ORIENTATION OF PRIMARY TERRESTRIAL ROOTS 313 
and below this zone. Sachs stated that the roots first showed this 
difference in the form of their curvatures a httle time before the roots 
in air had reached their maximum curvature. 
Now I have attempted to follow somewhat more closely than did 
Sachs the curvatures in air and in non-fluid media. Although it is 
my intention to study further the relation of the medium to the 
course of the primary curvature the results of my experiments have 
been so uniform that it is appropriate to report them in this con- 
nection. Instead of recording the course of the curvature of the roots 
in written notes and free-hand drawings, I made photographs of the 
roots at intervals during the primary curvature. The roots which 
grew in air were mounted on corks which were fixed in crystallizing 
dishes as already described (see page 278 of this paper) and throughout 
the course of the curvature the roots remained in the same position 
relative to gravity, their position not being changed while the photo- 
graphs were being taken. For comparison with air, moist sawdust 
was used instead of earth because the penetrability of the sawdust 
could be readily varied and because, when soil was used as a medium, 
fine earth particles frequently came to lie between the root tip and 
the glass wall of the Sachs's box in which the roots were grown. Thus 
the form of the root was obscured. Repeated experiments had shown 
that the curvatures in earth and in moderately compressed moist 
sawdust were not appreciably different. Seedlings of Vicia faba var. 
equina and var. major were employed and photographs were made 
at frequent intervals of the course of the curvatures in air, in loose 
sawdust and in moderately compressed sawdust. While the roots 
were curving most actively, the intervals between exposures were 
shorter than when the change in the form of the roots was less active. 
Thus in the case of one series of photographs of roots growing at a 
temperature of 23° to 24° C, the intervals between the exposures were 
2 hours, I hour, 2 J hours, 6 hours, and 12 hours. The source of 
illumination employed was a 100 cp. Nernst lamp. The light passed 
through a glass box 10 cm. thick, which was filled with water, before 
falling upon the roots. The time of exposure varied from | to 2 
minutes according to the magnification used. The magnification 
was the same for all the photographs of a given series. The magni- 
fications used in different series varied from 2 X to 4 X . The curva- 
tures of the roots were compared by superimposing the negatives and 
viewing them by transmitted light and also by mounting prints from 
