CHAP, ii.] The British Sylva 39 



in those whose crowns are limited to a more or less restricted 

 growing-space; because they have larger root-systems, can draw 

 greater supplies of nutriment from the soil, and enjoy better 

 opportunities for carrying out assimilation and for utilizing 

 their nourishment in the formation of ligneous tissue. On soils 

 similar as to quality the total production per acre in close- 

 canopied forest is not only greater than in the more open 

 forest, but the technical quality of the timber produced is also 

 much higher owing to the better dimensions attainable by the 

 bole or stem, and to freedom from knots formed where the 

 branches emerge from the trunk of the tree. The larger the 

 growing-space allowed to each individual tree, the greater is the 

 tendency to branching growth taking place at the expense of 

 the bole; hence, for the production of long straight stems, it is 

 necessary to maintain as close canopy as is consistent with the 

 general health and vigour of the crop until it has passed through 

 the pole-forest stages, during which its growth in height is most 

 active, although this again varies according to species of tree 

 and other factors. When crops attain their chief growth in 

 height after being maintained in canopy of normal density, 

 experience has shown that the stems rapidly increase in girth 

 when the canopy is interrupted, artificially, for the purpose of 

 permitting the individual trees to obtain larger supplies of light, 

 air, and warmth. This sylvicultural operation, although pur- 

 posely diminishing the number of individual trees per acre, 

 often leads to a greater increment and a more valuable yield 

 than if the crop had been allowed to remain growing in normal 

 canopy, but this is a subject that will receive full attention 

 subsequently (see Chapter X). As, in woods growing in close 

 canopy, the crown is circumscribed, and can for the most part 

 obtain only from above the light and warmth necessary for the 

 work of assimilation, the poles compete with each other for 

 a larger measure of exposure to these invigorating influences, 

 so that the growth in height becomes stimulated. And as, 

 owing to the crowns being limited to the upper portion of 



