276 
RICHARD M. HOLMAN 
cap of the roots growing in earth did not restrict the curvature of the 
"plastic" portion of the root to any extent. The "plasticity" of the 
root tip and the rigidity of the root cap, assumptions necessary to 
Hofmeister's explanation, are no longer tenable but, quite aside from 
that fact, the similarity of the curvatures executed by roots growing 
in water to the curvatures of roots in air is quite sufficient evidence of 
the incorrectness of Hofmeister's explanation. 
Sachs (1874, S. 444-447) also gave some attention to this problem 
and one section of his paper "Ueber das Wachsthum der Haupt- 
und Nebenwurzeln," bears the heading " Verschiedenheit der Kriim- 
mung in Luft, Wasser, Sand und Erde" and deals with the subject 
with which we are concerned. In another part (S. 455-456) of the 
same paper Sachs suggests certain reasons for this difference of be- 
havior. He attributes the differences in the course of the curvature 
of roots in earth or sand on the one hand and water or moist air on 
the other to four factors: 
1 . A more rapid growth of the under than of the upper side of the 
curved roots in air or water. This, he believed, caused the flattening 
of the curvature. 
2. The resistance which the earth or sand offers to change in the 
form of the curvature. 
3. A loss by the forward part of the root of the ability to curve 
further. 
4. The assistance supplied to the geotropic curvature of roots in 
earth or sand by a positive thigmotropic reaction resulting from the 
greater friction of the lower than of the upper side of such geotropically 
curving roots in their passage through the soil or sand, i. e., against 
the soil or sand particles. 
Elfving (1880, S. 32, ff.) performed experiments in which he 
compared the curvature in air of inverted roots and of roots placed on 
a centrifuge and subjected to a stimulus of 50 g. for 24 hours, the tips 
being at the beginning directed toward the axis of rotation. Ob- 
serving a more complete curvature of the latter than of the former he 
concluded that roots in earth probably possess a greater sensibihty 
for the stimulus of gravity than do those in air. 
Nemec (1901, a, S. 88-96 and 1901, h, S. 310-313) in two papers, 
devoted primarily to an effort to substantiate the statolith theory, 
reported experiments and observations on roots diverted from the 
normal position while growing in moist air. He subscribed to Sachs's 
