116 STRUCTURE AND FUNCTIONS OF LEAVES 



the newly formed starch from the green cells of leaves for use 

 in other parts of the plant body. Much of this food (carried 

 about in the form of a solution of sugar) is used for building 

 material, as suggested in Sec. 136 ; but a good deal of it is often 

 transported to parenchyma cells of the stem and the roots, 

 where it is changed back into starch for storage. This change 

 is accomplished by small structures known as leucoplasts in the 

 cells. Each leucoplast may cause a deposit, upon some part of 

 its outer surface, of successive layers which finally develop into 

 a complete starch grain. How the leucoplast is able to bring 

 about the change from starch to sugar is unknown. 



139. Transpiration. The process of giving off water in the 

 form of vapor from the stomata of plants is called transpiration. 

 It is not a mere drying up, such as occurs when a pile of sea- 

 weeds or a split stick of cord wood is exposed to dry air, but is 

 an important function of the leaves of most seed plants and of 

 the higher spore plants. In such forms as the cactuses (Fig. 50), 

 which are practically leafless, transpiration is performed by the 

 epidermis of the stem. 



As already mentioned (Sec. 36), ordinary terrestrial seed plants 

 are, during the active periods of their lives, continually absorbing 

 water through the roots. This water brings with it dissolved 

 salts from the soil, many of which are used in the tissue- 

 forming work of the plant body. Some of the water, but only 

 an insignificant portion of the wliole amount, is needed for 

 photosynthesis, and a good deal of it is useful in carrying the 

 soluble plant foods, such as sugars, to the growing parts; but 

 there remains a large excess of water to be excreted, and this 

 duty is mainly performed by the mesophyll, and its amount is 

 regulated by the epidermis of the leaves. The air within the 

 intercellular spaces of the mesophyll is surrounded by thin- 

 walled cells filled with watery protoplasm, and it must there- 

 fore be nearly or quite saturated with moisture. When allowed 

 to escape from the leaf this air rapidly carries off quantities of 

 watery vapor. 



