EXCRETION OF WATER 



has travelled up to the factories of the plant — the leaves — the 

 water has performed its carrying work and can be thrown 

 away to make room for fresh supplies of salt-solutions. In 

 the second place, transpiration helps to draw the salt-solution 

 up the stem, and even causes the root to absorb liquids more 

 quickly. It is easy to show that transpiration influences the 

 rate at which the root takes in water : for if we place a glass 

 bell-jar over a plant fitted into the apparatus figured on page 203, 

 the root will gradually absorb more 

 and more slowly as transpiration is 

 retarded by the increasing moisture of 

 the air under the bell-jar. Again, if 

 we remove the bell-jar and transfer the 

 plant and apparatus to a well-lighted 

 window, the roots will absorb more 

 rapidly than ever, because transpiration 

 has been accelerated by the strong light. 

 We can indeed use this fact as a rough 

 method for judging of the rate at which 

 transpiration is going on, merely measur- 

 ing the speed at which absorption by the 

 root, or the cut end of a stem, is pro- 

 ceeding. It is important to remember, 

 however, that the root does not neces- 

 sarily absorb the same quantity of water 

 as the leaf transpires. Plants not 

 watered, and plants whose roots are 

 kept colder than the shoot, are liable to 

 fade — that is, to lose more water than 

 they absorb. 



Fig. 244. — Apparatus for in- 

 ducing the excretion of drops 

 of water from cut shoots. Thf 



EXCRETION OF LIQUID WATER 

 FROM LEAVES. 



Early on a summer morning pearl- 

 like drops of water may be seen glisten- 

 ing on the leaves of many plants. 

 These are usually described as dew- 

 drops — that is, they are supposed to 

 have been derived from -the air. Frequently, however, this 

 description is incorrect, because the drops have been pumped 

 out from the leaves. For instance, water is thus excreted from 



clear space below the plant- 

 stem is occupied by water 

 inside the short arm of the 

 glass-tube : the water is being 

 forced into the stem bjr mer- 

 cury (which is shaded in the 

 iigure). 



