COMMON TREES 195 



nised at any time of the year by the old, black, woody 

 cones, which remain on the tree after the seeds are shed. 

 The catkins and cones of the next year are plainly visible 

 throughout the winter, and may be found even earlier. 

 The young twigs are sticky, resinous at a later time. 

 Foliage rather dense ; leaves dark-green, broad, blunt- 

 tipped, stalked, irregularly toothed ; buds set spirally, 

 stalked, protected by two scales, resinous ; old bark black. ' 



Poplars (Family of Catkin-bearers). Closely allied to 

 the willows, as the catkins and capsules show. The 

 poplars are often tall trees, especially the Lombardy 

 poplar. Leaves broad, long-stalked, often springing in 

 bunches from side-shoots. The chief groups of poplars 

 are : 



(i) Aspen-poplars. The upper part of the leaf -stalk is 

 flattened from side to side, and the leaf is on this account 

 ready to quiver with the slightest breeze. All the leaves 

 of a tremulous poplar do not quiver ; those which spring 

 from the upper side of the branch are smaller, have short 

 leaf-stalks, and move little. It is the lower, hanging 

 leaves which quiver, and they are enabled to do this by 

 the compression of the leaf-stalk in a direction perpendi- 

 cular to the plane of the leaf-blade. When the lower 

 leaves are agitated by wind they move chiefly from side 

 to side, the leaf-surface facing the light all the time, but 

 deviations from the rule are frequent. Possibly the shad- 

 ing of the lower leaves by the upper ones is thus avoided. 

 The nectaries common at the bases of the early leaves of 

 a poplar-branch are wanting in the tremulous leaves. 1 

 In aspen-poplars the leaf-buds are resinous and pointed, 

 the flower-buds blunt. White poplar, which belongs to 

 this group, has its leaves silky beneath ; old trunks of 

 white poplar have a grey bark, with large transverse 

 lenticels, reminding us of birch, and very different from 

 the fissured bark of the next group, though the base of 

 an old trunk of white poplar may be fissured too. In 



1 See Wiesner in Ludwig's Biologie der Pflanzen. 



