ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 305 
faba var. equina, I made measurements of the relative resistance 
offered to the advance of the root tip through certain of the media 
which I employed. The root model was of one fourth greater diameter 
than the root after which it was made. The rod was first forced into 
the medium for one centimeter of its length. Then weights were placed 
upon a pan fixed to the upper end of the rod until the rod had pene- 
trated a given distance into the material. The determinations given 
in Table VII are typical of those obtained in all the experiments. 
This very considerable difference in the penetrability of the media 
may be conceived of as influencing the form of the downward curva- 
ture in one of two ways; indirectly by making possible contact stimuli 
of different intensities or directly by mechanically assisting in the 
geo tropic reaction in some such way as I have already suggested. 
(See p. 282 of this paper.) 
It is possible to explain the differences in the rate of secondary 
curvature of roots in loose sawdust or sphagnum and in these media 
when compressed or in soil if we accept Sachs's and Nemec's assumption 
that roots of the species which we and they have employed are posi- 
tively thigmotropic. The more compact the medium the more 
resistance it would offer to the change in the form of the root which 
results from the geotropic reaction. The more resistance the medium 
offered the more intense would be the thigmotropic stimulus and the 
more intense the reaction. Thus the downward curvature, the sum 
of thigmotropic and geotropic reactions, would become more and 
more acute the greater the compactness of the medium. It would 
not, however, be so easy to explain in this manner the fact, already 
referred to, that roots which have been growing obliquely downward 
in air for several days without appreciable curvature promptly bend 
downward when placed in soil or other compact media. In this case 
we are at a loss to account for the initial downward curvature which 
must occur before the root receives that one-sided contact stimulus 
which Sachs's theory demands. Moreover the evidence for the thig- 
motropism of terrestrial roots of the species which Sachs, Nemec and I 
myself have used is indeed meager. The curvatures which Darwin 
(1880, p. 528 ff.) reported to be negative reactions to contact stimuli 
were shown by Detlefsen (1882, S. 627) and others to have been in all 
probability traumatropic curvatures. The positive thigmotropism 
of terrestrial roots, which here concerns us, was first asserted by Sachs 
(1874, S. 437, 438). He experimented with roots of Pisum, Phaseolus, 
