52 BOTANY 



Let us remove a twig with its expanded leaves to a con- 

 fined space, so that we examine the conditions of the air 

 around it to see what changes, if any, take place. Let 

 us shut it up in a well-dried bottle and keep it at its 

 accustomed temperature. We shall find after a short 

 time that the sides of the bottle become bedewed with 

 moisture, and a little later we shall see that the leaves 

 upon the twig and at least its upper part become wilted 

 and drooping. Part of the work of the shoot is clearly 

 to exhale watery vapour from its surface. 



FIG. 20. Section of leaf showing intercellular spaces and stomata. 

 The cells contain chloroplasts. X8o. 



If careful measurements are made of the total water 

 a plant gives off, it is found to be very considerable in 

 amount, and to be given off during the whole of the day 

 in quantities varying with the changing conditions sur- 

 rounding the plant. The structure of the leaf, to which 

 we must give later some careful attention, shows us 

 that the intercellular spaces which we observed in the 

 body of the root exist in even greater degree in the leaf 

 (Fig. 20) and yield an evaporating surface much larger 

 than the external surface of the twigs and leaves. 

 These internal channels communicate with the exterior 

 through small openings in the limiting membrane of the 



