242 Studies in Forestry [CHAP. xr. 



tracts underplanted, as compared with similar woods not 

 treated in this manner. There are, of course, circumstances 

 and situations in which undergrowth might make relatively 

 higher demands on the soil than are made by the standard 

 trees, as, for instance, any endeavour to underplant Scots Pine 

 with Beech on poor, dry, sandy soils, an attempt which would 

 be diametrically opposed to the true sylvicultural characteristics 

 of the latter kind of tree. 



Of at least equal importance to the amount of annual 

 increment on the standards, the quality of the timber produced 

 next demands consideration. Its technical utility is, apart from 

 such very special cases as formerly obtained when Oak-crooks 

 were in demand for ship-building, mainly determined by 

 (i) straightness of growth, (2) full-woodedness of the top end 

 or high-form factor, (3) freedom from knots and branches, and 

 consequent evenness of texture, (4) proportion of heartwood to 

 sapwood, and (5) height of specific gravity for with regard to 

 any samples of wood of the same species the heaviest is 

 ceteris paribus the most durable 1 , as its specific gravity is 

 dependent on thicker deposits of ligneous substance on the 

 cell-walls. This last point is determined by the proportion of 

 the denser summer zone to the more porous spring zone in 

 the annual ring, the former being formed when the assimilative 

 process is carried out most thoroughly during the hottest 

 months of the year. And, as the practical effect of the under- 

 wood is to delay the awakening and commencement of active 

 vegetation for at least a fortnight, owing to its maintaining the 

 soil and the root-systems of the standard trees cool and moist 

 under its shade, it therefore exerts a most beneficial influence 

 on the timber produced. When heavy thinnings or partial 

 clearances have been made, this mechanical hindrance to 

 insolation and to early stimulation of active vegetation is all 

 the more important, as, from the comparative isolation of the 

 standards, an early commencement of the transpiratory and 

 1 .Gayer, Die Forstbenutznng, yth edit., 1888, p. 66. 



