28 EXPERIMENTAL PLANT PHYSIOLOGY. 



23. Uses of Transpiration. In the economy of the plant 

 transpiration is of the greatest importance. Water and dis- 

 solved nutrient salts are carried to the leaves by the transpira- 

 tion stream. The greater part of the water evaporates, and 

 the remainder, with the salts, is formed into compounds useful 

 to the plant. In the leaves the simple " power of selection " 

 operates as in the roots, and only the salts they can use are 

 carried to them in quantity. It is probable that transpiration 

 serves other uses which are not yet clearly understood. The 

 suggestion has been made that it equalizes changes of tempera- 

 ture in the plant. 



24. Ascent of Sap. The forces concerned in carrying water 

 from the roots to the leaves are root-pressure, capillary action 

 of the wood-cells, imbibition, diffusion, expansion and contrac- 

 tion of the air-bubbles in the wood-cells, transpiration, and 

 osmotic action of the protoplasm of the wood-parenchyma 

 cells. 



In small herbaceous plants root-pressure is almost always 

 present, and it acts with a force sufficient (see paragraph 15) to 

 drive water to the leaves. In plants of this character the suc- 

 tion exerted by transpiration is also sufficient to carry water 

 upward to the desired height. (See Experiment 26.) The 

 other factors mentioned are of minor importance in such 

 plants. 



In trees, however, which may attain a height of 10 to 150 

 meters, the manner in which the necessary water-supply is 

 carried to the leaves becomes a question of great complexity. 

 Root-pressure is present in trees only during a limited period 

 at the beginning of the growing season and is almost entirely 

 absent in summer when the greatest amount of water is used. 

 Hence it cannot bear a very important part in the ascent of 

 sap. The transpiration of water from the leaves creates a 

 vacuum in the stem below, as has been demonstrated in 



