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 Yries and others. 
A summary and comparison of the hypotheses •—The work 
of Kraus, de Yries, 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. 
126 Sachs, von, F. G. J. Vorlesungen fiber Pflanzenphysiologie. Leip¬ 
zig. 1882. p. 623. 
127 1. c. 
