630 UTILIZATION OF BARK AND ITS CONSTITUENTS. 



it can be utilised commercially. In most species the forma- 

 tion of cork attains a thickness of only a few cells, whilst thin 

 layers of cork, with boundaries like oyster-shells, cut off parts 

 of the tissues from the deeper layers of bark, which turn red 

 or brown, and die. These flakes of bark contain scarcely any 

 cork, but are composed chiefly of the tissues of the inner bark. 



The inner bark of the shoot consists chiefly of the bast, 

 named from the occurrence in it of bast-fibres, hard and 

 soft bast. The former consist of very thick-walled, elon- 

 gated cells, which are sometimes solitary, sometimes in zones, 

 that alternate with zones of soft bast (Tilia) ; they are rarely 

 absent from the inner bark. Most of the inner bark, how- 

 ever, consists of soft bast, composed chiefly of sieve-tubes and 

 bast-parenchyma. The sieve-tubes are organs analogous to 

 the vessels of the wood, but are filled always witli aqueous, 

 plasmic contents, which by slow movement supplies nutriment 

 to the tissues. The bast-parenchyma, partly in strands 

 parallel to the axis (longitudinal parenchyma), partly in 

 horizontal bundles (medullary parenchyma) being a continua- 

 tion of the medullary rays of the wood, serves as a reservoir 

 for starch, sugar, tannin or turpentine, but also passes over into 

 other tissues. Thus, in many parenchymatous cells, crystals 

 appear of oxalate, or more rarely, of carbonate of lime, whilst 

 the plasmic contents disappear and the cell dies and is termed 

 a crystal-sac. Tannin accumulates in other parenchymatous 

 cells (tannin-sacs), as a refractive solution ; in medullary 

 parenchyma, ethereal oils, such as turpentine, camphor, 

 increase continually in quantity, whilst the other contents of 

 the cell continually diminish (turpentine and camphor sacs). 

 Often parenchymatous cells become converted into scleren- 

 chymatous or stone cells, their contents becoming attached 

 to their walls as a thickening, while the cell becomes either 

 spindle-shaped, or stellate, only a small part of the lumen 

 remaining. Sometimes the medullary-ray cells become stony 

 when they emerge from the wood, supplying an internal union 

 between bark and wood (beech). 



Elongated cells containing latex (fats and oil suspended in 

 water), also sometimes traverse the bark and are termed 

 laticiferous ducts (Finis) ; their contents are very important 



