CATKIN-BEARING TREES 279 



Poplar 



Poplars, like most Willows, are trees of damp places, and 

 they grow best in a deep moist soil. In such situations 

 several species are commonly planted, e. g. the White 

 Poplar (Populus alba), the young branches and leaves of 

 which are covered with white cottony hairs ; the Aspen (P. 

 tremula), whose orbicular toothed leaves are green and not 

 cottony ; and the Black Poplar, of which the tall Lombardy 

 Poplar is a conspicuous and easily-recognized variety. 



The Balsam Poplar is often planted, the opening buds 

 of which are very sticky. The Black Poplar (P. nigra, 

 Fig. 186) grows quickly and attains a height of ninety to 

 a hundred feet . Its long, slender, smooth branches curve up- 

 wards and form a loose, somewhat pyramidal, crown. The 

 long and pointed buds are covered by four scales, which 

 are modified stipules like the bud-scales of many trees. 



As in Willows, &c. (Fig. 78), the end bud dies, and 

 growth is continued by the next bud below ; therefore, 

 branching is sympodial, not, as in the Pine and Larch, 

 monopodial. 



The leaves are uprolled in the bud, and stipulate, but 

 when the bud opens, the stipules fall off along with the 

 bud-scales. The leaf-stalk is tough but very flexible, 

 flattened laterally, and the rhomboid or somewhat triangular 

 and toothed blade readily quivers in the wind. 



This modification, found in some other Poplars as well, 

 may be useful in two ways : (1) By moving readily with the 

 wind, the leaves will produce less strain on the branches ; 

 and (2) the movement of the blade will favour transpiration, 

 cause an increased upflow of sap, and therefore increased 

 food-supply, and, in trees growing in a deep moist soil, with 

 a good water-supply, will favour rapid growth. 



In the vicinity of the tree we often find young shoots 

 springing from the ground and resembling seedling Poplars. 



