Grossenbacher Radial Growth in Trees. 55 



The "normal" bark pressure was determined by stretching 

 rings of bark over a smooth cylinder by means of weights until 

 the bark had attained the length it had while still attached to 

 the tree. In his later work 113 the rings of bark were straightened 

 out and weighted at one end to determine the force required to 

 stretch the bark to its former length, for it was found that the 

 results obtained in this way were the same as those gotten with 

 the more elaborate apparatus. The bark pressure of conifers was 

 found to be usually under one-half an atmosphere and that 

 of broad-leaved trees about twice as great. In case of conifers 

 the pressure seemed to increase in fall on an average about 

 0.8 gm. per square millimeter of cross section, while the average 

 of similar measurements on a number of broad-leaved trees indi- 

 cated a decrease of pressure in fall equal to 12.5 gm. per square 

 millimeter of cross section. He maintained that the breaking 

 strain of bark is never reached by growth pressure. Bark pres- 

 sure was found greatest in regions of most rapid radial growth, 

 for instance on the side of excentric stems with the longer radius. 

 By using pressures from five to eight atmospheres the sum- 

 mer-wood type of radial growth was induced in spring on trees 

 having comparatively little difference in the size of spring and 

 summer-wood cells, while on trees having very marked differ- 

 ences between spring and summer wood it was practically impos- 

 sible to induce the formation of the summer-size of cells in 

 spring by increasing the bark pressure. In reducing the bark 

 pressure by means of longitudinal slits in the outer bark in sum- 

 mer, typical spring wood vessels developed in trees which nor- 

 mally have only a slight difference between size of spring and 

 summer wood cells; but in trees like Quercus and Fraxinus in 

 which a marked difference occurs between spring and summer 

 wood, the spring wood vessels could not be thus induced. 

 Krabbe therefore concluded that bark pressure remains practi- 

 cally the same throughout the growing season and that changes 

 in bark pressure could not be the cause of ring formation be- 

 cause it requires such a great increase to influence the size of the 

 wood cells. 



"'iJber das Wachsthum des Verdickungsringes und der jungen Holz- 

 zellen in seiner Abhangigkeit von Druckwirkungen. Abhandl. Kgl. 

 Akad. Wise. Berlin. 1884. Anhang. 1:1-80. 1885. 



