46 VEGETABLE PHYSIOLOGY. [LESSON 16. 



the stomata, are both unusually large, and present a very beautiful 

 view of this important structure. 



LESSOJST XYI. 



LEAVES. 



273. Leaves are expansions of the bark, developed in a sym- 

 metrical manner, as lateral appendages of the stem, and having a 

 connection with the internal part of the ascending axis. They 

 gradually expand in various ways, acquire vascular tissue, and ulti- 

 mately assume their permanent form and position on the axis. They 

 may be divided into aerial and submerged leaves, the former being 

 produced in the air, and the latter under water. 



274. Aerial leaves. These leaves consist of vascular tissue in 

 the form of veins, ribs, or nerves, of cellular tissue or parenchyma 

 filling up the interstices between the veins, and of a cuticular 

 covering. 



275. The vascular system of the leaf is continuous with the 

 stem, those vessels which occupy the internal part of the stem be- 

 coming superior, or placed on the upper surface, in the leaf; while 

 the more external are placed on the lower surface. The vascular 



FIG ^ system is well displayed 



in what are called skele- 

 ton leaves, in which the 

 cellular part has been 

 removed, and the fibro- 

 vascular left. 



276. The vascular 

 system of the leaf is dis- 

 tributed through the cel- 

 lular tissue in the form 

 of simple, or branched 

 veins. 



277. The parenchy- 

 ma (para, beside or 



Section, melon leaf. , , 



between ; chyma, any 



thing spread out, a tissue) of the leaf is the cellular tissue surround- 

 ing the vessels, and enclosed within the upper and under cuticles. 



