$902) THE RHEOTROPISM OF ROOTS 197 
Brassica nigra responds more slowly than Brassica alba but 
attains almost as good a percentage, and almost as great angles. 
__ The sensitiveness of its roots to light requires that it, too, be 
a kept in the dark in deter- 
«Mining its rheotropism. D> A @ a - 
Twenty-nine seedlings were 1 | BR / JF 4 
_ wed in two experiments with 
the revolving basins, the 2 a | 
—metemperturebeing 2, L) [| TTY ELT 
a the velocity so-™ to 450 per wis || ty te 
‘Minute, and the periods 9 | | | | 
and 15 hours. eeenty- > / } ad iy, 
four roots b iti 
(Sees ecame : 
and five ‘ eye Fic. 5.—Brassica alba. A tracing from 
ln | femained nlite | ae photograph, three-fourths natural -size. 
: "the €xperiment with the These seedli were set in glass tubes in 
longer period, allofthe roots rer te facilitate transfer for purposes of 
e ss hotographing. During the experiment the 
bering fift Positive, num- bar was held in a radius of the revolving 
oe ee een, were 45° to _ basin for 18 hours, water temperature 21°, 
from the vertical, The the range of velocities being 125°™ to 600 
sul gives ‘us 3 per cent perminute. At the conclusion, the seedlings 
were twisted around through an arc of 90° 
OS aie in order to show their angles in the photo- 
“a campestris was graph. The figure is fairly representative, 
ine two experim ents case that the tips declined slightly during oe 
- total 7 
a 
the preparation for photographing. 
a 
ots, 
with this plant. oe 
“a Was tested in two experiments, water tem- a 
velocity 60™ to 560™ per minute, and period 20° 
“2 four seedlings were used, of which thirty-two — 
to an angle of 15° to 80°, while two curved 
