380 BRITISH FLOWERING PLANTS chap. 



and rounded blades witli somewhat sinuate mar- 

 gins. Those on the radical shoots have shorter 

 stalks and subtriangular laminae ; and the whole leaf 

 is so arranged that the rain falling on it runs down 

 towards the petiole. Here are two small cups which 

 catch and hold it. The cells lining the cups have thin 

 walls, and secrete a resinous substance which swells up 

 when moistened. The thin-walled cells then absorb the 

 moisture, but in dry weather are protected by the 

 resinous varnish. 



The mediaeval explanation of the tremulousness of 

 the leaves was the touching legend that the Cross was 

 made of Aspen wood, and that the tree shivered ever 

 afterwards at the recollection. It has been suggested 

 that the movement helps to pump up the sap. Herbert 

 Spencer suggested that in all cases the movement of 

 leaves and branches by the wind was of use in this way, 

 but the object of the tremulousness of the leaves in 

 Poplars does not seem to be as yet clearly explained. 

 It is due to the insertion of the blade on the vertically 

 flattened leaf-stalk, as on a knife-edge. 



P. nigra (Black Poplar). — The leaves are orbicular 

 or rhomboidal, and rather coarsely toothed. They are 

 ciliate, and silky on the under side when young, after- 

 wards glabrous. The female catkins are lax, with scales 

 nearly smooth; the male flowers have 12-20 stamens. 



In most trees the stomata are situated mainly, if not 

 entirely, on the under side of the leaves. In the Black 

 Poplar, on the contrary, they are nearly as numerous on 

 one side of the leaves as on the other. Now, Avhy is 

 this ? If we compare the leaves of the Black and White 

 Poplar, we shall be at once struck by the fact that, 

 though these species are so nearly allied, the leaves are 

 very different. In the White Poplar the upper and 

 under sides are very unlike both in colour and texture, 

 the under side being thickly clothed with cottony hairs. 

 In the Black Poplar the upper and under surfaces 

 are — which is not frequent — very similar in colour and 

 texture. The petioles or leaf-stalks, again, are unlike 



