CH. XIl] ORIGIN OF CORK 95 



Currants, Birches, Cherries, Hazel, Ehns, Limes, Poplars, 

 &c. furnish excellent examples. 



As the epidermis cracks and dies, under the pressure 

 exerted by the thin layer of cork which is formed to replace 

 it, a tougher and more water-tight, elastic and resistant 

 covering to the delicate cortical tissues beneath results; 

 and since this cork is usually of some shade of pale brown 

 or grey, its hue, visible through the epidermis, or exposed 

 by the fissures, adds its effect in replacing the green tint 

 of the young shoot which prevailed hitherto, and which 

 was owing to the green chlorophyll of the cortex shining 

 through the translucent young epidermis. 



In by far the larger number of our trees the cork-layer 

 begins to form in the layer of cortical cells immediately 

 beneath the epidermis itself e.g. 



In these cases the growth of the cells in the radial 

 direction is followed by a number of divisions parallel 

 to the surface of the epidermis : the more superficial of 

 the daughter-cells thus formed become transformed into 

 cork, while those lying deeper either carry on the growth 

 and divisions, resulting in the development of more cork- 

 cells, or, in addition, add also to the cortical cells beneath. 

 In this way is gradually produced a layer of cork 

 (Periderm), varying in thickness according to the species 

 and to the length of the period of growth of the cork- 

 forming cells : this layer forms an elastic covering to the 

 cortex and comes more and more completely to the surface 

 as the true epidermis is sloughed off. 



