FACTORS OF FOREST PRODUCTION. 121 



water which is required for the biological processes, 

 including the transpiration of the leaves, and for 

 the composition of the wood, adding the hygro- 

 scopic water which is finally lost when the wood 

 seasons. Chemically water forms 48 per cent of 

 the wood substance, while 40 to 60 per cent more 

 is hygroscopically bound to it in the living tree, 

 and 8 to 12 per cent remains so in the wood after 

 seasoning ; the whole forest area, therefore, pro- 

 duces only 40 per cent of dry substance to 60 per 

 cent of water, so that the 8000 pounds annual 

 product on a fully stocked acre divides itself up 

 into 3000 pounds dry substance, 1250 pounds 

 chemically bound, and 3750 hygroscopic, water. 

 These are small quantities of water, but the tran- 

 spiration current requires many times more. Fig- 

 ures on this point are difficult to establish, as the 

 variations, by species not only, but from day to day, 

 in different seasons, are extremely great. An acre 

 of beech may some days transpire not more than 

 5000 pounds, other days four times that amount, 

 while agricultural crops seem to need from 50 to 

 100 per cent more. The interesting and impor- 

 tant point is that coniferous trees, especially pines, 

 require from one-sixth to one-tenth of what decidu- 

 ous-leaved trees transpire, which makes them espe- 

 cially valuable for dry soils and climates. The 

 silviculturist draws from these facts, regarding the 

 frugality of forest crops, the conclusion that he 

 need not like the farmer manure nor change his 



