318 Pfeffer. — Geo tropic Sensitiveness of the Root- tip. 
that perceives the geotropic stimulus, and, as a result, enables 
the adjacent parts, that are themselves not sensitive, to carry 
out the geotropic curvature. 
The geotropic reaction of the root was first conceived of 
in this way by Ch. Darwin 1 in the year 1880. Absolute 
proof of this view, to be sure, was not obtained by Darwin, 
nor has it been by the studies of other investigators 2 . As 
a matter of fact, however, Darwin’s assumption is right, as 
is irrefutably proved by investigations that have recently 
been conducted under my direction by Dr. Czapek. 
All previous investigations have been inconclusive because 
they were judged of by the results that followed cutting off 
the tip of the root. But by the infliction of such a wound 
the previous capacity for reaction may be suspended or 
destroyed. If then the root, after removal of the tip, no 
longer reacts geotropically, it is still wholly undetermined 
whether this result is occasioned by the want of the sensitive 
root- tip, or by the fact that through the infliction of the 
wound the geotropic sensitiveness has been suspended. 
That such a suspension of the capacity for reaction may 
occur is shown very clearly by investigations on heliotropic 
sensitiveness which Professor Rothert 3 conducted in the 
Leipsic botanical institute. It was found, among other 
things, that the young cotyledon of Avena sativa is helio- 
tropically sensitive throughout, but especially so at the tip. 
An heliotropic curvature of the uninjured leaf follows, there- 
fore, as well when the tip alone, as when the middle of the 
leaf alone, is exposed to light. But if a small piece is cut off 
from the leaf-tip, the leaf becomes indifferent to illumination 
from one side, and only after one or two days, that is after the 
reaction due to wounding has ceased, does the sensitiveness 
return. 
1 The Power of Movement in Plants, 1880, p. 523. 
2 Among those who have studied the question are Detlefsen, Wiesner, Fr. Darwin, 
Kirchner, Krabbe, Brunchorst, Fritsch. The literature is given by Brunchorst, 
Berichte d. deutsch. botan. Gesellschaft, 1884, p. 79. 
3 Berichte d. deutsch. botan. Gesellschaft, 1893, p. 387. 
