ANATOMY AND PHYSIOLOGY OF PLANTS. 227 



yet when partly dry, will become plump again in a few hours, if 

 plunged into water, the cuticle therefore must be very curiously 

 constructed, so as to admit of ready absorption, and very tardy per- 

 spiration. 



27. The epidermis allows also of the passage of air, 

 as is proved by experiments on the functions of leaves. 



28. The epidermis is usually transparent and co- 

 lourless, particularly in young and succulent twigs, and 

 consequently, in such cases, light is very readily ad- 

 mitted. 



When the epidermis is applied very closely to the cellular integu- 

 ment below it, which is the case in herbaceous plants and in young 

 stems and branches, the greater portion of the light is transmitted 

 through the cuticle and reflected from the cellular layer. 



29. In trees and shrubs which annually renew their 

 epidermis, it is found, when it begins to peel off, to be- 

 come more opaque, and does not transmit the light, but 

 reflects it from its own surface. 



By way of illustration, the whole cuticle of the plane is dark co- 

 loured, while the new is of a light green hue, and the old branches 

 of the currant are. of a dark brown, while the young shoots are of a 

 very light colour. 



30. In the permanent parts of the woody and peren- 

 nial plants, the old epidermis often disengages itself 

 spontaneously, but in herbaceous plants, and in the leaf, 

 flower, and fruit of other plants, it never disengages 

 itself spontaneously. 



Where the cuticle spontaneously separates, as in the currant, birch 

 and plane-tree, in which it appears to be undergoing a continual 

 waste and repair, it is again regenerated, even though destroyed by 



