l62 PLANT ORGANS OR PARTS OF PLANTS. 



to 40 cm. long. The individual pieces of bark vary 

 from I to 3 mm. in thickness. 



The inner side is dark-brown, almost to black at times, 

 the outer side is usually lighter — grayish-brown — in its 

 general tone. The differences in the amount of the cork 

 that has been scraped from the outer surface is the cause 

 for the variations in the color. When the cork has been 

 removed entirely, the bark may be reduced to i mm. in 

 thickness and have a clear reddish-brown exterior. 



The fracture is even, somewhat sharp and short, taste 

 and odor aromatic, at the same time astringent and 

 mucilaginous. 



Histology. — With a low power the cross-section is 

 reddish-brown and shows near the centre a strongly 

 refractile white line. On the outer side of this, near the 

 cork, small whitish spots can be noticed, and the inner 

 side shows fine radiating structures with numerous large 

 empty spaces in the inner bark. 



Under magnification of from 400 to 600 diameters, 

 the cross-section of the larger and thicker pieces shows 

 on the outside an even, many-layered brownish cork 

 sheath, the outer cork cells having thin walls ; the inner, 

 thicker walls, making what is usually called stone cork — 

 the * ' bork' ' of some writers. Just beneath the outer bark, 

 the parenchymatic cells of the middle bark are arranged 

 somewhat tangentially. Here and there are found stone 

 cells. These parenchymatic cells are usually rich in 

 starch. Irregular oval mucilage cells are also present in 

 this parenchyma. Marking off the middle bark from the 

 inner bark is a line of mixed stone cells and bast fibres, 

 which under the low power produces the white line. This 

 is not continuous, being broken here and there by numer- 

 ous thin-walled parenchymatic cells. 



The inner bark consists mainly of parenchymatic cells ; 

 these are smaller than those in the middle bark and are 

 traversed by the medullary rays, which vary from two to 



