58 Wisconsin Academy of Sciences, Arts, and Letters. 



sure its cells took on isodiametric forms which, were retained un- 

 til the bark 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 eases 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. 



"'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. 



