ON LEAVES. 57 



When the ribs of leaves are expanded upon the same 

 plane, the leaf is thin ; in succulent plants, which retain 

 moisture and evaporate but little, the cellular tissue, which 

 connects the vessels of the upper and lower surface of 

 the leaf, is thick and fleshy. 



The two surfaces of a leaf generally differ in appearance: 

 in the upper surface the ribs are the least prominent, and 

 the leaf is consequently the smoothest, and of the deepest 

 green. The under surface is more hairy, and abounds 

 with stomas or pores; the upper has fewer, or is sometimes 

 wholly deprived of them, excepting in aquatic plants, 

 whose leaves float on the water ; their upper surface being 

 alone exposed to the air, are alone supplied with stomas. 



But whether the two surfaces be similar or not, it is very 

 certain that their functions are different; for if you reverse 

 the leaf of a plant, and prevent it from resuming its nat- 

 ural position, it will wither and die. 



Emily. But corn aud grasses grow vertically, Mrs. B., 

 and can scarcely be said to have an upper and an under 

 surface ; though, it is true, they are greener and smoother 

 on one side than on the other. 



Mrs. B. All the gramineous family have a more equal 

 distribution of pores on either surface ; for growing near- 

 ly erect, and being therefore equally exposed to the air, 

 each surface can probably perform the same functions, 

 and these plants can bear the reversion of their leaves 

 better than any other. 



Floating aquatic plants, on the contrary, having no 

 pores on their lower surface, infallibly die if they are re- 

 versed without power of resuming their natural position. 



Caroline. It would be superfluous for aquatic plants 

 to be furnished with pores on their under surface, since 

 they could not evaporate into water. 



Mrs. B. Nor can they elaborate the sap without ex- 

 posing it, by means of the pores, to the atmosphere : but 

 we must complete the anatomical examination of the 

 structure of the leaf, before we enter upon its physiolog- 

 ical functions. 



296. What is said of the thickness of leaves'? 297. How do the 

 two sides of the leaf differ 1 298. What will be the consequence if 

 the leaf of a plant be reversed and kept from resuming its natural posi- 

 tion'? 299. What is said of the distribution of pores in gramineous 

 plants'? 300. What is said of floating aquatic plants'? 301. 

 And of their elaborating the sap 1 ? 



