532 POPULAR SCIENCE MONTHLY. 



ON THE PERCEPTION OF THE FORCE OF GRAVITY BY 



PLANTS. 



By FRANCIS DARWIN, F.R.S., Fellow of Christ College, 



PRESIDENT OF THE BOTANICAL SECTION OF THE BRITISH ASSOCIATION. 



TTTHEN I had the honor 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 summarize 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 move- 

 ments — namely, geotropism. I have said that the evidence is incom- 

 plete, 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 I 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 behavior 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 ' — i. e., to be executed at the 

 suggestion of changes in the environment, not as the direct and neces- 

 sary 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 suc- 

 cessfully through the difficulties of their surroundings. In the ex- 

 istence 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 con- 

 venient 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 rela- 

 tion 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 fash- 



