LEAVES AND THEIR WORK 



119 



young ones. In the mullein the entire plant forms a cone. The 

 old leaves near the bottom have long stalks, and the little ones 

 near the apex come out close to the main stalk. In every case 

 each leaf receives a large amount of light. Other modifications 

 of these forms may easily be found on any field trip. 



The Sun a Source of Energy. We all know the sun is a source 

 of most of the energy that is released on this earth in the form of 

 heat or light. Solar engines have not come into any great use as 

 yet, because fuel is cheaper. Actual experiments have shown that 

 vast amounts of energy are given to the earth. W T hen the sun 

 is in the zenith, energy equivalent to .one hundred horse power 

 is received by a plot of land twenty-five by one hundred feet, the 

 size of a city lot. Plants receive and use much of this energy by 

 means of their leaves. 



The Structure of a Leaf. Let us 

 now examine with some detail the 

 structure of a simple leaf of a dico- 

 tyledonous plant. 



A green leaf shows usually (1) a 



Palmately-veined leaf of the maple. 



The skeleton of a netted- 

 veined leaf : M.R., midrib ; 

 P., the leafstalk or petiole ; 

 V., the veins. 



flat, broad blade which may take almost any conceivable shape ; 

 (2) a stem or petiole which (3) spreads out in the blade in a 



