502 Forestry Quarterly 



must experience a minimum ; in close stand the ring width must 

 increase from root collar to tip. 



Guttenberg, however, denies the occurrence of a minimum in the 

 practical field, and other authors, among them Jaccard, combat 

 this mechanical explanation of tree form altogether, making a single 

 mechanical factor responsible for the laws of growth. 



According to Jaccard, the trees in comparison to the beam of 

 equal resistance are "over-dimensioned, " near the soil and near the 

 crown base, and towards the crown the shaft would have to be 

 relatively stouter than Metzger's formulae demand. This objec- 

 tion is in part met by the experience that the quality of the wood 

 with increasing height becomes poorer. 



Guttenberg has pointed out that in close stand the influence 

 of wind is very small. 



Jaccard considers the tree trunk "a shaft of equal water con- 

 ductivity." The unreliability of Metzger's formulae is increased 

 by the fact that they are derived from sticks with constant cross 

 section, while here we have to deal with bodies of constantly 

 varying cross section. 



Where the roots of a tree change into stem, at the root collar, 

 there is a place of sudden change in cross-section and a curvature ; 

 here the formulae based on the simple beam are not applicable. 

 In considering the static significance of the root collar, it is not any 

 more the physiologically admissible use of the wood, but the 

 admissible requirement of the soU that is at work. Tropical trees, 

 rooted in soft morass (mangrove), are supported by stilts which 

 transfer the pressure of the entire superior weight of the tree to the 

 subsoil. 



Other static conditions of various forms of vegetation are also 

 interestingly discussed. 



Technik und Naturwissenschaft. Centralblatt fur das gesammte Forst- 

 wesen, Jtily-August, 1915, pp. 254-72. 



Bailey and Sinnot have investigated the 

 Climate distribution of the various types of leaves 



and among the dicotyledonous plants in the 



Leaf principal plant-geographic regions of the 



Margins earth in the endeavor to throw some light 



upon the question of rigidity of leaf char- 

 acters and their modification by environmental factors. From the 

 floras of various regions, the authors tabulated the occurrence of 



