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. 



