304 
RICHARD M. HOLM AN 
there would be a greater tendency for carbon dioxide to accumulate 
and for oxygen to become depleted. The evidence in the literature 
in regard to the effect of increase of carbon dioxide and decrease of 
oxygen upon the geo tropic reaction of roots (see Pfeffer, 1906, pp. 
140, 143 and 145) would lead us to expect a flatter curvature in a 
more compact than in a less compact medium, if indeed the accumu- 
lation of CO2 and depletion of O2 reached a degree sufficient to affect 
the geotropic reaction at all under these circumstances. In water 
on account of the relative high solubility of CO2 as well as on account 
of the hindrance which the walls of the containing vessel would offer 
to diffusion the effect of the accumulation of O2 and accumulation of 
CO2 would in all probability be more extensive than in earth. Yet, 
in water, the root behaves very much as in air. 
Table VII 
Measurements of the Resistance Offered by Soil and Loose Moist Sawdust to Penetration 
by a Glass Rod Having the Form of a Primary Root of Vicia faba var. equina 
Weight Required 
Mean of Pre- 
ceding Values 
to Force Rod 44 
Plus Weight of 
Mm. into the 
Rod and Pan 
Medium 
Medium 
(4 Grams) 
Loose moist sawdust 6 days after being placed in a 
Sachs's box 
17 grams 
17 " 
15 " 
16 + 4 = 20 
17 " 
16 " 
Freshly prepared, loose, moist sawdust 
7 " 
8 " 
7 + 4 = II 
7 " 
6 " 
Moist sieved garden soil 6 days after being placed 
in a Sachs's box 
80 " 
80 " 
85 " 
86 + 4 = 90 
95 " 
ftr> " 
With compression, the resistance offered to the advance of the 
root tip in such media as sawdust or sphagnum would vary greatly 
and in earth it would be relatively high. In air or water, on the other 
hand, it would be entirely negligible. By means of a glass rod whose 
extremity was given the approximate form of the root tip of Vicia 
