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BOTANICAL GAZETTE 



[august 



subsequently were awarded to Arnold Engler and Paul Jaccard, pro- 

 fessors in the Federal Polytechnical School in Zurich. 



Engler 3 concerns himself with the effects of geotropic and heliotropic 

 stimuli upon the form and structure of arborescent plants. He is of the 

 opinion that old stout stems and branches of dicotyledons may develop 

 marked geotropic and heliotropic curvatures, but considers that, in the case 

 of the Coniferae, heliotropic bending is confined to the younger, more pliable 

 portions of the stem. He devotes considerable attention to the study of the 

 form and growth of broad-leaved trees on steep slopes, and concludes that the 

 terminal shoots, particularly during the earlier stages in the ontogenetic 

 development of the trees, tend to bend downhill toward more intensive 

 illumination upon that side. During subsequent growth these curvatures 

 are more or less completely neutralized by bending in the opposite direction 

 in response to geotropic stimuli. His numerous stem analyses show that 

 trees growing on steep slopes may be eccentric on the uphill side, the down- 

 hill side, or vary in their eccentricity at succeeding heights in the stem. Accel- 

 erated growth upon the uphill side is assumed to be due to geotropic stimuli, 

 regardless of whether the stem is concave or convex, and eccentricity on the 

 downhill side, as in Coniferae, to longitudinal compression upon the cambial 

 layer. He reaches similar conclusions in regard to the eccentricities of stems 

 and branches of trees growing upon level ground. In other words, accelerated 

 growth upon the upper sides of stems or branches is geotropic, whereas that 

 upon the under sides is due to longitudinal compression. The geotropic 

 stimulus ceases to act only when the terminal shoot occupies a vertical posi- 

 tion. Different parts of a tree may react differently toward light and gravi- 

 tational forces. Thus in the younger (higher) portions of a stem heliotropic 

 frequently overshadow geotropic stimuli, so that geotropic curvature and 

 eccentricity are confined to the base of the stem. In dicotyledons the 

 influence of gravity usually exceeds that of longitudinal compression, and 

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



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where it is not inhibited by negative geotropism. Lateral eccentricity occurs 

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

 tudinal compression affects the volume of secondary xylem but not its structure. 

 In ring-porous dicotyledons, "geotropic wood" is characterized by wider 

 vessels and a greater proportion of summer wood; but in diffuse porous 

 species, the wood of the upper and lower sides of stems and branches is of the 

 same general type. 



As evidence in favor of these views, Engler cites the crooked or curved 

 stems of trees growing under peculiar environmental conditions ; for example, 

 on steep slopes, displaced from the normal vertical position, in unilateral 



Longi- 



3 Tropismen und exzentrisches Dickenwachstum der Baume; Ein Beitrag zur 

 Physiologie und Morphologie der Holzgewachse. Schr. Stift. Schnyder von War- 

 tensee Zurich 21:1-106. Jigs. J-jo. 1918. 



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