2a 



tio* of height. We know that a = ^ (page 21), and 



« 

 C=c _£ (formula iv, page 12). In other words 



a 

 the sample tree of average hasal section is a true aver- 

 age tree. This case obtains only exceptionally in a 

 whole crop, but is always approximately true for the 

 groups of trees composing its several girth-dasses. 



(i) Height and form-factor a function of the girth and 

 hence variable, or height constant, but form-factor 

 variable and a function of girth. — The former con- 

 tingency may, as a rule, be assumed as always prevailing 

 in regular crops of one and the same age and orio-in ; 

 the former can very seldom liappen. In either con- 

 tingency only girth-classes shonld be adopted. 



(c) Height irregularly variable, the form-factor being a func- 

 tion of height. — la this case heiglit-classes must also 

 be formed. 



{£) Height and furm-f actor both irregularly variable, — This 

 is generally the case in irregular crops composed of 

 trees of various ages. Here also girth and height- 

 classes must be formed, and in addition several sample 

 trees shonld be measured for each class. 



B. GiKTH coNST^^"T. This is always true of girth-classes, 

 and may be assumed to be true also of girth gradations. 

 "We have three distinct eases as follows : — 



(a) Height and form-factor constant or both of them functions 



of girth. — In such a contingency every component 

 tree would be a correct average tree, but this is never 

 exactly the case even in the most r^ular crops. 



(b) Height constant or a function of girth, for m-f actor vari- 



flfife.-^-This is the ease generally met with. In order 

 to obtain a correct average form-factor sereral sample 

 stems must be measured. 



(c) Height variable and form-factor a function of height, — In" 



this case height-classes must be formed, thus redacincr 

 the conditions (for each height-class) to those of case 



