LEAVES. 



47 



the pores are more equally distributed on both surfaces, as in 

 the Iris. Those leaves which lie upon the water have no pores 

 on the under surface ; the upper surface performing the func- 

 tions usual! v belonging to the under side. 



84 1 The parenchyma appears to the unassisted eye a mass 

 of irregularly arranged cells; but by careful examination, aided 

 by the microscope, we find a remarkable regularity in the 

 arrangement of the cells. If we take a thin slice, made by a 

 vertical section, of the leaf of an Apple or Peach, and observe 

 it by a good magnifier, we shall find imme- Fig 39 

 diately beneath the cuticle, which consists of a 

 single row of cells, two or three layers of cylin- 

 drical celis arranged perpendicularly to the sur- 

 face, with very small intercellular cavities. Be- 

 tween them and the under surface are four or live 

 rows of similar cells, but differently arranged, 

 touching each other by their ends, and lying in- 

 clined to the surface of the leaf, forming com-, 

 paratively large cavities, particularly immediately 

 beneath thestomata. Fig. 39 exhibits a type of 

 the arrangement of dicotyledons ; that side of 

 the leaf which is furnished with stomata being 

 cavernous, and the opposite side more compact. 

 Those leaves which have the stomata equally 

 distributed on both surfaces, and those also which 

 have no stomata, have cells of the parenchyma 

 of the two surfaces similarly arranged. 



85. The veins of the leaf, which ramify in every direction 

 through the parenchyma, are composed of vessels inclosed by a 

 sheath of woody fiber. These veins serve two purposes, that of 

 giving form and support to the parenchyma, and affording 

 channels for the circulation of the sap to the various parts of 

 the leaf, and returning it to the stem. The veins are largest 

 where they enter the leaf, and decrease as they proceed and 

 ramify, till they are lost to our observation in the cellular tissue. 

 There are two separate venous systems in the leaf, one over the 

 other, connected by the extremities of the veins ; the upper one 

 being the system through which the sap passes into the leaf, 

 whiie the lower conveys it after elaboration back into the stem. 

 It is generally difficult, and often impossible, to distinguish these 



Internal structure of 

 a leaf. 



With those that lie on water ?— 54. What is the arrangement of the cel- 

 lular tissue in the section of an apple-leaf? How is that side of the leaf 

 containing stomata' — S5. What two purposes do the veins of leaves ac 

 complish ? How many systems ? How arranged i 



