472 



NA TURE 



[September 8, 1904 



zontal, and that as the organ is moved further and further 

 from the normal (in successive experiments) the geotropic 

 reaction ought to increase decidedly as the horizontal is 

 passed ; and this is not the case. 



T)iagcotrofiism. 



The diagram, Fig. 2, will serve to represent a diageo- 

 tropic organ in stable equilibrium. In spite of the fact 

 that it is at rest in the horizontal position, we must assume 

 that the tangential (horizontal) walls of the endodermis are 

 sensitive to the pressure of the statoplasts. For when the 

 organ is placed obliquely it has the power of returning, by 

 curvature, to the horizontal ; and this requires that the plant 

 shall distinguish up from down. If its apex is above the 

 horizon it must curve downwards, i.e., towards that side on 

 which the statoplasts rest on the external walls of the endo- 

 derm cells, and vice -versa if the apex is below the horizon. 

 But what signal tells the plant that it is not horizontal ? 

 This can only be effected by the statoplasts pressing on the 

 basal or apical walls, as in Fig. 3. 



The difficulty is increased by the fact that when a diageo- 

 tropic organ is fixed vertically, the apex being up or down,' 

 no curvature follows. This, according to the usual idea, 

 would mean that the terminal walls are not sensitive. But 

 the walls must be sensitive in some way, or the plant would 

 not react to the gravistimulus, as it undoubtedly does. The 

 only conclusion I can come to is that the position of tl),e 

 statoliths shown in Fig. 3, in which they rest partly on the 

 terminal wall and partly on the lateral (tangential) wall, 



must be capable of giving the combined stimulus," as above 

 suggested. 



Personally I do not attach great importance to the details 

 of how the statoliths act on the different walls of the cells, 

 although as part of the history of the inquiry I feel bound 

 to discuss it. The broad fact that the statoliths rest on 

 different parts of the cell-walls when the geotropic organ is 

 placed at different angles with the vertical seems to me 

 sufficient. The precise manner in which various reactions 

 are associated with the position of the statoliths may be 

 confessed to be for the present beyond our knowledge or 

 powers of imagination, and such confession need not weaken 

 the position of our theory. 



Finally, I desire to say a word on a subject having but 

 a remote connection with my theme. There is at the present 

 time a tendency to pay an increasing attention to what is 

 known as rectipetality or autotropism — viz., the inherent 

 capacity of rectilinear growth. In my Cardiff Address^ to 

 Section D I showed that rectipetality is really part of the 

 phenomena of circumnutation. We must believe that 

 rectipetality does not merely come into play in those com- 

 paratively crude experimental instances in which a geotropic 

 curvature is flattened out by means of growth on the 

 klinostat. We must believe that it also corrects curvatures 

 which arise from the slight irregularity of normal every- 

 day growth. This will imply that normal growth is built 



1 Czapek (08. p. 243). Noll (92, p. 37) had foreseen on th;oreical grounds 

 that this would prove to be the case See also Noll (30, p. 473). 



- In Noll's diagram of the stimulation-areas in a diageolropic organ the 

 obliquely placed areas seem to suggest a similarity to what is here given 

 Isee Noll (92, p. 29)]. But his stimulation areas in which only a single 

 statoliih occurs are not strictly comparable to cells containing 

 statoplasts. 



s F. Darwin (91). 



NO. I 8 19, VOL. 70] 



of a series of internal corrections ; in other words, of circum- 

 nutation. The point I wish now to emphasise is that the 

 stimuli, be they of geotropic or any other nature, should be 

 conceived as acting not on a stationary but on a moving 

 plant — acting, in fact, on the spontaneous correcting power, 

 whether we call it rectipetality, autotropism, or circum- 

 nutation. It is impossible to say how this consideration 

 might modify our speculations as to the manner of action 

 of the gravistimulus. It is quite conceivable that it might 

 not alter our theoretic views at all, but without more know- 

 ledge we cannot be certain. My only point at present is 

 that if we are led into contradictions or confusion by attempts 

 to analyse what goes on in the gravisensitive region accord- 

 ing to the statolith theory, such a result must not be held 

 to be fatal to the theory until we know inore of the problem. 



In conclusion — and to clear our minds of the doubtful 

 speculations in which I have entangled myself — I should like 

 to reiterate my belief in the general, though not the 

 universal, applicability of the statolith theory. I find it 

 impossible to doubt that, in the case of the higher plants, 

 sensitiveness to the pressure of heavy bodies will be found 

 to be by far the most important, if not the exclusive, means 

 by which gravity is perceived. We have seen that the 

 stimulus must depend on weight ; and since neither the 

 theory of radial pressure nor Noll's supposition of stimu- 

 lation by small unknown bodies lends itself to experimental 

 inquiry we are driven, as practical people, to test the views 

 of Haberlandt and N^mec. 



I base my belief partly on what I have already said, 

 namely, that geotropism, being an adaptive reflex action, 

 must during its development have been correlated, by that 

 mysterious bond which unites stimulus to reaction, with 

 some change, by which in the natural course of events it is 

 uniformly preceded. Now the most obvious change which 

 precedes geotropism is the disturbance of the falling starch- 

 grains. This fact, together with what we know of the dis- 

 tribution of statoplasts, would almost force conviction on 

 me. But this is not the whole of the evidence. We know 

 from Nemec's researches that the protoplasm, in the cells 

 assumed to be sense-organs, is sensitive to the pressure of 

 the statoplasts : and we know from zoological evidence that 

 heavy bodies resting on a sensitive surface can function as 

 a sense-organ for gravitation. Finally, the experimental 

 evidence, though not absolutely convincing, has not revealed 

 any absolute bar to our belief in the statolith theory, ancT 

 has brought to light a number of facts harmonising with* 

 it in a remarkable manner. It seems to me that the theory 

 of Nemec and Haberlandt may fairly hold the field until 

 a better theory of graviperception and a better theory of 

 the function of falling starch-grains are established. 



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ihe Connection between Geotropism pnd 

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(01) 

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(oi) . . On Growth Curvatures in Plants. Report of ihe 



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I 



