48 



THE LEAF 



cause is due the pyramidal shape assumed by plants like 

 the mullein and burdock, with large, undi- 

 vided leaves which the light cannot strike 

 through. The foliage on the upright stalk 

 that rises from these rosettes in spring con- 

 stantly diminishes from the ground upward, 

 giving the plant the general outline of a 

 sort of vegetable Eiffel 

 tower. The upper 

 leaves, too, will gener- 

 ally be found to assume 

 a more or less vertical 

 position so as not to cut 



9i.-Le^pyramid off tOO mUch Ught from 



of mullein. those below. 

 57. Heliotropism. — If there is any 9^.- a plant that has 



u „v w^ u _, been growing rear an 



doubt about the object of all these open window, showing 



J- 1 1 . . , ., -1 . , 1 J the leaves all turned 



careful adjustments it can be settled jo,,,;^^^ d^g jj i,t 

 by placing any healthy young potted 

 plant near a sunny window and at the end of a day or two, 

 observing the position of the leaves. Then turn the pot 

 round so that the leaves will face away from the light, 

 and again, after a few days, observe any change that has 

 taken place in their position. Try the experiment as often 

 as you like and with any number of different plants, the 

 result will be the same. This movement of plants in 



93- • 



- Rhubarb plant with leaves adjusted 

 for centripetal drainage. 



94- ■ 



-A caladmm showing centrif- 

 ugal drainage. 



