THE INNER ORGANIZATION OP TREES. 39 



anatomical structure. We see that the bark and pith are 

 composed of a number of bladderlike vesicles or cells of a 

 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 ex- 

 amination 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 chlorophyl, 

 and are united among themselves with an extraordinary 

 degree of force, they may be separated from the subjacent 

 tissues, without being detached from each other, as a con- 

 tinuous, transparent plate or membrane. In the young and 



