AMENTACE.^i; — CONIFER/E 



381 



those of P. nigra, presenting the pecuharity of being 

 much flattened ^t the end towards the leaf The 

 effect of the unusual structure of the petiole is that the 

 leaf, instead of being horizontal, as in P. alba and most 

 trees, hangs vertically; and this, again, 

 explains the similarity of the two sur- 

 faces, because 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 tha,n 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 would involve a certain 

 amount of waste ; but in the Black 

 Poplar, as Fig. 304 shows, the leaves, when hung in their 

 natural position, are quite detached from one another. 



Another peculiarity of the Black Poplar is that it 

 not only sheds its leaves, but also some of the twigs or 

 small branches. That these are thrown off, and not 

 merely broken, is clear, because they are thickened at 

 the base. It is said to have been only introduced into 

 England in 1758. 



Fig. 304.— Black Poplar 

 shoot. 



CONIFERS 



Wind flowers. The quantity of pollen is very con- 

 siderable, and may be seen falling from the trees in 

 yellow clouds. The grains are sometimes lightened by 

 the presence of two air-sacs. The flowers are sometimes 

 red in colour. This is perhaps a protection against 

 cold. The seeds of the Pines and Firs are generally 



