LOSS OF WATER 43 



should be made perfectly air-tight by means of a piece of 

 rubber tubing, about i inch long, extending over the end 

 of the glass tube. Fill the glass tube with water, then 

 invert it, and place the lower end in a dish of mercury, 

 having care that the water remains up in the tube far 

 enough to cover the end of the branch (Fig. 33). As 

 transpiration proceeds, the pressure of the atmosphere 

 will force the mercury up the glass tube as rapidly as the 

 water passes into the plant. This experiment is some- 

 times said to illustrate the " lifting power" of transpira- 

 tion, but from the explanation here given, it is seen that 

 the mercury is not lifted, but pushed up the tube by the 

 pressure of the outside air. This experiment should not 

 be regarded as illustrating more that it really does; it 

 does not, for example, explain the rise of sap in plants. 



41. Ascent of Sap. It is a well-known fact that, 

 although living leaves deprived of water merely become 

 wilted, dead leaves eventually dry up; they cannot supply 

 themselves with water, although evaporation is taking 

 place from their surfaces, and although the stem to which 

 they are attached is abundantly supplied. We must 

 conclude, therefore, that merely physical forces (imbibi- 

 tion and evaporation) are not sufficient to account for 

 the rise of liquids in stems. Recent experiments indicate 

 that, in this connection, much importance should be 

 attached to the secretion of substances by the leaf a 

 physiological process. 



We are familiar with such action in the secretion of 

 nectar by the nectar-glands of flowers (Fig. 34). Some 

 leaves (e.g., Colocasia aniiquorum) also secrete water so 

 rapidly that it falls in drops from their tips. It is 

 probable that, in transpiration, the protoplasm in the 



