40 THE INNER ORGANIZATION OF TREES. 



first year only in a few cases. In general, these cells die 

 toward the middle of summer, and the epidermis becomes 

 fissured and rent. The epidermis is therefore absent from 

 the old stems of forest trees. The pores must be regarded 

 as a variety of the epidermal cells. 



The Cork Cells or Tuberous layer. These cells lie imme- 

 diately beneath the epidermis. They are more or less flat, 

 tabular, and thin- walled cells of a brown color. They form 

 themselves under the epidermis, and usually appear first as 

 lenticels through the chinks of the epidermal layers. The 

 commencement of the cork formation can be observed 

 under the epidermis of the young branches of the oak, 

 birch, and beech, in the middle of summer, or at the com- 

 mencement of autumn. It is the cork which gives to the 

 trunk of trees their peculiar color and rugged appearance. 

 "When the cork-cells form, the epidermis speedily dies off, 

 and the cork supplies its place and in some measure its 

 function. It restrains, equally with the epidermis, the eva- 

 poration from the underlying cells, and affords them an ex- 

 cellent protective shelter from hurtful outward influences. 

 Cork invariably forms itself over the wounds of plants as a 

 protective envelope. Therefore, although the life of the 

 cork-cells is of short duration, they form when dead, a very 

 important tissue, and continue to be of considerable service 

 to the tree. The rough fissured bark on the outside of old 

 trees chiefly, consists of layers of dead cork-cells. 



The BastCells. These form the fibrous portion of the inner 

 bark, and develope vertically as elastic tubes, more or less 

 elongated and thick-walled, which usually lie together in 

 fascicles or bundles, and are united with considerable force. 

 It is these bundles which constitute the textile fibres in 

 flax and hemp, and in general in all plants cultivated for the 

 fabrication of clothing. The bast-cells are also a very 

 important tissue for the manufacture of paper and cord- 

 age. They elongate themselves with the parts of the 

 plant in which they originate without forming new cells, 

 and often attain to a considerable length. It is in the bast- 

 fibres that the sap descends after its elaboration in the 



