108 SURFACE OF TWIGS [CH. 



the colour of the corky layers themselves, and partly to 

 the transmittal of the green hue beneath, in a subdued 

 shade, through the more opaque periderm. Moreover 

 the shade of colour may be modified by the persistence, 

 relative opacity, and colour of the films or flakes of dead 

 epidermis and by hairs, secretions, and other peculiarities. 

 It is chiefly due to the gradual thickening and increasing 

 opacity of the periderm, however, that the colour of the 

 twigs changes as they become older, and pass into the 

 stage we term branches. With respect to the surface 

 characters of twigs, there are several points to be noticed. 



As already stated the predominating colours are greys, 

 olives and browns, more or less sharply marked off from 

 the green hue of the shoots ; but conspicuous exceptions 

 exist. 



Pure whites are not common, but they occur on the 

 branches of the Birch, usually mingled with patches of 

 grey and brown: grey whites are met with in Barberry, 

 Beech and Walnut. 



Greys are common. The silvery metallic greys of the 

 twigs of Sea Buckthorn and Elceagnus are due to scurfy 

 stellate hairs, which reflect the light ; and the glistening 

 and often silvery greys on twigs of Oak, Chestnut, &c., 

 are due to the as yet uncast epidermis, and a grey glint 

 is frequently seen on twigs of Ribes, Blackthorn, Cherries, 

 Hawthorn, &c., due to the same cause. 



Whitish greys, passing to ash-grey and dirty greys, are 

 common on twigs of Red and Black Currants, Buckthorn, 

 Gooseberry, Lonicera Xylosteum, Salix aurita, Yew, Oak, 

 Guelder Rose, Hornbeam, Rowan and Populus alba. 



Pale greys also occur in Robinia, Pyrus Sorbus, Birch, 

 Euonymus and the Dogwood; and in Elder, Rowan, 

 Cherry, Prunus Avium, Lycium, Symphoricarpos and 

 Currants. Dull greys are also found in the Beech, and 



