CHAP, x.] Stimulation of Increment 213 



Theodor Hartig was the first to assert l that, as the direct 

 and immediate consequence of the freer exposure to light and 

 air, without reference to the species of tree, stimulated incre- 

 ment on each individual tree invariably took place, though 

 subject to the influence of the other factors determining the 

 rate of growth. When any apparent exceptions to this rule 

 are met with, they are neither ascribable to differences in the 

 species of trees, nor to differences in respect of soil and situa- 

 tion, but are solely due to one or more of the above-indicated 

 causes. Species of timber, soil, and situation certainly exert 

 their influence as regards the extent to which increment may 

 take place ; but they are not of themselves the direct or primary 

 cause. There is, however, good reason for believing that deci- 

 duous trees are enabled by nature to avail themselves of 

 the direct and immediate increment to a somewhat greater 

 extent than those coniferous trees which retain their foliage 

 throughout the whole year. 



The enhanced increment need not assume the form of 

 broader annual zones of woody-fibrous tissue along the bole. 

 It may, and very often does, at first take the shape of con- 

 siderable changes throughout the root-system and the crown 

 of foliage, primarily and undoubtedly due to the freer ex- 

 posure to light, air, and warmth. This paves the way for the 

 succeeding form of enhanced increment, due to these changes 

 in, and increment of, the assimilative organs, which makes 

 itself more readily distinguishable throughout the stem and 

 branches. 



The extent to which the annual increment thus becomes 

 enhanced varies in any particular species of tree according to 

 the individual stem, its age and reproductive capacity, the soil, 

 situation, and exposure on which the crop grows, the density 

 of the canopy throughout the crop, and the development of 

 the crown of foliage borne by the individual stem. The 

 younger and sturdier the tree, the better developed its crown, 

 1 Lehrbuchfiir Forster, 1861, vol. i. p. 105. 



