LEAVES. 25 



enters the roots it becomes assimilated and lit for tlie pro- 

 duction of new cells, and that it passes upwards, forming 

 new wood or cells by a chemical process."* Observation, 

 however, has clearly established that in the leaves of 

 healthy trees chemical processes depending on light and 

 heat, and absolutely essential to the well-being of the tree, 

 ai'e continually going on, for trees shut out from the light 

 always make a feeble growth and have a blanched and 

 sickly hue, compared with the same species in the free 

 air and exposed to the rays of the sun. If one side or 

 portion of a tree is shaded or deprived of its full share of 

 light, it ceases to grow in its natural w^ay, and the shoots 

 are lean, slender, and imperfect. 



2d. Different Forms and Characters of Leaves. — ^The 

 different sizes and forms of the leaves of fruit trees, the 

 divisions of their edges, the absence or presence of glands, 

 the smoothness or roughness of their surfaces, are all 

 more or less serviceable in describing and identifying 

 varieties. 



The terms designating forms are seldom mathema- 

 tically correct, but merely made by comparison, for in- 

 stance — 



Oval (fig. 22), when about twice as long as broad, and 

 nearly of equal width at both ends. 



Oblong (fig. 23), three times or more, as long as broad, 

 and differing but little in width in any part. 



Lance Shaj)ed (fig. 24), lanceolate, when three or more 

 times as long as broad, and tapering gradually to a 

 sharp j)oint. 



Ovate (fig. 25), when twice as long as broad, tapering to 

 the apex, and widest towards the base. 



Ohovate (fig. 26), the inverse of ovate, the greatest dia- 

 meter being in the upper part. 



* Schleiden's Principles of Botaoy 



