52 



THE LEAF 



the morning. Can you think of any benefit a plant might 



derive from this habit of going to sleep ? 



In order to determine whether these changes are due to 



want of Ught or of warmth, put your plant in a dark 



closet in the middle of the 

 day, without change of tem- 

 perature. After several 

 hours note results. Trans- 

 fer to a refrigerator, or in 

 winter, place outside a win- 

 dow where it will be exposed 

 to a temperature of about 



103, i04r-lGround pea or peanut: 5° C. (40° F.) for Several 



103, in day position; 104, in night posi- hours, and See if any change 



takes place. Next put your 

 plant at night in a well-lighted room and note the effect. 

 If practicable, keep a specimen for several months in 

 some place where electric lights are burning continuously 

 all night, and try to find out whether it is possible to kill 

 a plant for want of sleep. 



64. Autumn Leaves. — When trees prepare for their 

 winter sleep the sap all retires from the foliage back to the 

 stem and roots, and the leaves, having no more work to do, 

 give up their chlorophyll and fall away. It is this breaking 

 up of the chlorophyll by the oxygen of the air, that gives 

 to the autumn woods their briUiant coloring, and not the 

 action of frost. After a wet season, when the leaves are 

 full of sap and nourishing juices, the chemical changes 

 attendant upon the withdrawal of the chlorophyll are more 

 active, and the changes of color more vivid than after a 

 period of drought, when the leaves wither and fall away 

 with little display of color. 



65. The Physiological Significance of leaf adjustment 

 will be evident if we consider that the process of food 

 manufacture is entirely dependent upon the action of chlo- 

 rophyll through the agency of light. Without this agency 

 no food can be produced, though its influence is not always 



