358 BOTANICAL GAZETTE [May 
hardly probable, therefore, that rheotropism stands in any bio- 
logical relation to water plants. 
Berg in his paper was unable to determine whether the 
velocity of water current had any effect on the response. Juel, 
_working only with Vicia sativa, obtained only straight or nega- 
tively bent roots in velocities as high as 1800™ per minute, 
while positive curves appeared in the majority of cases in veloc- 
ities from 1.8 to 1800™ per minute. The highest percentage 
and the strongest curves were found in velocities between the 
extremes named; the lower limiting velocity was not found. In 
my experiments data were obtained on the action of several spe- 
cies in varying velocities of water. Jt has been shown that 
velocities above 1000 per minute give generally negative ( mechan- | 
ical) curves, that the optimum velocity lies between roo™ and je 
per minute, and that velocities below 50° per minute bring fewer and 
slower responses with smaller angles. The remarkable result was 
_ obtained of finding the main root of Raphanus sativus rheotropi¢ 
in a velocity less than 1™ per second, while the directive influence 
ofa velocity of 2™ per second was no longer obeyed by = 
: ‘main roots of Brassica alba. It should be remembered, however 
that the character of the curve in these very weak currents di gee 
greatly from that shown in optimum currents, the actual 3 
n he latter becoming mere positive inclinations in th 
The latent period for rheotropism was stated by Pe 
for Zea mays one hour with the reading microscope; Ww 
: of Vicia sativa according to Juel is two hours. Table vi 
_ 272 of this paper shows the shortest latent period oo 
eight species of plants. There it is seen that mae? 
_ greatly in time of response, though the latent period for @ 
_ felatively long compared with that for geotropism. 
_ with the heliotropic response, the geotropic response 0° 
alba and Helianthus annuus is not much slower. — 
_Tatent period for rheotropism has not, however, been 0°" 
_ by anybody for any plant. In the first place, the xP 
_ have not been carried on in the optimum temperature 101 & 
and in the second place, all experiments, so far, hay 
