54 Wisconsin Academy of Sciences, Arts, and Letters. 



joint. Such transverse displacements or sharp double-bends 

 were found in about two-thirds of the fifty to sixty species ex- 

 amined. They were especially prevalent in Urticaceae, Apocy- 

 naceae, Asclepidaceae, Linaceae, etc., while in other families the 

 double-bends occurred only in certain genera. None were found 

 in the Rosaceae including the pomaceous group, nor in the Tilia- 

 ceae and Cupuliferae. 



It was held that the sharp bends are due to bark pressure, as 

 indicated by the fact that in the plants in which these bends 

 commonly occur the bast-fibers are but slightly or not at all 

 lignified. Hohnel held that if the double bends were not due to 

 growth or bark pressure they would not always appear at points 

 in the fibers where joints or breaks occur in the cells of the sur- 

 rounding tissues. The failure of the bends to become evident 

 until after the tissues are fully differentiated was taken to indi- 

 cate that bark-pressure becomes greater during the latter part of 

 the differentiation period. It also seemed that in case of Urtica, 

 Cannabis and Linum the bark pressure was often greater in the 

 lower part of the stem than above, for the angular bends were 

 frequently present on the fibers of the lower part while none oc- 

 curred in the upper. The transverse' displacements were found 

 to be made up of two successive sharp bends which were notice- 

 able in all layers of the wall. In many cases some of the layers 

 were actually ruptured. 



Krabbe 112 made extensive studies of bark pressure and tried 

 to obtain some quantitative measurements. He increased bark 

 pressure by encircling tree-trunks with a chain much like that 

 now used on bicycles, except that it was wider. One end of the 

 chain was fixed to an iron peg driven into the tree and the other 

 ran over a pulley and had a weight pan attached. A piece of tin 

 a little wider than the chain was placed about the trunk under 

 the chain to distribute the pressure more evenly and to reduce 

 friction. "Weights were put into the pans in accordance with the 

 determinations of bark pressure obtained before, and it was 

 found that the bark pressure had to be doubled and even quad- 

 rupled before any influence on the size of the cells or the thick- 

 ness of the yearly growth became evident. 



112 Krabbe, G. tJber die Beziehung der Rindenspannung zur Bildung 

 der Jahxringe und zur Ablenkung der Markstrahlen. Sitzungsber. 

 Akad. Wiss. Berlin 1882: 1093-1143. 1882. 



