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 
Fia. 39.—Course of No aes 
water into, an d by osmosis into the parenchyma cells. 
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 
lutea). 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 
ae ae. 
