BARK, 



[ 75 ] 



BARLEY. 



which it surrounds. The layers of liber 

 increase year by year so as to form a very 

 distinct fibrous layer. 



In the Cork Oak {Quercus Suber), the 

 bark of which, when young, does not differ 

 much from that of the Maple, the cellular 

 layer grows most in the earlier years, and 

 the epidermis is not destroyed until the 

 third, fourth or fifth; then the cork-substance 

 begins to increase in an im])ortant degree, 

 by the multiplication of its cells at the inner 

 side, bordering on the cellular envelope. New 

 layers of cork-cells are produced successively, 

 expanding much in the radial dii-ection. 

 They are thin-walled and destitute of con- 

 tents, of squarish form (pi. 38. figs. 16 & 17) 

 and soon become dry. The outer layers 

 being unable to expand sufficientlj^ to allow 

 the enlargement of the stem, tear irregularly 

 and give the surface of the stem a rough and 

 cracked aspect. On old stems we observe 

 that the formation of these layers has not 

 been continuous, but in successive groups or 

 sets, which causes the appearance of a darker 

 and more solid structure, composed of tabular 

 cells, at the points where successive sets of 

 layers adjoin, just as is the case at the lines 

 of union of the annual rings of wood in 

 Dicotyledonous stems. But these lines are 

 here very irregular. The cellular envelope 

 takes no share in the formation of the cork 

 of this tree. 



In the Birch {Betula alba), there is a very 

 decided distinction between the layers of the 

 cork-substance, namely, between the large 

 thin-walled colourless cells, and the denser 

 tabular cells forming the dark streaks in the 

 cork. The epidermis is succeeded here by a 

 periderm composed of tabular cells with 

 brown contents, corresponding to the darker 

 parts of common cork; in stems of 20 years' 

 growth, the bark presents as many as fifty 

 lamellae of this substance, which lamellae are 

 separated from each other by layers of the 

 lax, white cork-cells. The readiness with 

 which the latter structure gives way causes 

 the lamella3 to peel off in thin scales, and 

 these bring away a portion of the wdiite inter- 

 mediate structure on both faces, and thus 

 acquire their peculiar silvery aspect. 



In the Beech {Fagus sylvatica), where the 

 bark is smooth, even on old trees, the growth 

 takes place chiefly in the liber- layers, and 

 the cellular envelope and cork-substance 

 merely expand to make room for the enlarge- 

 ment of the stem ; the cork-substance is here 

 a periderm, i. e. composed of the flat, tabular 

 cells, not loose cork tissue. The Holly, Ivy 



and other smooth-barked trees are analogous 

 to this. 



The scaling off of the bark of the Plane 

 {Platanus occidentalis) arises from the for- 

 mation of layers of tabular peridermal cells 

 between the layers of liber ; the bark outside 

 the layers dries and falls aw ay by the tearing 

 of this peridermal layer. Here, therefore, 

 the periderm is produced from the cellular 

 envelope. 



In the Lime {Tilia), the Oak (Q. Robur) 

 and other trees, a similar production of peri- 

 dermal layers within the liber takes place, 

 but the lavers remain m situ for a long time, 

 and fall away irregularly, often persisting for 

 a considerable number of years as rugged, 

 many-layered scales. 



In many of the Coniferge (such as the 

 Scotch Fir and Larch), the peridermal struc- 

 ture is in like manner developed from the 

 cellular envelope; here, however, the cells 

 are not tabular, but parenchymatous, and 

 multiply and enlarge so as to form a thick 

 layer of cork-like tissue, which loses all 

 relation with the medullary rays. The tur- 

 pentine canals and liber-fibres engaged in 

 this corky periderm, become disturbed and 

 displaced by its irregular growth. 



In some plants, such as the Vine, the 

 Honeysuckle, &c., the bark is always string^-, 

 which arises from the formation of each annual 

 layer of liber being followed immediately by 

 the drying-up, and soon by the destruction, of 

 the layers of the preceding year, so that no 

 proper periderm, or suberous or cellular 

 layers exist here after the first year. The 

 same takes place in the thu'd or fourth year 

 in the Clematis. 



The inner layers of the bark are especially 

 distinguished by the presence of laticiferous 

 canals in those plants in which that tissue 

 exists ; in fact this appears to be in many 

 cases a modification of the liber tissue. 

 Further particulars are given on this head 

 under Liber, where also the intimate struc- 

 ture of the liber will be explained. See also 

 Lenticels and Cork. 



BiBL. Text-books on Structural Botany ; 

 Mohl, Entivick. des Korkes, &c., Vermischt. 

 Schrift. 1845, p. 212 ; Hanstein, Ueber den 

 Ban, Sf-c, der Baumrinde, Berlin, 1853; 

 Schacht, die Pflanzenzelle, p. 237 et seq. 

 1852. 



BARLEY. — One of the important cereal 

 grains, furnished by the Hordeum sativum and 

 its varieties (Monocot. Plants, N. O. Grami- 

 naceae). The starch of the albumen of the 

 seeds has a peculiar form, by which it may be 



