1902] THE RHEOTROPISM OF ROOTS 279 
: ward dip of 5° to 40°. This appearance is well illustrated 
in fig. 72. 
Both Berg (p. 27) and Juel (p. 529) have discussed this. 
_ phenomenon, but neither, to my mind, in a very satisfactory 
_ ftmanner. The former suggests that the apical 2™™ of the root 
_ May not be sensitive to the water stream stimulus, and hence 
__ this part does not take the direction of the growing zone, which 
_ Berg believes to be sensitive. The objection to the argument of 
Berg lies in the fact that it is not a question of sensitiveness but 
a question of response. The receptive tissue and the responsive 
_‘lssue may be, we know, quite widely separated in various 
‘imitable phenomena. Juel explains the S-shaped curve formed 
3 by the theotropic curve and the geotropic counter-curve as result- 
- ing from the competition of rheotropism and geotropism, the 
net being the stronger in the proximal part of the growing 
Fras the latter being the stronger in the distal 2™”. . 2 
‘ — proceeding with the discussion of the S-shaped curve, 
is at ag to State that it is not peculiar to rheotropism, but 
wn also in heliotropism. If seedlings of Brassica alba be set 
aia usual way with one-sided illumination, I have found 
gh se heliotropic curve reaches or surpasses 45°, that the 
*Pral 2 show a geotropic counter-curve like that seen in rhe- © 
ey adi now, we take into account the experiments of S 
ee “ag Nemec? showing that geotropic sensitiveness 1S 
ogy confined to the apical 1.5™™ to 2™™ of the root, and 
ae petiments of Czapek* showing that roots receive their 
i erotic stimulus when at an angle of 135° above oe | 
ony downward position, and that the effect of the 
aes diminishes constantly with the reduction of this angle, 
ey see that the geotropic downward dip of arheo- 
"Y curved root diminishes greatly the strength of the 
ws Untersuchungen iiber . Geotropisinas. Jahrb. Wiss. Bot. 9732433 [ 
78. 
» Wiss, ‘ogg “ Wahrnehmung des Schwerkraftreizes bei den eens 
aie, Wiss. Bot. 27 : 283. 
thine 
