1920] 



CURRENT LITERATURE 



161 



illumination, etc. ; such data as may be secured by critical field observations, 

 detailed stem analyses, and a careful study of the past environmental history 

 of the plants. Thus the method of attacking the problem consists in showing 

 that variations in form are closely correlated in each case with variations in 

 illumination, gravitational, or mechanical forces. Since it places a premium 

 upon circumstantial evidence and deductive reasoning, its ultimate success is 

 dependent upon the number of concordant facts that can be advanced in its 

 favor. Although Engler's data indicate that bending actually occurs in old 

 stems, they do not demonstrate in all cases that the curvatures are due neces- 

 sarily to particular stimuli. For example, in tall dense forests, the sudden 

 curvatures of trees toward gaps made by thinnings may be heliotropic; but 

 it is also conceivable that they may be due to a lack of rigidity in tall (13-18 m.) 

 very slender (8-13 cm.) trees. The reviewer has seen thinnings in lodgepole 

 pine forests in which the crowns of tall unsupported trees have bent over until 

 they touched the ground. Furthermore, if bending occurs in response to 

 heliotropic stimuli and is produced by the activity of living cells (parenchyma) 

 in the sapwood, as Engler supposes, the structure of the stem must be con- 

 siderably modified. No conclusive evidence is presented to indicate that 

 such modifications actually have taken place. 



In the second of the prize essays, Jaccard 4 attacks the problem of the 

 form and growth of trees from an entirely different angle. The first twelve 

 chapters of his memoir are devoted to a criticism of the Schwendexer- 

 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 " clear length" of the stem is, at successive heights, a shaft of 

 equal water-conducting capacity. Inasmuch as this portion of the memoir 

 is largely a recapitulation of former papers, which have been reviewed by 

 Grossenbacher 5 and others, it may be passed over without further comment. 

 The three succeeding chapters (pp. 101-169) are concerned with interesting 

 experiments, designed to test the effects of mechanical, geotropic, and helio- 

 tropic stimuli and various types of girdling upon the form and anatomical 

 structure of conifers and dicotyledons. A large number of young stems and 

 branches were subjected to various types of flexure (sustained or intermit- 

 tent). Their subsequent growth, form, and structure were 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 "redwood" is 

 formed unless the stem is allowed to remain in each posture for a certain period 



4 Jaccard, Paul, Nouvelles recherches sur l'accroissement en epaisseur des 

 arbres: Essai d'une theorie physiologique de leur croissance concentrique et excentrique. 

 Pub. Foundation Schnyder von Wartensee Zurich 23: i-xii+ 1-200. ph. 1-32. 



5 Grossenbacher, J. G., The periodicity and distribution of radial growth in 

 trees and their relation to the development of "annual" rings. Trans. Wis. Acad. 

 Sci. 18: part 1. 1915. 



