286 



BARKS 



to the naked eye, but is seen under the lens to be finely striated longi- 

 tudinally. Older pieces are more coarsely striated. The fracture is 

 short in the outer but fibrous in the inner portion. 



The section examined under a lens shows numerous minute tangen- 

 tially arranged groups of bast fibres. Under the microscope the cork 

 is seen to consist of but few rows of cells, the outer wall of which is 

 strongly thickened and bulges outwards ; one such row of cork cells 

 is formed each year. The bast contains no groups of sclerenchy- 







FIG. 142. Willow bark, showing, A, outer surf ace'; 

 B, inner surf ace. Natural size. 



matous cells. These characters sharply distinguish willow barks 

 from poplar and other barks. 



Willow bark often has a slight agreeable odour, and an astringent, 

 slightly bitter and aromatic taste. 



Constituents. The principal constituent of commercial willow 

 bark is tannin together with a little salicin. 



Salicin, C 13 H ]8 7 , is a crystalline bitter glusoside present in the bark of 

 various species of Salix and Populus. Emulsin hydrolyses it in aqueous 



solution to saligenin, C 6 H 4 < 



OTT 



(orthohydroxy-benzyl alcohol), and 



