LEAVES. 



47 



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

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

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

 tions usually belonging to the under side. 



81. The parenchynia appears to the unassisted eye a mass 

 of irregulariy arranged cells; but by careful examination, aided 

 by the microscope, we find a i-emarkable 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- 

 diately beneath the cuticle, which consists of a 

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

 drical cells arranged perpendicularly to the sur- 

 face, with very small intercellular cavities. Be- 

 tween them and the under surface are fuur or five 

 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 tlie stomata equally 

 distributed on both surfaces, and those also which 

 have no stomata, have cells of the parenchyma 

 of the tw^o surfaces similarly an-anged. 



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

 through the parenchyma, are composed of vessels inclosed by a 

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

 giving form and support to the parenchyma, and afibrding 

 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, 

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

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



Vv'";)i ihosfi tliat lie on waiter ? — 84. "Wliut is the arrangement of the col- 

 Irl: 1 tl. rue in rliu .sec-ion of an apple-leaf? How is that ^idc of tlie leaf 

 contuiuin<j stomata? — 85. What two purposes do the veins of leaves ao- 

 coraplislif How many systems ? How arranged ? 



