38 FAMILIAR TREES 



commonly cultivated American species, P. moni- 



lif'era, wrongly known as the Black Italian Poplar. 



This, and to a considerable extent the other species 



also, are now largely used for paper-pulp. From its 



rapid growth and softness. Poplar wood has come to 



be thought perishable ; but this is said to be only 



true when it is alternately wet and dry, according to 



the old saying — 



" Though heart of Oak be e'er so stout, 

 Keep me dry, and I'll see him out." 



When impregnated with a saccharine solution by the 

 new method known as Powellising, Poplar becomes 

 darker and harder and takes a good polish, so that it 

 might be employed as a cabinet Wood. 



The long, slender leaf-stalks, which are laterally 

 compressed, causing- the leaf-blades to hang obhquely 

 downwards and thus to quiver in the slightest 

 breeze, are characteristic of the whole group. Occur- 

 ring thus in a group of trees remarkable for rapid 

 growth, the power of draining marshy ground by a 

 wide-spreading system of roots, a spongy stem and 

 copious transpiration, it is not unnatural that Herbert 

 Spencer should have suggested a connection between 

 this constant leaf-movement and transpiration. To 

 draw the watery sap up into the leaf, transpiration is 

 constantly going on from its surface, and Spencer's 

 suggestion was that the swaying of bough and leaf in the 

 wind gives an upward thrust to the circulating liquids. 

 That evaporation may be unimpeded, the leaves are 

 spread out to the air on separate stalks, and in diverg- 

 ing directions. It has, however, been suggested that 

 this quivering serves only to shed water from the 



