634 JOURNAL OF FORESTRY 



the terminal shoots of dicotyls, particularly during the earlier stages 

 of ontogeny, tend to bend down-hill toward more intensive illumination 

 upon that side. Later these curvatures are more or less completely 

 neutralized by bending in* opposite direction in response to geotropic 

 stimuli. Eccentric growth upon the up-hill side is assumed to be due 

 to geotropic stimuli, regardless of whether the stem is concave or con- 

 vex, and eccentricity upon the down-hill side, as in conifers, to longi- 

 tudinal compression of the cambial layer. He reaches similar conclu- 

 sions in regard to the eccentricities of stems and branches of trees 

 growing upon level ground. In dicotyls, the influence of gravity 

 usually exceeds that of longitudinal compression, and accelerated 

 growth of the under sides of stems and branches is found only where 

 it is not inhibited by negative geotropism. Lateral eccentricity occurs 

 when these two factors, working in opposition, neutralize each other. 

 Longitudinal compression afifects the volume of secondary xylem but 

 not its structure. In ring-porous dicotyls "geotropic wood" is char- 

 acterized by wider vessels and a relatively greater proportion of sum- 

 merwood ; but in different diffuse-porous species the wood of the 

 upper and lower sides of stems and branches is of the same general 

 type. Engler suggests that the rapid bending of stout stems in re- 

 sponse to heliotropic stimuli is produced by the activity of living cells 

 in the sapwood. If this is so the structure of the stem must be con- 

 siderably modified. No conclusive evidence is presented, however, to 

 prove that such structural modifications actually occur. 



The first twelve chapters of Jaccard's memoir are devoted to a criti- 

 cism of the Schwendener-Metzger hypothesis, which holds that the 

 form of trees is determined largely by mechanical factors (wind and 

 gravity), and to an exposition of his own theory that the ''dear 

 length" of the stem is, at successive heights, a shaft of equal water- 

 conducting capacity. The three succeeding chapters, pages 10] -169, 

 are concerned with interesting experiments, designed to test the effects 

 of mechanical, geotropic and heliotropic stimuli and various types of 

 girdling upon the form and anatomical structure of Conifers and Di- 

 cotyledons. A large number of young stems and branches were sub- 

 jected to various types of flexures (sustained or intermittent). Their 

 subsequent growth, form and structure w^ere found to vary depending 

 upon the intensity and duration of the stimuli. Thus, if the stem of 

 an erect conifer is bent alternately to the north and south no "red- 

 wood" is formed unless the stem is allowed to remain in each posture 



