Periodical Literature 115 



as a function of the active crown; Metzger bases his theory on 

 static laws, recognizing the form as a reaction to wind pressure 

 — a girder or pillar constructed with a view of offering resist- 

 ance to bending pressure. Jaccard returns in part to the physio- 

 logical explanation and sees the cause in the requirement of 

 equal water conductivity throughout the annual ring. None of 

 these theories explains satisfactorily all the exhibitions of form, 

 varying from species to species, from age to age, from site to 

 site, from stem class to stem class. 



The author uses a large number of stem analyses of spruce, 

 fir, pine, larch, beech, oak, tabulated for comparison of diameter 

 and area development at different heights of trees of all sorts 

 of conditions, and finally draws the following conclusions. 



1. The form of stems unquestionably in the main is built ac- 

 cording to static laws, since the stem as carrier of the crown, 

 must be able to resist the pressure of air movements against 

 bending and against overthrow; but by no means is it necessary 

 to react so precisely as Metzger would make us believe. Nature 

 is wasteful, and even here does not work economically. 



2. Pressler's assertion that the food materials elaborated by 

 the foliage distribute themselves evenly along the shaft from 

 the crown downward, that, therefore, area and volume increment 

 at any cross section below the crown is approximately the same, 

 in the crown, however proportionate to the leaf surface above the 

 section, is — even if we leave out the unlawful accumulation at 

 the root collar — not entirely supported by the facts, since the 

 area increment even in the middle part of the stem general- 

 ly decreases upward and occasionally remains the same for a 

 distance. 



The same may be said regarding Jaccard's or Deccopet's 

 theory, that equal water conductivity requirement is determina- 

 tive, for this also supposes an equal area increment in all cross 

 sections from the crown downward, especially since Guttenberg 

 found that the width of the annual ring and, therefore, still 

 more the area increment, increases in the lower part of the stem 

 from 16 to 26 feet downward. 



3. The behavior of diameter increment in the stem at varying 

 heights differs with different species and, therefore, the result- 

 ing tree form must be considered as a characteristic belonging 

 to the species. 



