OUTLINES OF TUE PHYSIOLOGY OF NUTRITION 221 



and the exit of the water is regulated. Cells or whole 

 tissues which are thus distended by sap are said to 

 be in a turgid condition. The turgid parenchyma 

 cannot go on taking up water indefinitely; as fresh 

 supplies are taken in from the soil, a portion of the 

 cell-sap already contained in the tissue is continuously 

 forced out again. The sap thus expelled from the 

 cells under pressure does not leave the plant ; it is 

 pumped into the vessels and tracheides of the wood. 

 These elements, as we already know, only contain air 

 at a very low pressure, so that little or no resistance 

 is offered to the sap which enters them. As the 

 process goes on, the column of water in the vessels 

 rises, and when a cut stem bleeds it is from the open 

 ends of the vessels, or tracheides, that the sap escapes. 



Under natural conditions, if root-pressure acted 

 alone, the result could only be that all the wood of 

 the plant would soon become completely water-logged. 

 This actually happens in certain cases ; after a damp, 

 cool night, when little or no evaporation can take 

 place, we find that the leaves of many plants are 

 studded with drops of water, which we might very 

 easily mistake for dew. If wiped off, however, the 

 drops soon reappear in the same spots, and in fact 

 they come from within, not from without, being 

 excreted by the plant. When a plant is completely 

 gorged with sap, and no transpiration is possible, it 

 gets rid of its superfluous water in the liquid form, 

 and in many plants there are special glands for this 

 purpose. 



This state of things, however, is exceptional In 



