BARK. 233 
stone-cells, collenchyma), is termed the middle bark (green 
bark, Figs. 144 m, 149 m); and the epidermis (cork) the outer 
bark (Figs. 144 a, 149 a). 
Naturally, barks are found only in the dicotyledons and gym- 
nosperms. The name is used to designate the entire collection 
of cells located outside of the cambium. Nevertheless, in the 
case of monocotyledons the portion of tissue located outside of 
the nucleus-sheath is also, for the sake of convenience (but in- 
correctly) termed bark. 
In the middle bark, entire lines of cells are often found which 
are filled with crystals of oxalate. These frequently accompany 
Fic. 150,—Bast-fibres from Cinchona (China) Alba Payta, with impressions of the 
neighbouring crystal cells. 
the bast-cells, and are occasionally in such close contact with the 
latter that depressions are produced. The crystal-cells appear 
indented into the bast-cells, as in ‘* Cinchona (China) alba” * 
(Fig, 150) and in the bark of Aspidosperma Quebracho.” 
If the activity in the entire (cambial) thickening ring pro- 
gresses in a uniform ratio, then the separated elements of the 
wood and of the bark are arranged in strictly radial rows. 
In the wood, these rows mostly remain preserved (coniferous 
? Fitickiger, Neues Jahrbuch fiir Pharmacie, xxxvi. (1871), p. 293. 
* Hansen, “‘ Die Quebracho-Rinde.” Berlin, 1880. 
