52 



OBGANOGRAPHT. 



leaves which contain a number of hairs, and very small stomata 

 on their sides {fig. Ill, c). 



The number of stomata also varies considerably. The follow- 

 ing table will give some idea of their abundance in leaves, 

 and it will be observed that the number of stomata is always 

 greatest in those leaves where they are entirely absent from the 

 upper surface of these organs. 



Stomata 



n one square inch of surface. 



Upper surface. Lower surface. 



Stomata are not found upon all plants. They are absent 

 from the lower orders of Flowerless Plants, as the Algae, Fungi, 

 and Lichens. In the higher orders of Flowerless Plants they 

 abound, as in Ferns and their allies, while in Liverworts and 

 Mosses they appear to be confined to certain organs. They 

 exist more or less upon all Flowering Plants and their organs. 

 They are, however, far more abundant upon those organs which 

 are green, thus occurring especially upon leaves as we have 

 seen, and particularly on their under surface. On floating leaves, 

 however, we find them only on the upper surface. They occiir 

 also on the young green shoots of plants and on the parts of 

 the flower, and in the interior of the fruit of the Wallflower, and 

 on the seed of the walnut. In such plants as have no true 

 leaves, as the Cactacese, they abound upon the green succulent 

 stems. They are commonly only found on those parts which 

 are furnished with a true epidermis, and are accordingly absent 

 in roots and all submersed parts of plants. They are also 

 absent in pale parasitical plants, from the epidermis of plants 

 growing in darkness so as to be blanched, and from the ribs of 



