56 Wisconsin Academy of Sciences, Arts, and Letters. 
Gehmacher 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 
114 Gehmacher, A. Untersuchungen uber den Einfluss des Rinden- 
druckes auf das Wachstum nnd den Bau der Rinden. Stizungsber. K. 
Akad. Wiss. Wien. 88 Abt. 1:878-96. 1884. 
116 Hoffman, R. Untersuchungen uber 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. 
