86 , FOUNDATIONS OF BIOLOGY 



The water which enters the root hairs is given a start up 

 the^stem by 'root pressure' due to osmotic phenomena in the 

 multitude of cells of the root. This pressure in an actively 

 growing tree in spring may be nearly forty pounds to the 

 square inch. Passing from the xjlem of the root the 

 ascending water enters the similar region of the steru^ Here 

 the conducting vessels are of two kinds: namely, greatly 

 elongated single cells, known as TRACHEIDS, and DUCTS. 

 The latter are really tubes which have been formed by the 

 absorption of the contiguous walls of many long cells ar- 

 ranged end to end. The mature tracheids and ducts, though 

 originally derived from living cells, are devoid of protoplasm, 

 and form a series of non-living tubes extending up the stem 

 to the leaves, through which they are distributed in the veins. 

 This is the fluid-conducting highway from root -to lejf^ 



The supply of carbon which the plant needs is obtained 

 from carbon dioxide which enters the leaves through the 

 stomata. The water, containing various salts in solution, 

 which has been taken in by the roots, meets the carbon diox- 

 ide in the chlorenchyma cells; and it is here that these 

 raw materials are manufactured into food. Therefore, the 

 leaves are the organs specialized for assembling the materials 

 of the inorganic world and forming from them new chemical 

 compounds of such a character that they can be utilized as 

 building material for the plant body and as sources of energy 

 for carrying on the vital functions. In other words, the new 

 compounds are the food of the plant. As we know, through 

 the radiant energy of light a complicated series of chemical 

 reactions are initiated by which carbon, oxygen, and hydrogen 

 are united to form a sugar. In this process, free oxygen is' 

 evolved which may be used in respiration or liberated through 

 the stomata. Part of the sugar thus formed is directly util- 

 ized by the plant as fuel, and part is employed as the basis for 



