EFFECT OF SOIL ON FOREST VEGETATION. 37 



excluding or reducing the agencies which dissipate it, or, if 

 there is excess of moisture, by accelerating its consumption, 

 or by draining. 



The absolute quantity of moisture required annually by 

 each species is still under investigation, but experience has 

 shown, that a fresh soil with, as far as practicable, an even 

 and constant degree of moisture suits most of the species 

 enumerated above. For the rest, they may be classified as 

 follows : — 



Most Dwistiire in the soil demand .- Common alder ; next to 



this ash, most poplars and willows. 

 Moist soil like : Cembran pine, hornbeam, elm, lime, 



mountain ash, pedunculate oak. 

 Fresh soil like ; Silver fir, spruce, larch, beech, sessile oak, 

 Norway maple, sycamore, Weymouth pine sweet 

 chestnut. 

 On dry soil thrive : Corsican pine, Scotch pine, Austrian 

 pine, birch, acacia, aspen. 



4. A suitable Chemical Co)nposition. 



Apart from water and gases, the soil consists of mineral 

 and organic substances. These affect the development of the 

 trees partlj^ by providing nourishment and partly by determin- 

 ing the physical properties of the soil. Woody plants take by 

 far the greater portion of their nourishment from the air, more 

 especially carbon, but a certain portion, including the mineral 

 substances, is derived from the soil. Hence it is of importance 

 to ascertain the actual quantities of such substances in the 

 plant. The contents of mineral substances vary in different 

 parts of the tree ; thus wood taken from the stem generally 

 contains less than 1 per cent, of ashes (according to weight), 

 branches and twigs about 2 per cent., bark alone 2 — 3 per 

 cent., and leaves and needles from 4 — 6 per cent. Eber- 

 mayer* gives the following quantities of the more important 



* Physioloeische Chemie der Pflanzen, Volume I., patre 701. 



