3o6 
RICHARD M. HOLMAN 
Vicia faba and Zea. The roots were placed horizontally in air with a 
pin or piece of wood in contact with one side of the root at a point 
about I mm. behind the extremity of the cap. Positive curvatures 
were observed in the case of some roots. The concave side of these 
curvatures were toward the pin or piece of wood and the part of the 
root which had grown past the object lost the curvature. The curva- 
ture became permanent only when the part of the root behind the 
object had discontinued growth, the permanent curvature being at 
the point of contact with the "stimulating" object. Newcombe 
(1902, b, S. 243 ff.) by a large number of experiments carried out 
with great care, has demonstrated that when all possibility of injury 
to the root by the "stimulating" object is excluded, the roots of the 
species which Sachs employed exhibit no thigmotropic reaction what- 
ever. He looks upon Sachs's curvatures as traumatic in nature, 
resulting from direct injury to the tissue and consequent lessened rate 
of growth at the point of contact. 
Nemec (1901, a, S. 87) reported, as evidence of positive thigmo- 
tropism in the case of roots of Vicia faba, positive curvatures, resulting 
from placing on one side of the root tips droplets of plaster of Paris 
and water. Newcombe repeated these experiments (1904, S. 61, ff.) 
without obtaining any curvatures whatever. He employed only nine 
individuals and grants on that account that his negative results 
were no sufficient contradiction of Nemec's assertion. However, as 
Newcombe has pointed out, it is exceedingly difficult to determine 
whether such curvatures as Nemec reported are thigmotropic, chemo- 
tropic, hydrotropic or simply the result of prevention of growth of the 
tissue to which the dried drop of plaster is attached. 
I have not thought it necessary to repeat Newcombe's painstaking 
experiments but have endeavored to supplement them by determining 
whether a horizontally placed root which had previously grown for 
some time in an oblique position in air could be induced by contact 
"stimulation" to bend clear to the perpendicular in air. I used 
cylinders of different materials to furnish the contact "stimulus." 
These were in each experiment of such a diameter that the curvature 
of the surface corresponding to the secondary curvature of the roots 
when they were removed from the oblique position in which they had 
been growing in air and were placed horizontal without contact. 
Thus when the roots were placed horizontal and in contact with the 
cylinders in air the curvature of the root kept the under side in contact 
