Francis Darwin. 
69 
LECTURES ON THE PHYSIOLOGY OF MOVEMENT 
IN PLANTS. 1 
By Francis Darwin. 
V. —The Sense-Organs for Gravity and Light. 
All cases of stimulation depend on an influence of some kind 
affecting the protoplasm of the plant. Any arrangement which 
facilitates or intensifies the action in question may be called a 
sense-organ. Haberlandt writes 2 “ I describe as sense-organs or 
perception-organs all those morphological or anatomical con¬ 
trivances which serve for the reception of an external stimulus, and 
exhibit a more or less far-reaching correspondence between 
structure and function in this respect.” Haberlandt’s remarks are 
introductory to his treatise on the perception of contact-stimulus, 
in which the tropic element is not prominent; but they are also 
applicable to those contrivances by which plants orientate them¬ 
selves in regard to stimuli such as gravity and light. 
In the case of geotropism we want to know how (by what 
machinery) the plant perceives that it is not vertical. I have dis¬ 
cussed this question in the Presidential Address to Section K at 
the Cambridge meeting of the British Association, 1904. But in 
order to give you the present state of the question, it will be 
necessary to repeat, by way of introduction, the essential part of 
what is there given. If a plant growing vertically is turned through 90° 
so that the unsupported stem projects horizontally, it is clear that 
strains and compressions will occur in the tissues of the plant 
which were not before existent. It is quite conceivable that such 
strains might constitute a stimulus to geotropism. But it is also 
clear that this is not the only means of geoperception, since plants 
supported throughout their length are nevertheless geotropic. 
It has been suggested 3 that the change in hydrostatic pressure 
which would occur in a turgescent cell placed horizontally might 
originate a stimulus. Here again we can only say that such a 
thing is conceivable. A cell in the vertical position would have 
equal pressures on any two opposite points in the longitudinal cell- 
walls. But when such a cell is placed horizontal, the pressures on 
1 A Course of Advanced Lectures in Botany given for the Uni¬ 
versity of London at the Chelsea Physic Garden in the 
October term, 1906. 
3 Siunesorgane im Pjianzenreich, 1901, p. 9. 
3 Pfeffer, Pflanzenphysiologie, Edition I., 1881, Vol. i., p. 330. 
