56 Wisconsin Academy of Sciences, Arts, and Letters. 



Gehmaeher 114 also performed some experiments in the increase 

 and decrease of bark pressure on three to six-year-old trees and 

 shrubs. The outer cortex was slit in February and nearby on 

 the same stem a ligature of tightly wound wire was applied and 

 the stem allowed to grow until the end of the season. 



The number of cork cells varied inversely as the pressure and 

 their radial diameter was decreased by 11% under increased bark 

 pressure, while under reduced pressure an increase of 13% above 

 normal resulted. A similar effect was noted on the cortical 

 parenchyma cells except that both the radial and tangential di- 

 ameters were decreased under increased pressure and the inter- 

 cellular spaces were obliterated, while under reduced pressure 

 the cells became globular and the intercellular spaces were in- 

 creased in size above the normal. The difference between the 

 thickness of the cortical parenchyma under increased and that 

 under decreased pressure was enormous. In the wood the num- 

 ber of fibers increased and that of vessels decreased under added 

 pressure, while the number of bast fibers was greatly reduced by 

 increased pressure. Gemacher's conclusion was that it does not 

 require the enormous differences of bark tension to influence the 

 size of wood cells as had been maintained by Krabbe. 



Hoffman 115 also investigated the influence of pressure on cell 

 division and differentiation in the cambium of trees and con- 

 cluded that the forces which contribute to the development of 

 cylindrical stems rather than some other form are (1) bark ten- 

 sion and the consequent bark pressure, (2) radial-growth pres- 

 sure, and (3) the passive resistance of the wood. Cambial di- 

 vision and growth are said to continue only as long as growth 

 pressure exceeds bark pressure and it is thought that if bark 

 pressure is equal on all sides the axis must either be or soon will 

 become cylindrical on occurrence of continued radial growth. 

 This is shown by the fact that angular young shoots become 

 cylindrical on growing older. Even when the tension of the 

 bark is the same all around a branch bark pressure may be dif- 

 ferent at different points, being considerable at prominences and 



111 Gehmacher, A. Untersuchungen fiber den Einfluss des Rinden- 

 druckes auf das "Wachstum und den Ban der Rinden. Stizungsber K 

 Akad. Wiss. Wien. 88 AM. 1:878-96. 1884. 



116 Hoffman, R. Untersuchungen fiber die Wirkung mechanischer 

 Krafte auf die Teilung, Anordnung und Ausbildung der Zellen beim 

 Aufbau des Stammes der Laub- und Nadelholzer. Inaug. Dissertation. 

 Berlin. 1885. pp. 24. 



