THE LEAF. 63 



that is free from veinlets. Do these end freely or anas- 

 tomose ? Is there any apparent advantage in this ? 



Assimilation. 



The chief and characteristic function of green leaves is 

 assimilation, that is, the production of organized food sub- 

 stances. 



I. Examine the leaves of Elodea Canadensis under the 

 compound microscope. Study the form and position of 

 the chlorophyll bodies contained in the cells. Are they 

 equally numerous in all parts of the leaf ? Draw two or 

 more cells showing the chlorophyll bodies in place. Com- 

 pare with these the chlorophyll bodies of Mnium or other 

 common moss. 



II. Take fresh leaves of the Elodea that has been 

 growing in a jar of water exposed to sunlight. Place 

 them in strong alcohol and allow them to remain until 

 they have lost their color and the alcohol has turned 

 green. Mount for microscopic study and test with iodine 

 solution. Starch should be found in the chlorophyll 

 bodies. It may be demonstrated still more easily in the 

 chlorophyll bands of Spirogyra and other filamentous 

 algse. 



III. By an experiment best performed by the teacher 

 or by a pupil specially appointed, the necessity of light for 

 the production of starch, and the local nature of the pro- 

 cess of assimilation is demonstrated. Take a healthy 

 Tropseolum ( u nasturtium ") growing in a flower pot, and 

 place it in the dark for two or three days. Test one of 

 the leaves for starch, which by this time should have 

 disappeared. Now place the plant where it will be exposed 

 to the bright sunlight, having previously covered a part of 



