THE FOOD OF THE PLANT 17 



densities of the cell-sap and of the solutions the more 

 rapid will be the flow from the weaker to the stronger. 



The Work of Root-hairs. — For the study of root- 

 hairs, mustard or cress seeds may be germinated on 

 damp flannel. An examination of the radicles of the 

 seedlings shows them to be covered, at a little distance 

 from the tip, by very fine hairs. These are the root- 

 hairs and they last for a few days only. As the apex 

 of the root grows, new root-hairs are constantly bemg 

 formed just behind the tip, whilst those furthest from 

 the tip die away as the new ones grow. 



It is chiefly by means of the root-hairs that absorp- 

 tion takes place. 



It is not, however, easy to demonstrate the absorp- 

 tive power of the root-hair by any simple experiment, 

 since the young root is able to absorb even when 

 deprived of root-hairs. That this is so may be shown 

 by cutting off the radicle of a broad-bean seedling at a 

 point above the root-hairs. The seedling is not neces- 

 sarily killed by this treatment, but immediately puts 

 out secondary roots above the cut. 



Moreover, when seedlings are grown with their 

 radicles actually in water, root-hairs are usually not 

 formed at all. 



At the same time, under normal conditions of growth, 

 it is through the very thin wall of the root-hair that 

 osmosis takes place. 



The Ascent of Water in Plants. — So far then it has 

 been proved that a plant absorbs, by means of its roots, 

 water from the soil and substances that are dissolved 

 in the water. It has been seen that this solution 



f)asses up the stem and supplies the branches and 

 eaves. Something of the nature of the force which 

 causes the absorption of water and its passage through 

 the plant has been learnt. Of this passage of the water 

 there will be further discussion later, but it may be 

 stated at once that the rise of water in tall stems is by 

 no means fully understood. No adequate explanation 



