SECONDARY LIBER. 147 



been most extensively employed up to the present time are some 

 of the species of Abies, Betula, Populus, Tilia, and Liriodendron 

 Tulipifera (in the United States sometimes called "Poplar"). 

 The chemical processes depend (1) upon the solvent power of 

 caustic soda under pressure, and with heat, upon the so-called 

 intercellular substance which unites the cells, or (2) upon the 

 similar power of a sulphite, preferably maguesic, also under 

 pressure and with heat. 



413. Bark. A, Secondary liber. Each yearly addition to the 

 inner surface of the bark is seldom plainly distinguishable from 

 those which have preceded it, and hence we cannot determine 

 positively the age of an old tree by the layers of its inner bark. 

 The bast-fibres of a single year often cling together in a strik- 

 ing manner, forming bands or strips of considerable strength, 

 and in a few cases, notably that of Daphne Lagetta, there are 

 fine meshes between the fibres, so that the inner bark seems to 

 be composed of layers of delicate lace. 



A piece of thick bark of linden macerated for a while in water 

 becomes so softened that the younger portion of the inner bark 

 can be easily separated into the annual layers. Strips of the 

 coherent fibres form the Russia matting of commerce. The 

 strips often measure 2-3 meters in length, 2-5 cm. in width, 

 and .04- .08 mm. in thickness. Scattered among the individual 

 hard-bast fibres there are many parenchyma cells, some of which 

 plainly belong to the medullary rays, and others to the fibro- 

 vascular bundles. 



414. The bast-fibres, in a few instances, instead of being re- 

 tained upon the stem for an indefinite period, are separated early, 

 leaving the newer bast exposed. This is the case with some of 

 our species of Vitis, in which the bast becomes detached in the 

 form of long, loose shreds after the first year. 



415. The crystals found in bast are very abundant. They 

 are chiefly monoclinic, and occur both singly arranged in 

 rows and in clusters. 1 



416. The appearance and distribution of the fibres of bast 



1 De Bary gives the following list, taken chiefly from Sanio : 



Clusters of crystals in bast of Juglans regia, Rhus typhina, Viburnum Oxy- 

 coccus, V. Lantaua, Primus Padus, Punica Granatum, Ptelea trifoliata, Ribes 

 nigrum, Lonicera Tatarica. 



Single monoclinic crystals in bast of species of Acer, and the Pomacese, 

 Robinia, Cladrastis, Ulmus campestris, Berberis, etc. 



Single monoclinic crystals and clusters in bast of Quercus, Celtis, ^Esculus 

 Hippocastanum, Hamamelis Virginica, Morus, Salix, Fagus, Populus, Car- 

 pinus, Betula, Tilia, etc. 



