v.] POPLAR. 115 



If the surface of the leaf be examined with a 

 tolerably high power, small opaque spots will be ob- 

 served, resembling button-holes, with a thick rim or 

 border composed of two more or less curved cells, the 

 concavities being turned inwards. When dry, they 

 are nearly straight, and lie side by side ; but when 

 moistened they swell, become somewhat curved, and 

 gape open. It is difficult to realise the immense num- 

 ber of these orifices, or "stomata" as they are called, 

 which a single bush or tree must possess when we 

 remember that there are sometimes many thousand 

 stomata to a square inch of surface. In a large pro- 

 portion of herbs the two sides of the leaf are under 

 conditions so nearly similar that the stomata are al- 

 most equally numerous on the upper and on the lower 

 side. In trees, however, as a general rule, they are 

 found exclusively on the under side of the leaf, which is 

 the most protected ; they are thus less exposed to the 

 direct rays of the sun, or to be thoroughly wetted by 

 rain, so that their action is less liable to sudden and 

 violent changes. 



There are, however, some exceptions ; for instance, 

 in the black Poplar the stomata are nearly as 

 numerous on one side of the leaves as on the other. 

 Now, why is this ? If we compare the leaves of the 

 black and white Poplar, we shall be at once struck by 

 the fact that, though these species are so nearly allied, 

 the leaves are very different. In the white Poplar 

 (Populus alba), the upper and under sides are very 

 unlike both in colour and texture, the under side being 

 thickly clothed with cottony hairs. In the black 

 Poplar (P. nigra, Fig. 73) the upper and under surfaces 



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