QUALITY OF BARK. 639 



tannic acid is accumulated. As every year the cambium 

 produces a new layer of longitudinal parenchyma and sieve- 

 tubes, the quantity of tannin increases until the rhitidome 

 forms, when the external cortex is killed by the formation of 

 cork, so that much of the tannin is lost. When coarse bark 

 begins to form, the mass of bast and consequently of tannin 

 remains uniform, as just as much new bast as the cambium 

 forms is lost externally in dry bark. At this period it is best 

 to utilize the bark, for then worthless rhitidome continues 

 to increase. Kegarding the factors, which expedite the forma- 

 tion of rhitidome, Mayr's observations (cf. p. 629) have given 

 the most important, which are : rapid growth (favourable 

 climate and soil) ; action of sunlight (open crop, especially on 

 poor soil) ; tissue-tension at the base of branches (production 

 in open, branchy crops), von Schroder, Neubrancl and others 

 have proved the following production of tannin : 



25 years best oak-bark give up to 25*0 per cent. 

 40 18-0 



80 5-0 



/ 18 ,, spruce-bark ,, 5*0 ,, 



In crops 



J 35 15-0 



I 55 ,, ,, ,, 8'8 ,, 



55 11-0 



Suppressed 55 ,, ,, 8'0 



(e) Influence of Light. The effects of exposure to light 

 are seen clearly from the above ; too dense a crop, mixture 

 with shadebearing trees, overshading by standards, etc., reduce 

 the amount of tannin in the bark. Schuberg states that the 

 loss by overshading may be 35 per cent. Neubrand, there- 

 fore, recommends that oak timber should be grown on areas 

 not devoted to the production of bark. 



(f) System of Management. Coppice, with such a rotation 

 as may be determined from the preceding paragraphs, is the 

 system under which oak is grown to produce tanning bark. 

 If the maximum yield of the best quality of bark is desired, 

 the shoots are felled before rhitidome appears ; the rotation 

 is shorter the warmer the climate, it varies from 12 years to 



