ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 307 
with the cyHnder. The cyHnders were of three different sorts: glass 
tubes both smooth and ground, plaster of Paris cylinders well washed 
in water, and paraffine cylinders the surfaces of which were roughened 
by rolling in fine sand. Vicia sativa and Vicia faba seedlings were 
employed and proper precautions were taken to prevent hydrotropic 
reactions, the roots being frequently sprayed and the air around the 
roots being kept very moist throughout the experiments. The results 
were entirely negative. The roots in most cases remained in contact 
with the cylinders until the elongating regions of the roots were in- 
clined from 30° to 60° from the perpendicular. They then left the 
surface of the cylinders and grew nearly straight ahead. 
Altogether it may be said that there exists no conclusive evidence 
that the species Sachs and Nemec employed and those which I have 
employed are thigmotropic. We are certainly justified in assuming 
that thigmotropism if, as seems highly improbable, it exists in the 
case of terrestrial roots, is of sufficient intensity to account for the 
difference of secondary geo tropic curvature in air and earth. Still 
less can it account for the great difference in the curvature in loose 
and in compact sawdust or sphagnum. 
It remains for us to consider whether it is the direct mechanical 
influence of the medium upon the reaction of the root which brings 
about the reinforcement of the geotropic curvature. When a root 
about 2 cm. long is placed in air at an angle of from 45° to 50° from 
the normal position, a slight downward curvature takes place which 
is almost completely flattened. (This is shown at A in figure 2.) 
Thereafter the root elongates with curvature, if any, so slight as to 
be scarcely perceptible. The extreme tip, however, takes on the 
curvature already mentioned, which is maintained as the root elon- 
gates. This behavior of roots directed obliquely downward in air 
has been described earlier in this paper. If an obliquely directed 
root at the stage represented in figure 2, A 2, is planted in soil without 
changing its position relative to the perpendicular, it soon bends 
downward into the normal position. Now, as is clear from a glance 
at the figure referred to, it would be the upper side of such a root 
and not the lower side which would experience the greater friction 
against the soil particles as the curved tip is thrust forward through 
the earth by the increase in length of the region behind it. If such a 
root were thigmotropic in the sense in which Nemec and Sachs have 
maintained, the transfer from air, earth or other firm medium would 
