THE RHEOTROPISM OF ROOTS 355 
{fone considers the more common responses of roots, he will 
ecome convinced that rheotropism is none of these. 
cation of Czapek’s *4 hypothesis as to the means of 
ce the publi 
connection of rheotropism with geotropism. If the 
of gravitation is perceived by the radial pressure of 
$s of cells upon those more deeply lying, the pressure 
one side of the root in flowing water would stimulate the 
Pressure, and might call forth a pseudo-geotropic 
: theotropic curve. Since, however, it is almost certain 
gravitation stimulus is perceived by only the apical I to 
ne root, and the rheotropic stimulus is perceived by ~ 
Of the root, it is not probable that rheotropism is 
seotropism. : 
tropism is not necessarily connected with heliotropism ; 
* Species, é. g., Raphanus sativus, have been found rheo- 
are not heliotropic. Nor is rheotropism connected _ 
ism ; for, as demonstrated by Pfeffer,*s in hydro- : 
‘the apical 1 to 2™ of the root is sensitive. Rheo- — 
“annot be traumatropism ; for in the former the response — 
when the stimulus is perceived by the tip of the root — 
tt farther back. Aerotropism has been appealed to 
CC nt for various curvatures of roots in water, . 
il species of plants with rheotropic roots for 
> -Hnging submerged chambers of air within a ise | 
f the side of root-tips which were several co 
the surface of water. The confined air was in 
fated from the roots by parchment paper. a 
= was no membrane intervening, the submerged 
tilted So as to hold the air, while the root “ 
Within a few millimeters of the edge of the air-contat 
In None of these cases were the roots cause 
than 5 to 10° from the vertical directi 
brin 
= 
i ber Geotropismus. Jahrb. Wiss. Bot. 32: 224- . 2 : 
Rothert, Flora 79: 212. 1894; and Czapek, Jahrb. bese, 
