CHAP, x.] Stimulation of Increment 217 



of Hazel, Alder, and Willow, the swelling of the buds and the 

 flushing of the young leaves, the formation of rootlets, &c. In 

 fact, all the similar phenomena previous to the development of 

 independent cambial activity must be ascribed to these reserve 

 supplies of starchy and nitrogenous nutrients. The supply of 

 reserves is usually sufficient to maintain vegetation for one 

 complete year, and often for longer; indeed, in many cases 

 it amounts to 7 or 8 % of the total weight of thirty-year-old 

 timber crops. Diminution of light and foliage, consequent on 

 limitation of growing-space, must interfere with the utilization 

 of these reserve nutrients ; hence it is quite logical and reason- 

 able to expect that with more growing-space, and a consequent 

 increase of foliage, these reserves should be largely drawn upon 

 and utilized. 



That any stimulation of the productive capacity of the soil 

 takes place in consequence of the freer penetration of the 

 atmosphere, and the stimulus given to the formation of humus 

 or vegetable mould, is doubtful, or even more than doubtful ; 

 for, unless the canopy still remains comparatively dense, the 

 soil soon becomes covered with forest weeds, which often 

 really consume the mould, and tend to exhaust the soil, in 

 place of allowing the timber crop to have the full benefit of 

 the humus. 



The true physiological cause of the enhancement in incre- 

 ment, after heavy thinnings, or partial clearance of crops of 

 trees approaching maturity, is to be found in the stimulus to 

 the formation of starchy matter afforded by insolation ; for the 

 degree of activity of the assimilative organs is dependent 

 on the purity or brilliancy of the rays of light, as the decom- 

 position of the carbonic acid in the atmosphere can only take 

 place when the waves of light attain a certain length. Whilst 

 a crop is growing in full canopy, the assimilative activity of the 

 foliage forming the lower portion of the crown is extremely 

 small, owing to the low quality of the diffused light which is 

 alone available ; but, when the canopy is opened up to a suffi- 



