On Geotropism and the Localization 
of the Sensitive Region 1 . 
BY 
FRANCIS DARWIN, M.B., F.R.S, 
With Plate XXIX, 
T WO well-known cases of localization of heliotropic and 
geotropic sensitiveness are described in the Power of 
Movement in Plants. In the case of heliotropism the method 
is simple and the conclusion clear. The seedlings of Phalavis 
are extremely sensitive to light, but when little caps of an 
opaque material are placed on the tips of the plants they 
no longer bend towards the light, or only do so in a very 
slight degree, as Rothert 2 has shown. Now since, in this 
experiment, the part which is capable of curvature is fully 
exposed to light, it follows that the power of perceiving light 
resides chiefly in the tip, which thus plays the part of a nerve- 
centre specialized so as to receive a stimulus and to transmit 
it to the motor apparatus. 
In the case of the gravitation stimulus the case is not so 
simple: we cannot shelter part of a plant from gravitation 
as we can from light, and the only plan which at first 
suggested itself was the removal of the sensitive part by 
1 Paper read before the Botanical Section of the British Association, Dover, 
Sept. 1899. 
2 Cohn’s Beitrage, Bd. vii, 1894. 
[Annals of Botany, Vol. XIII. No. LII. December, 1899.] 
