46 



water. This escape of water in the form of water 

 vapour is known as transpiration. Let us see how 

 this is effected. On the underside (as a rule) of the 

 leaf occur innumerable minute apertures known 

 as stomata (Fig. 18). These are in communication 

 with a complicated system of spaces between the 

 inner cells of the leaf, through 

 which latter also run the vascular 

 cords. The excess water evaporates 

 into the intercellular spaces whence 

 it escapes to the exterior through 

 the stomata. 



Not only does water vapour 

 escape by the stomata but air 

 enters by them also, and one of 

 the constituent gases in the air is 

 carbon dioxide. In this way water, 

 mineral matters and carbon di- 

 oxide gas are brought into the 

 immediate neighbourhood of the 

 chloroplasts. 



These raw materials are, how- 

 ever, already fully oxidised and, 

 as we have seen above, are, in 

 that condition, useless as sources of 

 energy. In the form of carbon 

 dioxide the affinity of the carbon for oxygen has 

 been completely satisfied, as also that of hydrogen 

 for oxygen when in the form of water, and the same 

 is true of most of the other salts absorbed and carried 

 upwards by the water. The potential energy of 

 position of separation has already been liberated by 

 the union of oxygen with these other elements, so 

 that to make the elements again valuable as stores 

 of potential energy the combined oxygen must be 



FIG. 18. Stomata. 

 (X 250.) 



