n6 



POPLAR. 



[CHAP. 



arc, which is not frequent, very similar in colour 

 and texture. The petioles or leaf-stalks, again, are 

 unlike ; those of P. nigra presenting the peculiarity 

 of being much flattened at the end towards the leaf. 

 The effect of the unusual structure of the petiole is 

 that the leaf, instead of being horizontal as in the P. 

 alba and most trees, hangs vertically, and this again 

 explains the similarity of the two surfaces, because 



FIG. 72. Black Poplar (P. nigra). 



the result is that both surfaces are placed under nearly 

 similar conditions as regards light and air. Again, it 

 will be observed that, if we attempt to arrange the 

 leaves of the black Poplar on one plane, they generally 

 overlap one another ; the extent is larger than can be 

 displayed without their interfering with one another. 

 In foliage arranged like that, for instance, of the Beech, 

 Elm, Sycamore, or, in fact, of most of our trees, this 



