1900 ] MECHANISM OF ROOT CURVATURE 43 
preferable to that of injury for showing the effect of stimuli on 
the relation of axial cylinder to the cortex on the concave side 
of the curving root. I have therefore used gravitation as the 
stimulus in the following experiments. Seedlings of Cucurbita 
Pepo with roots 5-20™ long were placed in a horizontal position 
in moist air, and when the curve had begun to show or before, 
the roots were split in the growing region and the upper or con- 
vex half of the tip was removed. For control observations 
roots were left whole or the lower half of the tip was removed. 
Thirty-four roots with one half of tip removed were placed in a 
horizontal position in moist air with the cut face upward. In 
two to four hours ten had curved slightly upward, six had 
curved slightly downward at tips, the others, eighteen in num- 
ber, remained straight. In six to eight hours fifteen had curved 
more or less upward, eight downward, and eleven still remained 
straight. After twenty-four hours eleven were curved upward, 
and eighteen were curved downward. 
As control for these, thirteen roots were laid in a horizon- 
tal position without injury. They all turned downwards within 
six to eight hours. Also seventeen roots were split and laid 
horizontal with cut face down. They likewise had all turned 
downward within six to eight hours, and some of them had 
formed a curve of 90° within two hours. There was, therefore, 
a very remarkable difference between the behavior of those 
roots which had the cut face of the split tip turned down and those 
which had the cut face turned up. While the former began at 
once to curve in toward the cut face,and the curve was com- 
pleted rapidly, the latter nutated up and down for some time, 
and even at the end of twenty-four hours after splitting it was 
still undecided in which direction some of them would curve. 
In the thirty-four roots observed a larger number at first started 
to curve upward, and a few continued to curve in that direction. 
Others of this number later reversed the curve and finally 
turned downward. They sometimes showed a double curve, the 
split part farthest from tip turning up, but nearer the tip turning 
down. The same root often nutated up and down several 
