The roots, flowers, and capsules of the chinchona-trees have a bitter taste with tonic properties, but the upper bark is the only part which has any commercial value.[26] The bark of trees is composed of four layers--the epiderm, the periderm, the cellular layer, and the liber or fibrous layer, composed of hexagonal cells filled with resinous matter and woody tissue. In growing, the tree pushes out the bark, and, as the exterior part ceases to grow, it separates into layers, and forms the dead part or periderm; which in chinchonas is partially destroyed, and blended with the thallus of lichens. The bark is thus formed of the dead part, or periderm, and the living part, or derm. On young branches there is no dead part, the exterior layers remaining entire, while the inner layers have not had time to develop. In thick old branches, on the contrary, the periderm or dead part is considerable, while the fibrous layer of the derm is fully developed. In preparing the bark the periderm is removed by striking the trunk with a mallet, and the derm is then taken off by uniform incisions. The thin pieces from small branches are simply exposed to the sun's rays, and assume the form of hollow cylinders, or quills, called by the natives _canuto_ bark. The solid trunk bark is called _tabla_ or _plancha_, and is sewn up in coarse canvas and an outer envelope of fresh hide, forming the packages called _serons_. The character of the transverse fracture affords an important criterion of the quality of the bark. Cellular tissue breaks with a short and smooth fracture, woody tissue with a fibrous fracture, as is the case with the _calisaya_ bark. The best characteristics by which barks containing much quinine may be distinguished are the shortness of the fibres which cover the transverse fracture, and the facility with which they may be detached, instead of being flexible and adhering as in bad barks. Thus, when dry _calisaya_ bark is handled, a quantity of little prickles run into the skin, and this forms one of its distinguishing marks.[27]