82 



HOW GREEN PLANTS MAKE FOOD 



the starch test). A test then shows that starch is present only in 

 the portions of the leaves exposed to sunlight. From this experi- 

 ment we infer that the sun has something to do with starch making 



Diagram of experiment to show that sunlight is necessary for starch making. 

 Read the text carefully and then explain this diagram. 



in a leaf. The necessity of air for starch making may also easily 

 be proved, for parts of leaves in a plant treal^d as in the previous 

 experiment, if covered with vaseline, will be found to contain no 

 starch, while the parts of the leaf without vaseline, but exposed 

 to the sun and air, do contain starch. The part of the air used 

 in starch making is carbon dioxide, which is always present in the 

 atmosphere in very small amounts, less than 4 parts in 10,000 in 

 fresh air. 



Air is necessary for the process of starch making in a leaf, not 

 only because carbon dioxide gas is absorbed but also because the 

 leaf is alive and must have oxygen in order to do its work. This 

 oxygen it takes from the air. 



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 work of a mill. In this case the mill is 

 the chlorophyll 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 product is starch,^ and a certain by-product 

 (corresponding to the waste in a mill) is also given out. This 

 by-product is oxygen. To understand the process better, we must 

 refer to the diagram of a small portion of the leaf (page 77) . Here 

 we find that the cells of the green layer of the leaf, under the upper 

 epidermis, perform most of the work. The carbon dioxide is 



1 A simple sugar is manufactured and then transformed into starch. 



