THE INNER ORGANIZATION OF TREES. 39 



variable form, which are united among themselves and form 

 a continuous mass ; and that the woody portion of the stem 

 consists of thick-walled cells, among which are numerous 

 openings exhibiting quite a distinctive character. The exa- 

 mination of a longitudinal section of the same shoot proves 

 that these openings are the mouths of vessels of a cylindri- 

 cal form, and that the thick- walled cells are sections across 

 tubes which taper to either extremity, terminating in a point. 



Attempts have been made by botanists to classify these 

 different species of cells according to their outward form, 

 which have failed to give satisfaction : their form being too 

 variable to admit of a classification of them on such a basis. 

 A much better character, because a more permanent one, is 

 afforded by their physiological peculiarities. Guided by 

 this principle, we may readily distinguish, in the stem of 

 the beech-tree, six different species of cells. In the bark, 

 three species, viz., the cells of the epidermis, those of the 

 corky or tuberous envelope, and the bast-cells ; in the 

 wood, two species, the fibre-cells, and the vasiform or duct- 

 cells ; and in the pith and medullary rays, one species, 

 common parenchyma. 



Each of these species of cells carries its own life, has its 

 own peculiar period of growth and vital activity ; and its 

 cells differ morphologically as well as chemically from the 

 cells of the neighboring tissues. 



THE BARK. 



The Cells of the Epidermis clothe the outer part of the 

 bark of trees during the first year of their life, and subse- 

 quently the surface of those green herbaceous shoots which, 

 are annually put forth from the branches. These cells are 

 in form, flat and tabular, and as they are without chloro- 

 phyl, and are united among themselves with an extraordi- 

 nary degree of force, they may be separated from the sub- 

 jacent tissues, without being detached from each other, as a 

 continuous transparent plate or membrane. In the young 

 and tender condition of the stem of trees the epidermal 

 cells are of considerable importance ; but they survive the 



