58 Wisconsin Academy of Sciences, Arts, and Letters. 
sure its cells took on isodiametric forms which were retained un¬ 
til the hark pressure became appreciable again and then reverted 
back to the elongate form normal to the species. It is held that 
the upper and lower edges of a wound do not produce callus as 
copiously as the lateral ones because of the lesser reduction of 
bark pressure, and the death of the cut cells which extend some 
distance above and below the wound. 
From his experiments in which ligatures were applied to 
stems Sorauer 116 concluded that slow radial growth combined 
with high bark pressure results in twisted grain and that a re¬ 
duction of bark pressure below normal not only induces more 
cells to form from the cambium, but cells having a greater di¬ 
ameter and a reduced length. 
Newcombe 117 found that when external conditions prevent 
growth, the unfinished tissues remain unaltered and thin walled; 
that mechanical resistance or pressure prolongs the differentiat- 
tion period, the cells remaining smaller and thinner walled. 
The occurrence of numerous cocoons of bag-worms on various 
species of trees and the fact that the narrow silken bands by 
which they are attached to the twigs are often too strong for ra¬ 
dial growth pressure to break, afforded von Schrenk 118 an occa¬ 
sion for a study of the effects of excessive pressure on radial 
growth. In most cases the silken bands encircling the twigs are 
burst early in the summer of the year following the time of the 
attachment of the bags. In some instances in which the liga¬ 
tures were too strong to be ruptured by the thickening twigs the 
transfer of elaborated food was eventually checked and an en¬ 
largement developed on the distal side of the constricting band. 
In other cases the ligature was sufficiently distended by growth 
to permit of some food transfer and resulted in the formation of 
welts on both sides of bands. In some instances the pairs of 
welts fused above the ligatures and reestablished normal connec¬ 
tion and pressure. In arbor vitae the wood fibers of the first 
116 Sorauer, P. Handbuch der Pflanzenkrankheiten. Dritle Auflage. 
1:764-66. 1909. 
117 Newcombe, F. C. The influence of mechanical resistance on the 
development and life-period of cells. Bot. Gaz. 19:149-57; 191-99; 
229-36. 1894. 
118 Schrenk, von, H. Constriction of twigs by the bag-worm and in¬ 
cident evidence of growth pressure. Ann. Rpt. Mo. Bot. Gard. 17:153-81. 
1906. 
