2S6 VEGETABLE ORGANOGRAPHY. 



the external membrane becomes mo^e firm. Thus, in 

 each leaf, the cuticle which is easily raised, is that 

 where one ought to expect to find the stomata. These 

 two laws, in appearance contradictory, modify one 

 another, and it is from their combination that all the 

 different degrees of adherence of the cuticles of leaves 

 result. 



The two surfaces are often very different from each 

 other ; generally the upper surface presents the nerves 

 scarcely projecting, and consequently has a more even 

 appearance ; it has a less quantity of hairs ; it is fre- 

 quently devoid of stomata, or has a less number than 

 the lower surface ; consequently it has also its cuticle 

 more adherent, whence it results that its colour is of 

 a more intense green. These characters of the upper 

 surface of leaves are especially very decided in most 

 trees — the Pear, &c. for example. The lower sur- 

 face presents most frequently opposite characters to the 

 preceding; its nerves are more projecting, and it bears 

 a greater number of hairs ;* it alone is provided with 

 stomata, or at least it bears a greater number than the 

 upper surface ; lastly, its cuticle is less adherent, whence 

 it results that the colour is generally paler. 



The differences between the two surfaces present 

 peculiar phenomena in floating leaves, such as those of 

 the Water Lily : these have the upper surface shining 

 and green, and the lower pale and dull, but nevertheless 

 the upper surface, being alone exposed to the air, alone 

 has stomata. 



Not unfrequently the two surfaces almost exactly 

 resemble each other, both as regards the number of the 



* This observation, which is generally true, is not so in all cases ; thus, 

 we sometimes see Astragalus Hijpoglottis with its leaflets hairy above and 

 smooth beneath. The leaves of Passerina hirsuta are woolly on the upper 

 surface and smooth on the lower ; but the number of these exceptions is 

 very limited. 



