54 Studies in Forestry [CHAP. m. 



The influence of the sum total of the factors dependent on 

 soil and situation find their expression in the amount of timber 

 produced per acre, and in the quality of the timber, since this 

 latter must determine its adaptability for technical purposes. 

 Practically speaking, the best and the most convenient general 

 standard for estimating the quality of timber is to be found in 

 the specific weight, as, for one and the same kind of ivood, the 

 heavier specimen is the more durable 1 , owing to the thick 

 deposits of ligneous substance in the cells. This does not, 

 however, apply to any comparison of different species ; for the 

 light wood of conifers is more durable than heavy Beech 

 timber. High specific gravity, length, straightness, and full- 

 woodedness of bole, with freedom from knots and branches, 

 are the best indications that soil and situation are eminently 

 suitable to the woodland crops covering them. 



III. Capacity for bearing Shade. 



All trees require a certain amount of light in order to carry 

 out the work of assimilation of the carbonic acid which forms 

 so important a part of their nutriment. Hence the want of a due 

 intensity of light exerts an injurious influence ; otherwise there 

 is no reason why the density of the foliage in the interior of 

 the crowns of trees should not be as great as it is near the 

 circumference. It is true that excess of light may tend to 

 paralyse the action of chlorophyll ; but, so far as the forest trees 

 of Britain are concerned, they are exposed to no danger from this 

 cause in our climate; they will thrive all the more vigorously the 

 more light they receive the benefit of. But, with regard to the 

 measure of light necessary for the performance of the assimi- 

 lative functions by the foliage or, in other words, to the 

 capacity for bearing shade marked differences occur among 

 the various species of trees ; and though affected by soil 



1 Gayer, Die Forstbenutzung, 7th edit., 1888, p. 66. 



