108 LJSaSONS WITB PLANTS 



tops of pines and spruces, any more than in maples or oaks 

 (Figs. 12, 13, 14). The pupil can determine how long they persist 

 by simply looking. He knows how to distinguish the age of any 

 branch by tbe yearly "rings" and by its method of branching. Let 

 him follow a limb back, and see for how many years the leaves 

 persist. 



118. From all these observations, it is apparent 

 that the leaf- idea is a most variable one in the 

 plant structure. The leaf normally serves as foli- 

 age : that is, it performs certain functions in pro- 

 moting the life and growth of the plant ; but 

 branches may perform this function just as well. 

 The leaf may be modified endlessly, and may cease 

 to be a functional part (that is, cease to play any 

 part in the life -processes of the plant). If it is 

 impossible to determine what are leaves and what 

 are not, in certain cases, then it is likewise im- 

 possible to define a leaf ; but practice in making 

 a definition may be useful in fixing the modifi- 

 cation of the leaf- idea in the pupil's mind. It 

 is more important to dwell upon differences or 

 modifications than upon uniformities ; for, as al- 

 ready said, variations are among the greatest 

 facts in nature. 



118o. Leaves, then, may have two types of offices, — the physio- 

 logical type and the morphological type. By one office they aid the 

 plant to live and grow. By the other they afford tendrils, fly-traps, 

 and other devices, and they sometimes appear to be useless, as if they 

 were either remnants or incidental variations. In the former capa- 

 city, they are organs; in the latter, they are members. In other 



