Plants in Relation to Man 517 



where it will have the sunlight for several hours. At the end of the 

 time remove the covered leaf and another that has been well exposed 

 to the sunlight. Treat them with boiling water and alcohol as pre- 

 viously described, and test with iodine solution. What does the test 

 reveal? In what parts of the leaf is the presence of starch indicated 

 by the color? In what is a little or no starch shown? What do you 

 find as to the extent of the starch in the uncovered leaf? What do 

 these facts signify? 



The uncovered leaf and the exposed parts of the other leaf 

 show by their color that they contain starch. The protected 

 parts indicate by their reaction the absence of starch. The 

 facts seem to mean that leaves manufacture starch when they 

 are exposed to light. In leaves kept in darkness not only is 

 there no starch produced but that which was formed during 

 the day is removed. If this evidence is not sufficient, test and 

 compare leaves cut off such plants as the bean, clover, or 

 geranium early in the morning with those removed late in the 

 afternoon. Observe plants which are kept in shady places, or 

 in greenhouses where the light is poor, as during a long spell 

 of cloudy weather. 



Photosynthesis. The chloroplasts or manufacturing centers 

 of the leaf produce sugars and starch only in sunlight. In the 

 dark or at night the starch is removed from the leaf to other 

 parts of the plant. This process of fixing carbon or combining 

 it with water to form carbohydrates (page 129) by means of the 

 energy derived from the sun is sometimes called photosynthesis, 

 meaning " putting together by means of light." The term 

 emphasizes the dependence upon the light of green plants in 

 manufacturing food. An idea of the tremendous work done 

 in breaking up carbon dioxid by the chloroplasts with the aid 

 of the sun's radiant energy may be obtained from the fact that, 

 outside the leaf, carbon dioxid can be decomposed by heat 

 energy of a temperature of about 2372 F. 



Proteins. Sugar, starch, and other carbohydrates are not 

 the only foods manufactured by plants. There is a different 

 group of foods, in the most important of which is found nitrogen 



