i68 Chapters in Modern Botany CHAP. 



but that they close when drooping for lack of sufficient 

 moisture begins, and also that they are often closed at 

 night and open during the day. But the proof of this takes 

 us beyond the region of simple experiment to that of micro- 

 scopic observation. 



By observing the rapid drooping of cut flowers, and 

 their recovery when placed in water, or the need of con- 

 stantly filling up our hyacinth-glasses, or more precisely by 

 fixing a leafy shoot in a narrow bent tube of water, and 

 noticing how the water diminishes when the plant is kept 

 in a warm room, and by other more delicate experiments, 

 we can convince ourselves that the leaves of plants give off 

 water-vapour into the air. So much of the water which 

 is absorbed by the roots is indeed used in building up 

 organic substances and in growth, but by several weighings 

 of plants it is easy to prove that much of the water simply 

 disappears into the air. And when we consider that the 

 surface of a leaf is often thick-skinned, and sometimes var- 

 nished with wax, and notice, for instance, how an apple 

 without its skin dries up much sooner than one which is 

 intact, and remember how much more numerous the little 

 apertures or stomata are on the under than on the upper 

 surface, we are warranted in concluding that it is by the 

 stomata, and therefore especially by the under surface of 

 the leaf, that the escape of water-vapour or transpiration 

 takes place. Nor is it difficult to prove this by direct 

 experiment. Thus we recognise another of the uses of 

 leaves ; they are the organs of transpiration, and the little 

 apertures on their under surface, which close in drought 

 but open when moisture is abundant, are the regulators of 

 this. 



But we have not yet completed our experiments. Let 

 us place a number of growing seedlings in a glass retort 

 through the cork of which two glass tubes are passed. 



