450 Forestry Quarterly. 



deciduous trees usually on the windward side ; the side of attack, 

 i. e., the tension side exhibits usually a long-fibred break, showing 

 that here it gave way last. 



Metzger adduces the straight, cylindrical, vertical shaft form 

 and the uniform, bricklike cell structure of the tracheids in con- 

 iferous wood as suggesting its structure for compressive strength, 

 the opposite conditions in deciduous growth. 



Interesting references are made to the derivation of climbing 

 and creeping varieties from treelike dicotyls, or else the reverse, 

 the ancestors of the treelike dicotyls winding themselves on the 

 historically older conifers. From that period until to-day the 

 diametrically opposite principle of epinastic and hyponastic struc- 

 ture of one-sidedly loaded members has persisted. Unfortunately, 

 as Dengler points out, this position is not so generally supported 

 as the author seems to imply, for of 92 branches of oak, 27 were 

 found epinastic, 40 hyponastic, in beech 88 and 49, in pine 28 and 

 123 respectively, and these differences often on the same individ- 

 ual. Roots, which also exhibit epinastic and hyponastic struc- 

 ture vary similarly. 



In explaining annual ring structure by statical and mechanical 

 laws, it is pointed out that resistance to the force of winds in the 

 crown tests the bending strength of the stem. At the end of the 

 period of vegetation force and resistance must be in proportion. 

 In the spring when by increase of crown the proportion is dis- 

 turbed, an interior tension of cambium cells is created which re- 

 sults in the formation of the annual ring of corresponding 

 breadth, or rather strength. The wide-lumened tissues are needed 

 for physiological purposes; the effect on strength is greatest if 

 with the same amount of material the wide-lumened elements are 

 disposed on the inside, the narrow-lumened, thick-walled ones on 

 the outside. This mode of disposition has become an inherited 

 quality. While Metzger then claims for increase of the wood 

 body (diameter growth) direct mechanical causes for the division 

 into spring and summer-wood, he relies upon teleological explana- 

 tions. He refuses to accept Schwarz's explanation who refers 

 the formation of summer-wood to longitudinal pressure, which 

 stimulus during spring-wood formation is offset by other forces. 



To this position Dengler also takes exception with good rea- 

 soning, and altogether, acknowledging the ingenious and interest- 

 ing character of the discussion, and the priority of Metzger in 



