STRUCTURE AND WORK OF PLANTS 15 



In summing up the structures of the leaf we may say that 

 it usually consists of a petiole and a blade. The outer portions 

 of the blade both above and below are the epidermis; in ad- 

 dition to the ordinary epidermal cells the epidermis contains 

 special structures, the stomata ; within the epidermis are the 

 veins and masses of green tissue ; the green tissues are made 

 up of more or less compact cells in which, in addition to other 

 cell contents, are plastids, which contain the green coloring 

 matter, chlorophyll. 



16. Leaves: material for leaf work. In connection with the 

 discussion of roots and stems it was found that water is carried 

 into the leaves. In the soil are many substances which are 

 dissolved by the water, just as common salt or sugar would 

 be. When water is taken up into the plant some of these sub- 

 stances that are in solution also enter. In this way there may 

 be carried into the plant compounds containing such things as 

 nitrogen, potassium, phosphorus, sulphur, and iron. Through 

 the surface of the leaf, chiefly through the stomata, the plant 

 secures carbon dioxide. This is a gaseous substance which 

 exists in the atmosphere in the ratio of about 3 parts in 10,000 

 of air. Inside the leaf, therefore, there is a supply of the so- 

 called raw materials for food, as water, carbon dioxide, and 

 substances that were in solution in soil water. 



17. Leaves: food manufacture. Carbon dioxide and water 

 must undergo change before they can be used in nourishing and 

 building up the plant. The sun shines upon the leaf and the 

 chlorophyll absorbs some of the energy from the sun's rays. 

 This energy serves in some way as yet unknown to break 

 up the compounds water and carbon dioxide into the carbon, 

 hydrogen, and oxygen of which they are made. The carbon, 

 hydrogen, and some of the oxygen immediately unite again ; 

 not, however, into the compounds carbon dioxide and water, 

 but into new compounds. These rapidly pass through several 

 changes and may finally become sugar and starch. At present 

 the changes before starch and sugar are formed are not all 

 known. Some of the oxygen resulting from the breaking up 



