58 MOVEMENT OF MATERIALS IN PLANTS 



in the chlorophyll-containing cells to make food, and if 

 the nitrates and other dissolved substances are also used 

 in food manufacture, they will be pulled upwards by a 

 certain amount of force. In the ducts and tracheids 

 themselves, containing longer or shorter columns of water, 

 tensions arise or exist which lift the columns of water. 

 Water and dissolved substances may be pressed from ad- 

 joining cells into the ducts and tracheids in the roots. 

 Thus several or many forces are combined in bringing 

 about the hfting of great quantities of water. Under 

 ordinary conditions of nature enough water is lifted to 

 the top of the tallest trees to supply their topmost leaves. 

 In the case of large trees the quantity of water may 

 amount to several tons per year. 



The Structure of Leaves. — The leaves of the usual 

 land plant are broad, fiat, thin expanses of green tissue. 

 The structure of leaves (Fig. 13) is by no means so simple 



Fig. 13. — Cross-eoction of I^af (ca.stor bean) showing the upper 

 and lower epidermis, pt^forated by ffiiarded openings (Htomata) and 

 enclosing the mesophyll, consisting of a palisade layer of compact 

 cells and the siK)ngJ' portion Inflow. 



as their form. Thoy are completely covered by a single 

 layer of epidermal cells which are flat, rather thin, with 

 outer walls which are water-proofed, though the inner 

 and side walls are freely permeable to water. This single 



