TRANSACTIONS OF SECTION K. 763 
Section K.—BOTANY. 
PRESIDENT OF THE Section—Francis Darwin, M.A., M.B., F.R.S. 
THURSDAY, AUGUST 18. 
The President delivered the following address :— 
On the Perception of the Force of Gravity by Plants. 
Wuen [ had the honour of addressing this Association at Cardiff as President 
of the mother-section from which ours has sprung by fission—I spoke of the 
mechanism of the curvatures commonly known as tropisms. To-day I propose to 
summarise the evidence—still far from complete—which may help us to form a 
conception of the mechanism of the stimulus which calls forth one of these 
movements—namely, geotropism. I have said that the evidence is incomplete, 
and perhaps I owe you an apology for devoting the time of this Section to an 
unsolved problem. But the making of theories is the romance of research; and 1 
may say, in the words of Diana of the Crossways, who indeed spoke of romance, 
‘The young who avoid that region escape the title of fool at the cost of a celestial 
crown,’ I am prepared for the risk in the hope that in not avoiding the region 
of hypothesis I shall at least be able to interest my hearers. 
The modern idea of the behaviour of plants to their environment has been the 
growth of the last twenty-five years; though, as Pfeffer has shown, it was clearly 
stated in 1824 by Dutrochet, who conceived the movements of plants to be 
‘ spontaneous ’"—2.e., to be executed at the suggestion of changes in the environment, 
not as the direct and necessary result of such changes. I have been in the habit of 
expressing the same thought in other words, using the idea of a guide or signal, by 
the interpretation of which plants are able to make their way successfully through 
the difficulties of their surroundings. Inthe existence of the force of gravity we 
have one of the most striking features of the environment, and in the sensitiveness 
to gravity which exists in plants we have one of the most widespread cases of 
a plant reading a signal and directing its growth in relation to its perception. 
I use the word perception not of course to imply consciousness, but as a convenient 
form of expression for a form of irritability. It is as though the plant discovered 
from its sensitiveness to gravity the line of the earth’s radius, and then chose a 
line of growth bearing a certain relation to the vertical line so discovered, either 
parallel to it or across it at various angles. This, the reaction or reply to the 
stimulus, is, in my judgment, an adaptive act forced on the species by the struggle 
for life. This point of view, which, as I regret to think, is not very fashionable, 
need not trouble us. We are not concerned with why the plant grows up into the 
air or down into the ground; we are only concerned with the question of how the 
plant perceives the existence of gravitation. Or, in other words, taking the 
reaction for granted, what is the nature of the stimulus? If a plant is beaten 
down by wind or by other causes into a horizontal position, what stimulative 
change is wrought in the body of the plant by this new posture? 
