CARBOHYDRATE MAKING 157 



on one side and then on the other. In the horse-chestnut, where 

 the leaves come out opposite each other, the older leaves of an up- 

 right branch have longer petioles than the younger ones. In the 

 case of the dandelion, a rosette or whorled cluster of leaves is found. 

 Here the leaves are arranged spirally on a very short stem. Leaves 

 with long petioles are nearest the ground while those with shorter 

 petioles alternate with them, filling the space. In the mullein the 

 entire plant forms a cone. The old leaves near the bottom are 

 very large, and the younger ones near the apex are much smaller and 

 come out close to the main stalk. In every case each leaf receives 

 a large amount of light. 



Practical Exercise 7. Bring into class as many examples of various leaf 

 arrangement as possible. 



The sun a source of energy. We have already learned that green 

 plants are the great food makers for themselves and for animals. 

 We are now ready to learn how green plants make food. We know 

 the sun is the source of most of the energy that is received on this 

 earth in the form of heat and light. Every one knows what 

 " burning glass " will do when it focuses the sun's rays on a piece 

 of paper. Solar engines have not come into any great use as yet, 

 because fuel is cheaper, but some day we undoubtedly shall harness 

 the energy of the sun to do our everyday work. Experiments have 

 shown that as much as 80 per cent of the radiant energy falling on 

 certain green leaves is absorbed. Part of this energy is used by 

 the leaf; but part is changed to heat, raises the temperature of 

 the leaf, and is later lost to the air if the air is cooler than the leaf. 

 Regulation of this temperature is obtained in much the same way 

 as in our own bodies, by evaporation of water. We perspire ; the 

 leaf passes off water vapor, largely through the stomata. 



Relation of light and air to starch in leaf. We can readily test 

 how light affects the amount of starch found in a leaf. We do this 

 by pinning strips of black cloth, such as alpaca, over portions of 

 several leaves of a growing hydrangea which has previously been 

 placed in a dark room for a few hours, and then putting the plant 

 in direct sunlight for an hour or two. We remove the partly cov- 

 ered leaves, boil them to soften the tissues, and extract the chloro- 

 phyll with wood alcohol (because the green color of the chlorophyll 



