98 



Sucking- 

 Force in 

 Leaves. 



Transpira- 

 tion. 



in solution are continually flowing by osmosis into the living 

 cells of the leaf, each of which, like the algal cell, is busily 

 engaged in the sunlight in manufacturing starch and in building 

 up protoplasm to replace that lost by respiration, yet, as the 

 water which enters is again continually exhaled in transpiration 

 and as the salts which entered with the water are continually 

 being seized on by the protoplasm, made to combine with other 

 substances and built up into protoplasm, or otherwise changed, 

 the attraction exercised by the cells for water and these par- 

 ticular salts in solution remains unchanged, so long as the vital 

 processes mentioned continue actively, and water and 

 salts are in consequence continually withdrawn by the living 

 cells of the leaves from the vessels and tracheids with which they 

 are in contact. In this way a backwardly-acting sucking force is 

 developed which, in some way not yet clearly understood, extends to 

 the roots and causes fresh supplies of water and salts to continually 

 pass into the xylem elements of the vascular bundles from 

 the root-hairs. So much, however, is certain that there is no 

 enormous pressure acting from below which forces the water up 

 to the summit of high trees, and that, although the suction 

 force acting from above is considerable, it also by itself is not 

 sufficient to account for the transpiration current. The 

 most recent researches show that, if the living cells in the wood 

 are killed, the latter soon loses its power of conducting water, 

 and it is thought that the living cells of the medullary rays 

 and wood parenchyma which are in contact with the vessels 

 and tracheids assist in helping on the flow from point to point. 

 It has, however, been ascertained that the water current ascends 

 mainly in the cavities of the vessels and tracheids and not in 

 their walls, and by compressing transpiring shoots and thus 

 diminishing the sectional area of the cavities of their elements 

 the water current can be greatly reduced. It has also been 

 ascertained that the water current ascends in the younger 

 layers of wood and that when heart- wood is present it takes 

 no part in the conduction of water. 



91. In large thin leaves the 



living cells are not shut off from the outer air so 

 effectually as are those in the stem and branches which 

 are soon protected by an outer coat, several layers of 

 cells in thickness, of cork, or other more or less impervious 

 material, and hence, although the outer walls of the epidermal 

 cells of such leaves are usually more or less thickened, a certain 

 quantity of water is continually abstracted from the leaf by 

 evaporation. This process also is largely promoted owing to 



