74 PLANT PHYSIOLOGY 



is present in thin or thick films around the soil particles, 

 the entry being molecule by molecule. It passes by 

 osmosis from cell to cell through the cortex- of the root 

 until the tracheary tissue of the vascular bundle is 

 reached. It enters these vessels (just by what force is 

 not clear) and ascends through them (also by what force 

 is uncertain). Some of it is taken out 

 by osmosis, by various parenchyma 

 cells (e.g. medullary rays) bordering the 

 tracheary tissue and passed osmot- 

 ically to the various tissues at that ap- 

 proximate level, but the bulk passes 

 on out into the leaves where it is taken 



Fio. 39. Course of , . , , n 



water into, a n d by osmosis into the parenchyma cells. 



through a land plant. ,_, , 11 u i J.T. i 



From the cells bordering the larger air 

 spaces, it evaporates into these and passes as vapor out 

 through the stomata. 



110. The evaporation of water from a wet membrane 

 (e.g. cell wall) makes available a large amount of energy 

 for lifting up water to replace that evaporated. It has 

 been shown that the energy thus available in a leaf is 

 many times more than that necessary to lift the water 

 up to the tops of the highest trees (150 meters). How- 

 ever, though the energy is ample, the air pressure at sea 

 level is only sufficient to lift water not quite ten meters 

 into a vacuum. The osmotic pressure developed in 

 roots that are rapidly absorbing water is enough oc- 

 casionally to lift water to a height of eleven meters in the 

 grape and even twenty-five meters in the Birch (Betula 

 luted). The distance that this root pressure will lift 

 water plus the height air pressure will lift water into a 

 vacuum fall far short of the distance water must be 

 lifted in tall trees. It has been suggested that perhaps 

 the cohesion that exists in water in narrow vessels 



