64 Wisconsin Academy of Sciences , Arts, and Letters. 
The different degrees of hydrostatic pressure assumed by Rus- 
sow as the cause of the difference between spring and summer 
wood has apparenty also been implied by Hartig, Mer and 
others in speaking of growth force, etc., but even more than in 
the former case do the few qualitative tests need to be replaced 
by quantitative measurements before the validity of the idea 
could be tested. 
Hartig has collected a mass of observational and even some 
indirect quantitative data that seem to support his hypothesis 
that the relative abundance of elaborated food determines the 
thickness of cell walls and that the relative intensity of the 
transpiration stream determines the length of the radial diam¬ 
eter of wood cells, but the experiments of Jost, Lutz and others 
show that although food and water may be present in great 
abundance very little or no radial growth occurs when termi¬ 
nal growth is prevented. 
Wieler’s hypothesis that the abundance of metabolized food 
in the cambial region in spring induces the formation of spring 
wood and its reduction, summer wood is also lacking in that it 
does not account for the cessation of radial growth on the re¬ 
moval of the elongation structures. Besides, the experiments 
with which he assumes to have made his contention probably in¬ 
volved too many unknown variables to afford even a satisfactory 
test of the hypothesis. 
The results obtained by Morgulis 128 in his experiments in al¬ 
ternately feeding and starving salamanders tend also to make 
one skeptical regarding the value of the hypotheses of both 
Hartig and Wieler as explanations of ring formation because 
Morgulis found “That the rate of growth is independent of the 
amount of nutrition ’ ’ and that ‘ ‘ The impulse to grow plays the 
leading part’’ and “determines the degree of utilization of the 
nutriment.” Finally, he found too that “From all that has 
preceded, the conclusion can be drawn that periodic starvation 
is more detrimental to the organism than acute starvation fol¬ 
lowed by a liberal supply of food. In the former case the in¬ 
dividual remains below the level of the normally fed animals; 
in the latter case, on the contrary, provided the inanition has 
128 Morgulis, S. The influence of protracted and intermittent fasting 
upon growth. Amer. Nat. 47: 477-87. 1913. 
