40 THE INNER ORGANIZATION OP TREES. 



tender condition of the stem of trees the epidermal cells 

 are of considerable importance ; but they survive the 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 re- 

 garded as a variety of the epidermal cells. 



The Cork Cells or Suberous 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 commencement 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 evaporation from 

 the underlying cells, and affords them an excellent protec- 

 tive shelter from hurtful outward influences. Cork invari- 

 ably 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 Bast Cells. 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 cordage. 

 They elongate themselves with the parts of the plant in 

 which they originate without forming new cells, and often 



