278 Hooker.—Hydrotropism in Roots of Lupinus albus. 
level, and was filled with a few drops of concentrated sulphuric acid. 
This produced a psychrometrical difference on the two sides of the roots, 
which, however, could not be measured as the hygrometers were too large. 
The whole apparatus was enclosed in a covered jar to obviate disturbing 
air-currents and placed in a dark cupboard. As the roots grew into the 
water they were pulled out of it by means of the pins to which they 
were fastened, and in this way never more than 2 mm. were immersed. 
Of 94 roots treated in this way, 50 bent away from the sulphuric 
acid, 8 towards it, and 36 remained straight or bent to one side. Although 
these results indicate that hydrotropic reactions may occur if the root-tips 
do not receive a direct stimulus, they nevertheless were considered uncon¬ 
vincing. 
An attempt was made to grow the roots in water first, to accustom 
them to that medium before the experiment just described was begun, but 
roots thus grown were found to be very insensitive to hydrotropic stimuli, 
and the greatest difficulty was met with in obtaining straight roots. 
E. The last experiment was repeated, with the difference that paraffin 
oil was substituted for water in the bottom of the glass vessel. The 
floating paraffin tray was fixed about 2 mm. from the two rows of im¬ 
mersed root-tips. This apparatus was enclosed in a jar, the air of which 
was kept as nearly as possible saturated. Of 117 roots, 81 bent away 
from the sulphuric acid, 12 towards it, and the remaining 24 stayed more or 
less straight. The reactions required eight to nine hours. These results 
were accepted as decisive. 
In discussing the perceptive region of roots, Rothert ( 26 ) made four 
categories to include all possibilities : 
1. Only a relatively short tip may be sensitive. 
2. The whole end from the growing region to the apex may be 
sensitive, the tip however to a greater degree. 
3. The whole end from the growing region to the apex may be equally 
sensitive throughout. 
4. The whole end except the tip may be sensitive. 
The second category has been shown to apply to hydrotropism. 
Molisch ( 24 ) demonstrated that the tip was highly sensitive, and the 
previous experiment shows that the region above the tip is also capable of 
receiving hydrotropic stimuli, but since more time was required for the 
reaction this region is probably less sensitive. The cells appear to lose 
their hydrotropism in proportion as they lose their embryonic qualities. 
It seems possible that some connexion may exist here, for differentiation of 
the root-cells would naturally render them ineffective for other purposes 
than those for which they become modified. 
6 . Nature of hydrotropism. Darwin ( 4 ) considered hydrotropism to be 
precisely the opposite of traumatropism, for in the one case the roots bend 
