THE LEAF 73 



gives off moisture which escapes into the outside air 

 through the minute openings in the surfaces of the leaf. 

 These openings are able to open and close according to 

 conditions, and so regulate the rate at which water can 

 be given off. When the air is dry they become smaller, 

 and so hinder the escape of water. We shall see, too, 

 from our practical work that light has an important 

 effect, and that plants give off more water when exposed 

 to the light than when in the shade. When cuttings of 

 plants are being taken the shoots are separated from 

 their roots and cannot obtain much water. It will be 

 clear now why, under such circumstances, some of the 

 leaves should be cut off and the cuttings placed in the 

 shade* 



We can easily measure how much water a single 

 leaf or a whole plant gives out during a certain time, 

 and experiments for doing so are described in the 

 practical work on this chapter. 



Another point of interest is to find out whether this 

 water is given off equally from the upper and lower 

 surfaces of the leaf, or more from one surface than the 

 other. An experiment to enable this to be ascertained 

 is also described. 



In a long drought there is often insufficient water to 

 counterbalance that given off by the leaves, and, 

 although the pores may be closed, there is danger of 

 injury to the plant from excessive loss of water. To 

 prevent this, some leaves for instance, of many grasses, 

 and particularly those which grow in dry, sandy places 

 have the power of curling themselves up so as to cover 

 the pores (stomata) with the over-arched leaf-blade, 



