114 WATER-ABSORPTION AND TRANSPIRATION. 



151. Transpiration. Everyone knows that a leaf , plucked 

 from a living plant, becomes dry and withered after a time. 

 A Bean seedling becomes limp when pulled up and allowed 

 to " wilt," but recovers when set in water. 



Have you noticed that wherever plants are enclosed by 

 glass e.g. in greenhouses, or bell-jars covering plants 

 moisture often collects on the glass? Does this moisture 

 come from the moist earth, or from the plants, or f ro n both ? 

 It is easy to show by experiment that a healthy and vigorous 

 plant gives off water- vapour, which escapes chiefly from the 

 leaves. This escape of water-vapour from a plant is called 

 transpiration, and the current of water which passes from 

 roots to leaves is called the transpiration current. 



The root absorbs a very dilute solution of salts, hence the 

 excess of water must be got rid of by evaporation from the 

 leaves. Transpiration is, however, something more than 

 mere evaporation, for the leaf can control the rate at which 

 the water vapour is given off. 



Before returning to the subject of root absorption, we shall 

 study the transpiration current. 



152. The Transpiration Current. The amount of 

 water conveyed upwards from the roots to the leaves is 

 often very great, the water flowing upwards at from 6 ins. to 

 6 ft. per hour in trees (as much as 20 ft. per hour in climbing 

 plants with long slender stems, e.g. cucumber). 



If the stem of a Vine or similar plant is cut in spring, 

 large quantities of watery sap escape from the cut end 

 attached to the roots, 1 and if a tube filled with mercury or 

 water is attached to this cut end, by stout rubber tubing, 

 the escaping sap may support a column of mercury several 

 inches high, or one of water several feet in height. 



A Sunflower, 3J ft. high and with a surface of 5,600 square 

 ins., has been found in summer to exhale a pint of water a day, 

 which is about a cubic millimetre from each square inch. 

 Excessive and unchecked transpiration is, however, highly 

 dangerous, and hence arises the importance of an impermeable 



1 This only takes place when the plant is fully charged with water. 

 Hence, in summer, no "bleeding" is shown, although if all the leaves 

 are removed it ma}' commence after a longer or shorter interval. 



