182 BARKS. 



Neither vessels nor tracheids are to be found in barks except 

 in those few instances in which the bark contains abnormally 

 developed bundles. These elements therefore would, if found in 

 a powdered bark, indicate adulteration, but care must be taken 

 in drawing such a conclusion, as commercial barks sometimes 

 retain small portions of the wood of the tree yielding them, 

 from which they can with difficulty be separated. Amongst cell 

 contents starch and calcium oxalate are perhaps the most 

 important. Starch has already been alluded to, and the 

 student will be well acquainted with the means by which 

 starches may be distinguished from one another. 



Calcium oxalate may be entirely absent (sassafras bark), or 

 it may be present in one or more distinct crystalline forms 

 (prismatic, rhombohedral, rosette, acicular, sandy, etc.). 

 In some barks it assumes only one form, but in many there 

 is more than one present ; the rosette and the prism are most 

 commonly found in the same bark, but in cusparia the prism is 

 associated with bundles of acicular crystals (raphides) ; the latter 

 are generally agglomerated into bundles and enclosed in large 

 oval cells. Usually thb Crystals are irregularly scattered through- 

 out the parenchyma of the bark, but in some instances they are 

 restricted in their distribution. Thus, in canella bark, each cell 

 of the medullary rays contains a rosette crystal of calcium 

 oxalate, while the bast parenchyma is free from it; in the 

 pomegranate similar crystals are arranged in regular concentric 

 lines in the bast, but are not found in the medullary rays. 



When two forms of crystal, such as the rosette and the prism,, 

 occur in the same bark, the rosettes are generally scattered 

 through the parenchyma, but the prisms are restricted to the 

 cells abutting on the sclerenchymatous cells or bast fibres ; these 

 crystal-cells are often superposed so as to form long, axial rows. 



Sclerenchymatous tissue is sometimes absent, sometimes pre- 

 sent in considerable quantity ; in the latter case the size, shape 

 and thickness of the wall may afford valuable indications of 

 identity. Secretory tissue of any kind is characteristic ; suffi- 

 cient reference to tissue of this kind has been made wher 

 dealing with leaves. 



The presence or absence of sclerenchymatous fibres (bast 

 fibres) is important. They are more generally found in stem- 

 barks than in root-barks. If present, the details of the elements, 

 their thickness, the thickness of the wall, the number audsbape- 

 of the pits should be observed. 



The following are the chief tissues met with in barks : 



(1) Cork. The cells of which this tissue is composed are 

 usually rectangular in transverse section, taugentially elon- 



