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- 
naeeae, 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. tiber die Beziehung der Rindenspannung zur Bildung 
der Jahrringe and zur Ablenkung der Markstrablen. Sitzungsber. 
Akad. Wiss. Berlin 1882: 1093-1143. 1882. 
