56 MOVEMENT OF MATERIALS IN PLANTS 



hairs, only a short distance back of the tip, is almost 

 directly under what may be called the eaves of the dome 

 of foliage. There results from this position the contact 

 of the absorbing root-hairs with those particles of soil 

 most likely to hokl water on their surfaces. The ab- 

 sorption of water and of matter dissolved in the soil 

 water, taking place through the hairs in contact with 

 soil particles, must be equalled by the transfer of these 

 substances from cell to cell until they are used. 



Movement of Water and Dissolved Substances. — 

 The movement of water and of solutes from the root- 

 hairs to and through adjacent cells finally carries them 

 to the vascular or conducting tissues. These, with the 

 mechanically strengthening fibrous tissues, form the core 

 of the root. Into the cells and vessels, the so-called 

 tracheids and ducts, of the wood the absorbed water and 

 solutes are discharged by the adjoining cells. Through 

 the vessels of the wood of the roots the solution is carried 

 to the similar tissues forming the wood of stem, branches 

 and leaves. Thus there is a conducting system for the 

 transfer of water and raw materials which is continuous 

 from tip to tip of the higher lanil plant. Through this 

 the solution is moved by means not yet perfectly under- 

 stood, but with a speed which, except in severe conditions, 

 insures the plant against wilting. 



The Structure of Stems. — The stem of the ordinary 

 land plant gives mechanical support and furnishes the 

 paths along which the substances absorbed by the root 

 reach the leaves in which food manufacture is mainly 

 carrieti on. Strengthening and conducting tissues, there- 

 fore, make up the bulk of the stems, branches, twigs, and 

 even stalks of leaves, flowers, and fruits. Stems, living 

 and growing in the air, are only feebly supported by it 

 and hence tlevelop mechanical tissues quite different from 

 those of roots. The place and mode of growth of stems 

 is also correspondingly different. Stems are formed of 

 segments called joints, or nodes and inteniodes, and 



