238 THE SOLID COMPONENTS OF PLANTS. 



cellular, and varying in different plants. The elder 

 Saussnre concieves the epidermis to be a fine, trans- 

 parent, unorganized pellicle. The pores, by which 

 the insensible perspiration escapes, are so minute, that 

 they are quite invisible, and with difficulty permit the 

 passage of air through them. Thus, if an apple be 

 put under the receiver of an air-pump, and the air 

 withdrawn, the cuticle of the apple will be lacerated 

 by the dilation of the air contained in the pulp of the 

 fruit. There are oblong pores also in the cuticle of 

 herbaceous plants in particular, as was first observed 

 by Decandolle, who named them cortical pores. The 

 size of these is considerably greater than that of the 

 former ; and varies in different plants. 



The epidermis seems to be entirely destitute of 

 longitudinal vessels. When applied very closely to 

 the cellular layer below it, the greater portion of the 

 light is transmitted through it and reflected from the 

 cellular layer, and not from the transparent substance 

 of the cuticle ; so that the color of herbaceous stems 

 or twigs is that of the cellular layer, and not of the 

 cuticle itself; yet in trees and shrubs, which annually 

 renew the cuticle, as the Plane, Birch, Currant, and 

 others, the epidermis, when beginning to peel off, be- 

 comes more opaque and does not transmit the light, 

 but reflects it from its own surface. Thus the old cu- 

 ticle of the Plane (Platanus) is dark colored, while 

 the new is of a light green hue ; the stem of the Birch, 

 from which layers of epidermis are continually peeling, 

 is white, while the young branches are brown ; and 

 the old branches of the Currant are dark brown, while 

 the young shoots are a very light green. In some 

 plants, instead of being thrown off in plates, or in 

 layers, the old cuticle is cracked and reduced into 

 powder. 



Although the epidermis is not cast off from all plants 



