96 PERIDERM AND BARK [CH. 



In Roses, Apples Pears, Willows, Viburnum Lantana 

 and some others, the cork is traced to a different origin, 

 in so far that the first divisions take place in the epi- 

 dermal cells themselves, so that the primary cork -layer is 

 from the first superficial, instead of having to push off the 

 dead epidermis proper before becoming so. 



In Pines, Larch, Ribes, Robinia, Honeysuckle, Berberis, 

 Philadelphus, Spircea, &c., on the other hand, the first cell- 

 divisions resulting in cork-formation take place more or 

 less deep down in the cortex ; so that when the first cork- 

 layer is completed since it is always impervious to 

 water it excludes from any further participation in 

 water-supplies and food-materials from within, all those 

 cells which lie to its outer side. Consequently all the 

 cortical cells on its exterior, as well as the splitting and 

 dying epidermis cells, die and are soon sloughed off. 



The depth in the cortex at which this cork-layer 

 (periderm) may begin to form varies in different species. 

 In some, indeed, e.g. Yew, Heaths, Vine, &c., it may lie so 

 deep that the region of the vascular bundles themselves is 

 invaded, and practically all the cortical tissues may be 

 starved to death and sloughed off. 



All these tissues cut out by the periderm, and con- 

 sequently deprived of water and food-supplies from within, 

 die and shrivel up as more or less brown or grey or other- 

 wise dull coloured debris, and the totality of them is 

 termed Bark. 



An interesting phenomenon in connection with the 

 replacement of the epidermis by periderm is the formation 

 of the cork-warts, or Lenticels, so prominent on branches of 

 Elder, Robinia, Rhamnus, Alder, Walnut, Pyrus, Poplars, 

 Prunus, Chestnut, Maples, Pear, Myrica, Horse-chestnut, 

 and more or less so in all European trees and shrubs 

 except Clematis, the Vine, Philadelphus coronarius, Rubus 



