 hairs (except when they come into contact with a moist surface), 

 but they have a more or less well-developed velamen (see p. 155)> 

 which serves as a means of absorbing water- vapour and gases from 

 the air. Sometimes these roots contain chlorophyll, and act as 

 assimilatory organs : this is also the case to a remarkable degree 

 with the roots of the (aquatic) Podostemaceee (see p. 63). 



The primary internal structure of the root is remarkably con- 

 stant through all the various groups of plants. It is always mono- 

 stelic with the vascular bundles compacted into an axial cylinder 

 (p. 171): between the axial cylinder and the epiblema is a con- 

 siderable parenchymatous cortex, the cells of which take up from 

 the absorbent cells the water and substances absorbed, and transfer 

 them to the tracheal tissue of the bundles for conduction to the 

 upper parts of the plant. 



The growing-point is usually provided with a root-cap (pp. 61, 

 154) to protect it as it makes its way through the soil. 



In many cases the root becomes adapted to serve as a depository 

 of reserve plastic materials : such are the tuberous roots (p. 63) of 

 various plants, in which secondary growth in thickness (see p 195) 

 produces a large amount of parenchymatous tissue, in the cells of 

 which the plastic substances (starch, etc.) are deposited. 



The physiological adaptation of the root is even more remark- 

 able in its properties than in its structure, as is shown by its 

 irritability to the action of various stimuli. Thus the action of 

 the force of gravity causes roots (at least primary roots) to grow 

 towards the centre of the earth (positive geotropism) : the action of 

 light, as a rule, causes the growing root to curve away from the 

 source of light (negative leliotropism) : a moist body causes the 



