148 MINUTE STRUCTURE off THE STEM. 



are so characteristic in certain kinds of bark that they may be 

 used for identification. An example is given below. 1 



417. B, Cork, which has already been described in part in 

 Chapter II., plays a very important part in the structure of older 

 bark. Its relations to the cells which produce it, and to the 

 epidermis which it displaces at an early period of its growth, will 

 be plain from an examination of Fig. 117. In its production 

 there are periodic arrests of activity just as in the case of wood, 

 and hence in cork-tissue of firm texture it is possible to detect 

 the lines of annual demarcation. When the cork of the cork- 

 oak has reached a merchantable thickness (usually in ten to fifteen 

 years), it is removed down to the phellogen, or cork cambium, 

 and from this tissue new growths begin. 8 



1 "The liber is traversed by medullary rays, which in cinchona are mostly 

 very obvious, and project more or less distinctly into the middle cortical tissue. 

 The liber is separated by the medullary rays into wedges, which are constituted 

 of a parenchymatous part, and of yellow or orange fibres. The number, color, 

 shape, and size, but chiefly the arrangement of these fibres, confer a certain 

 character common to all the barks of the group under consideration. 



" The liber- fibres are elongated and bluntly pointed at their ends, but never 

 branched, mostly spindle-shaped, straight, or slightly curved, and not exceed- 

 ing in length 3 mm. They are consequently of a simpler structure than the 

 analogous cells of most other officinal barks. They are about J to J mm. 

 thick, their transverse section exhibiting a quadrangular rather than a circu- 

 lar outline. Their walls are strongly thickened by numerous secondary depos- 

 its, the cavity being reduced to a narrow cleft, a structure which explains 

 the brittleness of the fibres. The liber-fibres are either irregularly scattered 

 in the liber-rays, or they form radial lines transversely intersected by narrow 

 strips of parenchyma, or they are densely packed in short bundles. It is a 

 peculiarity of cinchona barks that these bundles consist always of a few fibres 

 (three to five or seven), whereas in many other barks (as cinnamon) analogous 

 bundles are made up of a large number of fibres. Barks provided with long 

 bundles of the latter kind acquire therefrom a very fibrous fracture, whilst 

 cinchona barks, from their short and simple fibres, exhibit a short fracture. 

 It is rather granular in Calisaya bark, in which the fibres are almost isolated 

 by parenchymatous tissue. In the bark of C. scrobiculata a somewhat short 

 fibrous fracture is due to the arrangement of the fibres in radial rows. In 

 C. pubescens the fibres are in short bundles, and produce a rather woody frac- 

 ture" (Fluckiger and Hanbury, Pharmacographia, p. 317). 



2 As noticed in 246, the inner layer of cork-meristem may give rise to paren- 

 chyma cells containing chlorophyll. Of these cells Sanio says : "They never 

 become cork-cells, but are truly parenchymatous ; they are filled with chloro- 

 phyll, starch, and sometimes with crystals. They never become lignified, but 

 the wall remains as unchanged cellulose, and, in short, they are true cortical 

 cells. Since, then, they owe their origin to the activity of the cork-meristem, 

 but behave throughout their whole subsequent development precisely like the 

 cells of the cortex, they may be called cork-cortex cells. When they form a 

 distinctly defined layer, the term Phelloderm is appropriate " (Pringsheim's 

 Jahrb., 1860, p. 47). 



