2O2 Studies in Forestry [CHAP. ix. 



That, at the same time, a qualitative enhancement takes 

 place along with the quantitative increment of the timber is 

 clear from the researches of R. Hartig, who asserts 1 that in 

 general, without variations according to the species of tree, 

 the quality of the timber grown in the comparatively full 

 exposure to light and air is better than that formed under 

 other conditions. This is due to the fact that the extent to 

 which transpiration is carried on through the foliage does not 

 increase part passu with the enhanced rate of increment, as 

 the conductive capacity of the vessels in the new zones of 

 wood is relatively smaller, in consequence of the ligneous 

 deposits within the cellular tissue being thicker and denser. 

 And, further, when underplanting is simultaneously carried 

 out, the undergrowth keeps the soil and the root-system cool, 

 and thus retards, for at least a fortnight, the awakening of active 

 vegetation in spring; hence the latter is only entered upon 

 when the weather has become somewhat warmer, and when 

 the assimilative and constructive processes can consequently 

 be performed more thoroughly and energetically, owing to their 

 taking place under more favourable conditions as to warmth 

 and sunshine. 



To determine whether or not a partial clearance is likely to 

 be financially remunerative, practical figures, based on past 

 average results for crops of similar kinds on soils of similar 

 quality, are requisite. Where these are available, the reckon- 

 ing is very simple by means of the following equation : 



When C = the cubic contents that may be expected from the mature 



crops with ordinary treatment only, 

 V = the value of same per load, or per cubic foot, at time of 



harvesting the mature crop, 

 /.-.. tq outturn in cubic contents from ordinary thinnings from 



now till harvesting of the mature crop, 

 v a ---- Vg = the sale-price of same per load, or per cubic foot, 



capitalized until the clearance of the mature crop. 



1 Centralblatt fur das gesammte Forstwesen, 1888, pp. 8, 363. 



