AMENTACEiE 



379 



snow. The advantage of a toothed edge in snowy 

 regions has been already indicated. On the other liand, 

 teeth would retain moisture, and in wet regions, there- 

 fore, an entire edge tapering to the stalk is an advantage, 

 because it carries oif the moisture more rapidly. (See 

 also Cerastium alpinum, p. 28.) 



S. lanata. — This is also an Arctic and Alpine species. 

 The stomata are protected by thick, soft, silky wool. 

 This not only serves to prevent the stomata from be- 

 coming clogged, but also keeps a layer of comparatively 

 warm air, thus protecting the leaf from extreme cold. 



POPULUS (Poplar) 



Dioecious, and, as a rule, wind flowers ; they are, 

 however, visited by bees for the sake of the pollen. 

 We have three species. In P. alba the under side of 

 the leaves is white, while in the other two both sides 

 are green. P. tremula (Aspen) has orbi- 

 cular or rhomboidal leaves with large 

 teeth. In P. nigra (Black Poplar) 

 the leaves are ovate triangular, tapering 

 to a point, and with small teeth. It is 

 not a true native. The seeds have a 

 tuft of silky hairs which serve the same 

 purposes as in the Willows. While in 

 most species the stipules are caducous, 

 or fall with the leaf, those of the ter- 

 minal leaf in the Poplar are persistent, 

 and assist in protecting the bud during 

 winter. In P. alba and P. tremula 

 the young shoots are downy, the female 

 catkins dense, the braeteal scales fringed 

 with hairs, and the stamens 4-12 in 

 number. On the other hand, the leaves 

 of P. alba are white and cottony underneath ; those of 

 P. tremula silky or smooth. 



P. tremula (Aspen). — The leaves are green and 

 glabrous underneath. There are two kinds. Those 

 on the upper part of the tree have long petioles. 



Fig. 303. — Populus 

 nigra. Terminal 

 "winter bud, sliowing 

 a pair of persistent 

 stipules [st] belong- 

 ing to a leaf of last 

 season. x 2. 



