ORIENTATION OF SECONDARY TERRESTRIAL ROOTS 413 
from photographs of roots in earth and loose moist sawdust cultures. 
Within a few hours after they had been displaced from their limiting 
angle, the roots in both media showed a very distinct curvature of the 
tip which was more distinct in the case of roots in loose sawdust than 
in the case of those in earth. As the roots in the former medium 
continued to grow, the curvature of the tip was continually flattened 
behind and was constantly reformed at the very extremity of the root, 
as in the case of roots in air. The flattening was not, however, com- 
plete and there remained a slight permanent curvature. In some 
cases this permanent curvature was so slight that it was perceptible 
only after the root had elongated three or four centimeters. The 
curvature of the tips of roots in soil was almost completely fixed and 
as the roots elongated an acute permanent curvature resulted which 
brought the root tip into a position not much (5°-20°) removed from 
the former limiting angle. Thus a secondary root in earth generally 
reached within 24 hours at 15° to 16° C. a position which was attained 
by a root in loose sawdust only after 3 to 5 days. The secondary roots 
on the lower side of a horizontally placed main root often grew for 
2 or 3 days in either of the media without showing any tendency to 
curve upward. This was due apparently to two factors. The first 
of these is the relatively slight angle these roots formed with the normal 
position of the secondary roots. The second is the still unexplained 
tendency of secondary roots which are displaced downward from their 
limiting angle to react less promptly and intensely than secondary 
roots which have been equally diverted above the normal position of 
rest.^^ The downward directed secondary roots, however, curve more 
promptly upward when growing in earth than when they are in loose 
sawdust. 
The very close parallel which exists between the behavior of 
secondary roots under stimulus of gravity in different media and 
primary roots under the same conditions indicates that the part 
played by the medium in both cases is the same. After flattening of 
the primary curvature mechanical resistance to change in the root's 
form and to the advance of the root is necessary to complete reaction 
and within certain limits the greater this resistance is the more 
promptly the reaction is completed. As in the c'ase of the primary 
12 Sachs (1. c. p. 627) noted the increase of the bounding angle following each 
curvature. 
13 Czapek, 1. c. p. 328 ff. 
