Grossenbacher — Radial Growth in Trees. 63 



diameter of summer-wood cells results from a reduced supply 

 of food to the cambial region; nevertheless, it is held to be a 

 more likely contention than that maintained by Hartig to the 

 effect that summer-wood results from an increase in the supply 

 of metabolized food. 



In this paper "Wieler cited similar experiments by Sachs 126 in 

 support of his conclusions, although Sachs noted that the fre- 

 quent addition of abundant nutrient solution failed to induce 

 more growth in small pots. Sachs held the dwarfing in small 

 pots to be due to a crowding of the root system into mats in such 

 a way as to greatly impair their absorptive functions. 



The relation of rest and food supply to the production of 

 wood rings: — Mer 127 held that the winter rest of the cambium 

 and its consequent great activity in spring in connection with 

 the abundance of plastic materials at that time are the causes 

 of the production of large-celled spring wood. The cell walls 

 of spring wood are thought to remain relatively thin because 

 the food transfer through such a thick differentiating zone of 

 cells is comparatively slow, and the thick walls of summer wood 

 cells are assumed to be due to slow rate of cambial division or 

 to- the thinness of the differentiating zone and consequent ready 

 access of organic food to its cells. The sudden and consider- 

 able decrease in the radial diameter of the peripheral few rows 

 of wood cells in a year's growth is held to be due to an arrest 

 of their development as a result of enfeebled cambial activity 

 rather than to an increase of bark pressure as maintained by 

 Sachs, de Vries and others. 



A summary and comparison of the hypotheses: — The work 

 of Kraus, de Vries, Nordlinger, Detlefsen, von Hohnel, Ge- 

 macher, Hoffman, Kny, Newcombe, von Schrenk, and Sorauer, 

 have made it apparent that pressure on the cambium affects the 

 rate of cell division as well as the size differentiating wood cells 

 may attain, but owing to the fact that no method has as yet 

 been developed by means of which quantitative measurements of 

 bark pressure can be made it is impossible to determine just 

 what relation bark pressure has to the production of "annual" 

 rings. 



12 ' Sachs, von, F. G. J. Vorlesungen fiber Pflanzenphysiologie. Leip- 

 zig. 1882. p. 623. 

 127 I. c. 



