Grossenbacher Radial Growth in Trees. 53 



cells had become two to three times that produced in the normal 

 portions of the ring. Similar experiments also showed that the 

 amount of radial and tangential growth of cells differentiating 

 from the cambium is inversely proportional to the pressure ex- 

 erted on them. It also seemed that pressure acts as a selecting 

 agent in determining the proportion of vessels to wood fibers; 

 i. e. the greater the pressure the fewer the vessels and the more 

 numerous the wood fibers to be produced. De Tries concluded 

 therefore that bark pressure influences the rate of cambial di- 

 vision as well as the relative size cells may attain during differ- 

 entiation. Since bark-growth follows the enlargement of the 

 wood cylinder it was thought evident that bark pressure is 

 greater toward the end of the radial-growth period than at' its 

 beginning. For these reasons de Vries held that a seasonal 

 change in bark pressure is the chief cause of seasonal growth ap- 

 pearing as "annual" rings. 



In some later experiments, while studying wound wood, he 109 

 found on lifting loose strips of bark with a knife on the concave 

 side of young tree-trunks held in a bent position, and then tying 

 it in place again in such a way as to prevent evaporation, that 

 numerous large vessels developed in the new wood produced un- 

 der the strips. He reiterated his former conclusion that bark 

 pressure is an important factor in determining the size of wood 

 cells and that it is largely responsible for the difference between 

 spring and summer wood. 



That bark tension does occur on enlarging stem structures had 

 been shown by Kraus 109 as well as by Nordlinger 110 but neither 

 of them secured quantitative results of value. 



The influence of pressure on cambial activity and cell differ- 

 entiation have since been investigated from various viewpoints 

 and have led to different conclusions. Hohnei 111 found sharp- 

 angled transverse displacements in the bast fibers of many 

 Dicots at points where neighboring cells make an abrupt uneven 



109 Vries, de, H. Ueber Wundholz. Flora. 34:2-8; 17-25; 38-45; 

 49-55; 81-88; 97-108; 113-21; 129-39. 1876. 



109 Kraus, G. Die Gewebespannung des Stammes und ihre Folgen. 

 Bot. Zeit. 25:105-19; 121-33; 137-42. 1867. 



110 Nordlinger, H. Spannt die Baumrinde im Sommer nicht? Kritische 

 Blat. Forst-u. Jagdwiss. 52: (1) :253-55. 1870. 



111 Hohnei, von, F. Ueber den Einfluss des Rindendruckes auf die 

 Beschaffenheit der Bastfasern der Dicotylen. Jahrb. Wiss. Bot. 

 15:311-26. 1884. 



