Grossenbacher — Radial Growth in Trees. 69 



Pick 139 regarding the action of the enzymes which coagulate 

 blood and milk. 



The fact that stems and branches of trees are more pliable and 

 easily bent while in the midst of active spring growth than they 

 are at any other time, indicates that perhaps some enzymotic 

 softening of the mature wood occurs during the period of most 

 active growth. The upward bending of a branch on a decapi- 

 tated conifer also argues for the presence of some softening 

 agent during the time of most vigorous growth because -of the 

 fact that such branches often bend in response to gravity at 

 places where lignification had previously occurred. In other 

 words, it seems that one of the most important factors in the 

 production of large wood cells in spring and smaller ones in 

 summer may be the presence of enzymes which retard lignifica- 

 tion and prevent rapid thickening of the walls and thereby per- 

 mit growth or hydrostatic pressure to develop large cells in 

 spring; while the absence or inactive condition of those enzymes 

 induces rapid thickening and early lignification of the walls in 

 summer and thus checks the enlargement of summer-wood cells. 

 It may be that the idea of growth force expressed by Detlef- 

 sen, Mer and others as well as "the impulse to grow" em- 

 phasized by Morgulis imply the same sort of notion as that ad- 

 vanced in the above scheme regarding the possible relation of 

 enzymes to ring formation, but in any case the hypothesis is 

 only a guess based on rather suggestive indirect evidence. Mer's 

 conclusion that the winter rest of the cambium induces its 

 greater activity in spring seems to have something in common 

 with the outcome of some feeding experiments by Morgulis, to 

 the effect that in subjecting salamanders to alternate periods of 

 fasting and liberal feeding a greater growth resulted than by 

 more frequent and abundant feedings. A theory to account for 

 wood rings must also make use of the evidence brought out re- 

 garding the effect of variations in bark tension both longitudinal 

 and transverse, as well as of the influence of the transpiration 

 stream as suggested by Hartig and more recently elaborated by 

 Jaccard in his discussion of the distribution of radial growth. 

 It should be remembered, however, that transpiration is per- 

 haps greater during the time summer-wood is formed than it is 

 while spring wood develops; to say that larger cells are pro- 

 duced in spring to meet the higher water requirements of the 

 approaching summer explains nothing. 



180 Fick, A. Ueber die Wirkungsart der Gerimmngsfermente Archiv 

 Gesam. Physiol. Mens. Thiere. 45:293-96. 1889. 



