1 88 OSMOSIS ; ABSORPTION OF WATER 



buds are opening. At this season, the warmth of the soil 

 encourages very active absorption by the roots and the water 

 taken into the plant finds no outlet : the vessels and tracheids 

 of the young wood throughout the plant become, therefore, 

 gorged with water and cutting into the stems allows the water 

 to escape. Later in summer, however, when the leaves are 

 expanded, the water absorbed by the root and forced into the 

 vascular cylinder, travels through the stem and into the leaves, from 

 whence it escapes into the air in the form of vapour as described 

 in the next chapter. The rapid loss of water from the leaves 

 results in the removal of large quantities of water from the 

 cavities of the vessels and tracheids and these latter elements 

 of the wood are then found to contain considerable amounts 

 of air as well as water ; plants cut at this season do not 'bleed.' 



Moreover, the evaporation of water from the leaves goes on so 

 rapidly that a partial vacuum is created and a negative pressure 

 is set up in the vascular system of the plant; under such 

 conditions, instead of water being pressed out with considerable 

 force from the cut stump of a plant connected with its root, 

 the stump is found to absorb any water given to it, and not 

 until it has become saturated can a positive root-pressure be 

 detected. 



Root-pressure and 'bleeding' are not confined to trees and 

 shrubs, but are observable in greater or lesser degree in many 

 plants when evaporation of water from the leaves is retarded 

 or prevented. They may be as readily observed in many 

 herbaceous plants, such as the sunflower, potato, tobacco, dahlia 

 and maize, as in woody plants. 



The force of root-pressure is usually highest in the afternoon 

 and lowest in the early morning. Like other vital processes, it 

 is influenced by external conditions : an increasing temperature 

 of the soil increases it. 



Although the pressure set up by the osmotic activity of the 

 parenchymatous cells of the cortex and other parts of the root 



