94 



THE ASPENS AND COTTONWQOIXS POPULUS (Tourn.) L. 



This genus comprises about 27 species native to the north temper- 

 ate and arctic zones, of which number 19 are native to North America 

 and 4 to Pennsylvania. 



The trees belonging to this genus have many common names, as 

 Aspens, Cottonwoods, Poplars, or Popples. Although some of them 

 are called Poplar, still they are in no way related to the well-known 

 Yellow Poplar or Tulip Tree winch belongs to the Magnolia family. 



The leaves of some of the representatives become very conspicuous 

 on account of their trembling or quaking habit. This fluttering of 

 the leaves, even when only a slight breeze is at hand, is due to their 

 laterally compressed leaf-stalks. The buds of a few species are evi- 

 dently resinous and often pungent. Possibly no group of trees, ex- 

 cept the Willows, is so well equipped to disseminate its seeds. The 

 seeds are very light, produced in great abundance, and furnished 

 with a dense covering of long white hairs which aid in their dispersal. 



The wood of the members of this genus is just beginning to be of 

 commercial importance. It was formerly despised but is now used 

 for various purposes, especially for paper pulp. These trees have 

 some valuable merits in that they grow very fast, often on situations 

 where other species refuse to grow, especially in wet places, and may 

 easily be reproduced by cuttings, sprouts, or seeds. 



In addition to the 4 species described and contained in the sub- 

 joined key, a few other species are rather common throughout the 

 State especially as ornamental trees. The White or Silver-leaf 

 Poplar (Populus alba L.) is a native of Europe and Asia but very 

 common as an ornamental tree. It can be distinguished by its lobed 

 leaves, covered by a dense white persistent wool on the lower surface, 

 and by its twigs, usually covered with white cottony felt which rubs 

 off easily. The Lombardy Poplar (Populus nigra var. italica Du 

 Koi) is frequently cultivated in this State. It can best be distin- 

 guished by its form (Fig. 36). The lateral branches are almost erect 

 forming] a high but ina'rrow cprcywni., The leaves have flattened 

 petioles, are finely toothed, smooth, and sharp-pointed. The Balm 

 of Gilead (Populus candicans Ait.), sometimes regarded a variety of 

 the Balsam Poplar, is occasionally found as a cultivated tree and 

 frequently escapes cultivation. It can be recognized by large- resi- 

 nous buds, reddih-brown twigs, and its ovate leaves with round or 

 channeled petioles and heart-shaped base. The leaves of the closely 

 related Balsam Poplar (Populus balsamifera L.) do not have a cor- 

 date base. 



