DESCRIPTION OF PLATE VI 



Illustrating the Histology of a Typical Bark Showing all of the Tissues 

 Which May be Found in a Bark. A, Longitudinal section in the radial direc- 

 tion but not showing the medullary rays. B, Transverse section, i. Outer 

 bark. The demarkation between outer and inner bark is not always distinct. 2. 

 Inner bark. 3. Beginning of wood tissue, a, Epidermis. Always wanting in tree 

 trunks and older branches, b, Cork tissue, c, Bark parenchyma. Cell-walls are 

 usually not suberized and the cells may contain various inclusions such as crystals 

 of calcium oxalate, tannin, starch granules and resin, d, Groups of sclerenchyma 

 cells. These, when present, normally predominate in the outer bark, e, Crystal- 

 bearing fibers which usually accompany the bast fibers. /, Bast fibers. These, 

 when present, normally predominate in the inner bark. The fibers may occur singly 

 or in groups, g. Cambium, h, Wood fibers, i, Ducts. Usually of the typically 

 porous type, k, Medullary rays. 



An excellent typical bark having all of the histological elements indicated in 

 Plate VI is Rhamnus purshiana. The demarkation between outer bark and inner 

 bark is well defined in Ulmus and Quillaja. 



