444 New combe . — The Sensory Zone of Roots . 
half-circle before it reaches the row of seedlings in the other 
half of the diameter. The speed of revolution (this being 
twelve times to the minute) is rapid enough to overcome in 
large measure the tendency to eddies and cross-currents. 
The constancy of response by the roots indicates a constancy 
in direction of water-stream. 
A second criticism that might be advanced against the 
method here employed may be found in the assumed possibility 
of a current through the tubes, even if a very slow one, thus 
bringing the current against the elongating zone. This notion 
is worth considering, for it has been shown that a velocity of 
only one centimetre, or less, per minute is sufficient to call 
forth a rheotropic response in Raphanus sativus 1 , and a 
velocity of 2 cm. per minute will call forth a response in the 
roots of Zea Mays 2 , Vida sativa 2 , and Brassica alba l . There 
are three reasons why this possible explanation of the cause of 
the curves will not suffice, (i) The glass-tubes with the seed- 
lings were kept as nearly vertical as the eye could detect. If 
the glass-tubes were vertical in the water, no current could 
pass through the tubes, though there might be a little move- 
ment of the water a slight distance within the opening of each 
end of the tube. In all my experiments the lower end of the 
root was kept three or more millimetres (usually 10 to 1 2 mm.) 
above the lower end of the tube. It seems impossible to 
assume that any movement of water in a constant direction 
could have taken place within the tube at such a distance 
from its opening. ( 2 ) If the tubes were in some cases slightly 
inclined from the vertical direction, thereby causing possibly 
a very slow stream through the tubes, only those streams 
which flowed from above downward through the tubes could 
have caused the curves* of the roots ; for a stream flowing from 
below upward through the tube would keep the root from 
bending, the tendency of the rheotropic response being to turn 
the root- tip toward and parallel with the current of water. It 
1 Newcombe, Bot. Gazette, xxxiii, 1902, p. 177. 
2 Juel, Untersuchungen iiber den Rheotropismus der Wurzeln. Jahrb, f. wiss. 
Botanik, xxxiv, 1900, p. 515. 
