THE LEAVES 87 



from the tree in September, and a slicing cut is made 

 through the base of the petiole and the twig, the 

 separatory layer may then be seen with a magnify- 

 ing glass. Trees which usually shed their leaves 

 are sometimes seen to retain them during the winter. 

 This is usually due to the fact that the formation of 

 the separatory layer has been interrupted by cold or 

 drought. 



112. The length of life of leaves. The leaves 

 of the conifers and other evergreen trees may remain 

 in place on the branches two or three years, or even 

 longer. After a time, however, they become dam- 

 aged by the wind, and by insects or other agencies, 

 and are cast off. An evergreen tree is casting its 

 leaves almost continually, but as it loses but a few 

 at a time it is not noticeable. Quite a deep layer of 

 needles may be found underneath almost any pine 

 tree. 



113. Growth of leaves. The greatest variation is 

 shown in the method and rapidity of growth. All 

 parts of the organ do not grow with equal rapidity, 

 as may be seen by the following test : Mark off 

 intervals with India ink on the petioles and midribs 

 of leaves of sunflowers, narcissus, and any other con- 



