September 8, 1904] 



NA TURE 



467 



the region of hypothesis 1 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 1S24 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 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, bv the interpretation of which 

 plants are able to make their way successfully through the 

 difficulties of their surroundings. In the 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 bodv of the plant bv this new posture? 



It is conceivable in the case of a stem supported by one 

 end and projecting freely in the air that the unaccustomed 

 state of strain might act as a signal. The tissues on one 

 side (the upper) are stretched, and they are compressed 

 below : this might guide the plant ; it might, in fact, have 

 evolved the habit of rapid growth in the compressed side. 

 This is only given as an illustration, for we know that the 

 stimulus does not arise in this way, since such a plant, 

 supported throughout its length, and, therefore, suffering 

 no strain, is geotropically stimulated. The illustration is 

 so far valuable, as it postulates a stimulus produced by 

 weight, and we know from Knight's centrifugal experiment 

 that weight is the governing factor in the conditions. Since 

 we cannot believe that the stimulus arises from the strain 

 as affecting the geotropic organ as a whole, we must seek 

 for weight-effects in the individual cells of which the plant 

 is built. We must, in fact, seek for weight-effects on the 

 ectoplasm ' of those cells which are sensitive to the stimulus 

 of gravity. 



If we imagine a plant consisting of a single apogeotropic 

 cell we shall see that the hjdrostatic pressure of the cell- 

 contents might serve as a signal. 



.■\s long as the cell is vertical the hydrostatic pressure of 

 the cell-sap upon the ectoplasm at C (Fig. i) is equal to 

 that at D. But the pressure on the basal wall, B, differs 

 from that at A (the apical wall) by the weight of the 

 column AB. If the plant be forced into the horizontal, the 

 pressure at A and B becomes the same, while the pressure 

 at C no longer equals that at D, but differs by the weight 

 of the column CD. Here undoubtedly is a possible means 

 by which the plant could perceive that it was no longer 

 vertical, and would have the means of distinguishing up 

 from down. So that if it were an apogeotropic plant it 

 would need to develop the instinct of relatively accelerated 

 growth on the side D, on which the pressure is greatest. 



What is here roughly sketched is the groundwork of the 

 theory of graviperccption - suggested bv Pfeffer ' and sup- 

 ported by Czapek,* which I shall speak of as the radial 

 pressure theory, and to which I shall return later. 



t See Noll's ingenious reasoning by which he makes it clear that the 

 stationary ectoplasm, not the flowing endoplasm, is the seat of stimulation. 

 Noll (88). 



- I propose this term in place of gt0acsthesia, which does not lend itself to 

 the formation of adjectives, or the hybrid vtorAgeofiercefiiiott. Ry not using the 

 form " geo "we avoid any necessary connection with geotropism, and may thus 

 use terms compounded o(^-az't for phenomena other than those of curvature. 



3 Pfeffer (Si). -1 Czapek (98), (01). 



It is obvious that there is another consideration to be 

 taken into account, namely, that cells do not contain cell- 

 sap only, but various bodies — nucleus, chloroplasts, crystals, 

 &c. — and that these bodies, differing in specific gravity 

 from the cell-sap, will exert pressure on the physically lower 

 or physically higher cell-walls according as they are heavier 

 or lighter than the cell-sap. Here we have the possibility 

 of a sense-organ for verticality. As long as the stem is 

 vertical and the ape.x upwards the heavy bodies rest on the 

 basal wall, and the plant is not stimulated to curvature ; 

 but if placed horizontally, so that the heavy bodies rest on 

 the lateral cell-walls, which are now horizontal, the plant 

 is stimulated to curve. This is known as the statolith 

 theory. 



It seems to me quite certain that the stimulus must 

 originate either in the weight of solid particles or in the 

 weight of the fluid in the cells, or by both these means 

 together. And for this reason. Take the statolith theory 

 first. There undoubtedly are heavy bodies in cells ; for 

 instance, certain loose, movable starch-grains. Now, 

 either these starch-grains are specialised to serve the purpose 

 of graviperception or they are not. If they are so 

 specialised, cadit quaestio ; if they are not, there still 

 remains this interesting point of view : the starch-grains fall 

 to the lower end of the cells in which they occur ; therefore, 

 shortly before every geotropic curvature which has taken 

 place since movable starch-grains came into existence, there 

 has been a striking change in the position of these heavy 

 cell-contents. Now, if we think of the evolution of 

 geotropism as an adaptive manner of growth we must con- 

 ceive plants growing vertically upwards and succeeding in 

 life, others not so behaving, and consequently failing. 



There will be a severe struggle tending to pick out those 

 plants which associated certain curvatures with certain 

 preceding changes, and therefore it seems to me that, if 

 movable starch-grains were originally in no way specialised 

 as part of the machinery of graviperception, they would 

 necessarily become an integral part of that machinery, since 

 the act of geotropism would become adherent to or associated 

 with the falling of the starch-grains. 



This argument must in fairness be applied to any other 

 physical conditions which constantly precede geotropic 

 curvature ; it is therefore not an argument in favour of the 

 statolith theory alone, but equally for the pressure theory, 

 and cannot help us to decide between the two points of view. 



Are there any general considerations which can help us 

 to decide for or against the statolith theory? I think there 

 are — namely, (i) analogy with the graviperceptive organs 

 of animals ; (2) the specialisation and distribution of the 

 falling bodies in plants. 



(i) Berthold ' (to whom the credit is due" of having first 

 suggested that Dehnecke's falling starch-grains might 

 function as originators of geotropic reaction) is perhaps 

 somewhat bold in saying that " the primary effect of 

 gravity " as regards stimulation must depend on the passive 

 sinking of the heavier parts. Noll, too," says that Knight's 

 experiment depends on weight, and not the weight of com- 

 plete parts of the plant-body, but of weight within the 

 irritable structure. I cannot see that these downright 

 statements are justified on direct evidence, and I accord- 

 ingly lay some stress on the support of zoological evidence. It 



1 " Protoplasmamechanik," 1S86, p. 73. I was direcled to this passage 

 by Pfeffet's discussion ("Pflanzenphysiologie," ed. 2, ii., p. 641). 



- Rerthold's remarks seem not to have received much notice, and it was 

 not till the publication of Noll's " Heterogene Induction," 1892, that a form 

 of the statolith theor>' was at all widely recognised as a possible explanation. 



^ "Heterogene Induction," p. 41. 



NO. 1819, VOL. 70] 



