170 



THE LIFE OF THE PLANT 



that the loss in weight is due to evaporation from the 

 plant. We can also take two glass bells of equal size, a 

 little smaller than the leaf the evap- 

 oration of which we are going to 

 investigate, so that the leaf can be 

 held tightly between them (carefully, 

 of course, so as not to crush it, but 

 /y at the same time in such a way that 

 the greased edges of the bells are 

 tightly fixed to it) . Under each bell 

 we place some substance which 

 greedily absorbs water vapour, such 

 as sulphuric acid for instance, so 

 that it may absorb the water which 

 evaporates from the leaf. By weigh- 

 ing the vessels containing the sul- 

 phuric acid we shall find out the 

 amount of water it has absorbed. 

 In this way we can solve many 

 interesting problems. We learn, for 

 instance, that it is the lower side of 

 the leaf, i.e. the one which, as we 

 have seen, bears the stomata, 1 that gives off water vapour 

 most actively. It appears that these stomata are to 

 be looked upon as the regulators of evaporation. 

 When the plant is saturated with water the slit-like 

 aperture of the stoma opens wide (fig. 48 b) and 

 evaporation increases ; but as soon as the leaves begin 

 to fade, whether on account of too much evapora- 

 tion, or too little water, the stomata contract and 

 almost close (fig. 48 a) ; evaporation decreases and the 

 plant revives. We also learn from similar experiments 

 that leaves with a bright shiny surface evaporate less 

 than leaves such as grass ; this is the reason why plants 

 with shiny leaves apparently stand dry, torrid climates 

 more easily. Lastly, such experiments teach us that 



1 See chapter v. 



FIG. 48. 



