122 



LEAVES AND THEIR WORK 



The chloroplasts, hy means of the energy received from the sun, manu- 

 facture starch out of certain raw materials. These raw materials are 

 soil water, which is passed up through the bundles of tubes into the 

 veins of the leaf from the roots, and carbon dioxide, which is taken 

 in through the stomata or pores, which dot the under surface of 

 the leaf. 



Light and Air necessary for Starch-Making. — If we pin strips 

 of black cloth, such as alpaca, over some of the leaves of a growing 

 geranium, place the plant in a sunny window for two or three days, 

 and then remove some of the covered leaves after a day of bright 



simlight, we find after ex- 

 tracting the chlorophyll 

 with wood alcohol (because 

 the chlorophyll covers up 

 the contents of the cells) 

 that starch is present only 

 in the portions of the 

 leaves exposed to sunlight. 

 From this experiment we 

 infer that the sun has 

 something to do with starch- 

 making in a leaf. The ne- 

 cessity of air for starch- 

 making may also easily be 

 proved, for the parts of leaves covered with vaseline will be 

 found to contain no starch, while parts of the leaf unvaselined 

 but exposed to the sun and air contain starch. 



Air is necessary for the process of starch-making in a leaf, 

 not only because carbon dioxide gas is absorbed (there are from 

 three to four parts in ten thousand present in the atmosphere), 

 but also because the protoplasm of the leaf is alive and must have 

 oxygen. This it takes from the air around it. 



Comparison of Starch-Making and Milling. — The manufacture 

 of starch by the green leaf is not easily understood. The process 

 has been compared to the milling of grain. In this case the mill is 

 the green part of the leaf. The sun furnishes the motive power, the 

 chloroplasts constitute the machinery, and soil water and carbon 

 dioxide are the raw products taken into the mill. The manufactured 



Starchless areas in leaves caused by excluding 

 sunlight by means of strips of black cloth. 



