STEUC'TUitE AND WORK OF PLANTS 15 



In summing ii[) the structures of tlie leaf we may say that 

 it usually (•(insists of a jwlitile and a hladf. Tlic outer portions 

 of tlie Made botli al)o\'c aiul li(d(jw are the ijiidcnnlx; in ad- 

 dition to the ordinary epidermal cells the epidermis ccmtains 

 special structures, — the stoiiintn ; within the epidermis are the 

 veiim 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 pJitntiJii, which contain the green coloring 

 matter, chloroplnjU. 



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

 discussion of ro(jts and stems it was found that water is carried 

 into the leaves. In the scjil are many suljstances which are 

 diss(.il\'ed by the ^^"ater, just as c(_immon salt or sugar would 

 l)e. When water is taken up into the plant some of these sub- 

 stances tliat are in solution also enter. In this A\-ay there may 

 be carried into tlie plant compounds contaming such things as 

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

 the surface of the leaf, chiefly tlrrough the st(jmata, the plant 

 secures carbon dioxide. Tliis is a gaseous substance which 

 exists in the atmosphere m the ratio of about 'i 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 '\^•ere in solution in soil water. 



17. Leaves: food manufacture. Carbon dioxide and water 

 must undergo change before the}' 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 ^^et unknown to break 

 up the compounds water and carbon dioxide iirto 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 



