POWER CONCERNED IN ASCENT OF WATER 



117 



It seems that the paUsade and spongy cells in the leaves must 

 be exercising a powerful osmotic pull on the water in the tracheal 

 elements in the veins; and this 

 pull communicated downward 

 might explain the negative pres- 

 sure in the water highways when 

 transpiration is rapidly going on. 



Although the medullary rays 

 and wood parenchyma are in 

 direct contact with the tracheal 

 elements and take water from 

 them (Fig. 60), it is now very 

 doubtful whether the living cells 

 in the rays and wood paren- 

 chyma are essential factors in 

 water ascent, for many liters of 

 water have been drawn up 

 through the trunk of a tree after 

 the tissues in the trunk have 

 been killed by poisonous solu- 

 tions to the height of sixty feet. 

 Again, when a branch with 

 foliage is placed in a solution 

 of indigo-carmine, a stain which 

 does not at all enter living cells, 

 the dye is found nevertheless to 

 rise in the tracheal elements 

 along with the water; and it may 

 be inferred from this that the 

 water holding the dye in solu- 

 tion has risen without being 

 drawn into, and forced along by, 

 living cells. Osmotic intake by 

 the roots, capillarity, osmotic suction by the mesophyll cells of 

 the leaves, acting together, seem to be the chief forces concerned 

 in water ascent. 



Fig. 60. — Diagram to show the path 

 of the water as it rises to, and escapes 

 from, the leaves. 



