September 8, 1904] 



NA TURE 



471 



on the concave side is not the result of compression pro- 

 duced by increased growth on the convex side, but rather 

 an independent reaction. It is necessary, therefore, to 

 inquire what theoretical conclusions may be fairly made as 

 to the stimulation correlated with such a mechanism of 

 curvature. Noll' uses the term "Reizfeld," or "stimu- 

 lation-area," to express the regions in which graviperception 

 occurs. The distribution of these areas is expressed in 

 diagrams which serve as shorthand methods of recording 

 the geotropic reactions of various organs. All such ways 

 of clarifying and expressing our ideas of the laws of per- 

 ception are useful. I must confess that I do not find Noll's 

 terminology easy to use, and I prefer to express the same 

 ideas in terms of the distribution of the pressure of stato- 

 liths on the different parts of the ectoplasm of the gravi- 

 sensitive cells. 



Imagine an apogeotropic shoot placed in the horizontal 

 position as shown in longitudinal radial section in Fig. 2, 

 where C and C are the cortical tissues and the seat of 

 motile power ; E and E' the endodermis, the supposed region 

 of graviperception ; M, the central tissues, which do not 

 concern us. 



The fact that the statoliths now rest on the horizontal 

 (tangential) walls differentiates the horizontal from the 

 vertical position of stable equilibrium. But what circum- 

 stance is there that can be conceived to originate curvature 

 in one direction more than another? It can only be that 

 in the endodermis E on the physically upper side the stato- 

 liths rest on the inner tangential wall, whereas in E' they 



rest on the outer wall. This view agrees with Noll's hypo- 

 thesis of the arrangement of stimulation-areas. There is no 

 difficulty in believing that the inner and outer tangential 

 walls have different individualities: \"6chting's work ~ on 

 transplantation seems to indicate that this is the case. And 

 if this analogy with formative polarity is not allowable, we 

 must still insist that the presumption is in favour of E and 

 E' in Fig. 2 being in different conditions, since we have 

 certainly no right to assume that the outer and inner walls 

 are identical in what we have called their individuality. 



It is not here necessary to go into the question whether 

 the radial walls of the endodermis are or are not sensitive, 

 since the problem of geotropism in its broad outlines is not 

 concerned with it.^ 



The Position of Maximum Stimulation. 

 This problem involves the question whether an orthotropic 

 organ in the vertical position is or is not freed from stimulus. 

 We will first take the question as to the existence of a 

 stimulus in the normal (i.e., not the inverted) position. One 

 of Pfeffer's* arguments for the existence of a stimulus is 

 as follows. A root having been allowed to curve from the 

 horizontal to the vertical position is placed on a klinostat, 

 and after a time the curve disappears. It is therefore 

 assumed that there existed a geotropic stimulus keeping the 

 root curved until the stimulus in question was rendered 

 inoperative by the klinostat, when the rectipetality of the 

 root could have free play. But it is not a necessary con- 

 clusion that while the root is strictly vertical any stimulus 

 is acting. If from some internal cause the root leaves the 

 vertical, the ordinary geotropic curvature depending on the 



1 Noll (92, p. 19). '■* Vcichling (92, p. 151). 



3 See the discussion in H.lberlandt (03, p. 467). 



^ Pfeffer (93, p. 19). I am only concerned with this special point, not with 

 PfefFer's general argument. 



NO. 18 19, VOL. yd] 



stimulation of the tangential walls will come into action 

 and bring the root back to the vertical. To translate into 

 the language of the statolith theory, it is not necessary to 

 assume that the lower walls of the graviperceptive cells are 

 sensitive to the pressure of the statoliths — the sensitiveness 

 of the tangential walls will suffice. The experiment above 

 mentioned does not therefore seem to prove that an ortho- 

 tropic organ in stable equilibrium is stimulated. But it is 

 quite conceivable that a stimulus might be originated by the 

 loss of pressure on the lower wall, for this would be a well- 

 marked change in the internal condition of the cell, and 

 therefore might become associated with a reflex. Thus, 

 when an organ is placed horizontal the stimulus from the 

 pressure of statoliths on the lateral walls (now horizontal) 

 may be combined with, or in some way influenced by, the 

 loss of pressure on the terminal wall of the cell which was 

 formerly horizontal. But if the absence of pressure on a 

 cell-wall acts in this way are we not bound to consider the 

 pressure (when present) as a stimulus? I think we are, and 

 therefore, though I do not think that the particular experi- 

 ment referred to supplies the necessary evidence, I hold the 

 lower wall of an orthotropic cell to be sensitive to the 

 stimulus of statoliths, though such stimulus cannot be of a 

 directive nature. 



Since an organ when accurately inverted ' and prevented 

 from circumnutating receives no impulse to curve, it is 

 assumed that the normally upper cell-wall (which is now 

 b*low) is not stimulated. According to the statolith theory 

 it is inconceivable that the organ should curve, since uniform 

 pressure on the horizontal terminal wall cannot determine 

 the direction in which such curve shall begin. 



But though no directive stimulus seems to be a possible 

 result of uniform pressure on the end-walls, it does not follow 

 that such pressure has no effect. It seems to me that such 

 a striking change as pressure on a wall which in normal 

 circumstances does not receive pressure may very well modify 

 the result of the normal stimulation of the lateral walls of 

 the cell. 



Czapek - has shown that with both stems and roots the 

 gravistimulus is greater when the organ is removed from 

 the normal vertical position by 135° than when it deviates 

 from the normal by 4^°. In the case of an apogeotropic 

 shoot the position of the starch in the endoderm is given 

 in Fig. 3. The pressure of the starch on the lateral walls 

 is the same in the two cases. In i., however, the starch 

 rests partly on the basal wall (B), while in ii. it rests, to 

 the same degree, on the apical wall (A). On the usual 

 assumption that the basal and apical walls are insensitive, 

 there is nothing to differentiate i. from ii. I cannot help 

 suspecting that the pressure on the apical wall does in some 

 way affect the sensitiveness of the tangential walls. If the 

 pressure on the wall (A) was in itself the decisive element we 

 should expect the stimulus to increase as the angle increased 

 — from 135° to nearly 180° — which is not the case. From 

 my point of view we can dimly understand why 135° should 

 be the position of maximum stimulation. It would be the 

 result of a compromise, being a position in which the com- 

 bined pressure on both lateral and apical walls was as high 

 as possible ^ — a mean, in fact, between full pressure on the 

 lateral walls (as in the horizontal position) and full pressure 

 on the apical walls (as in the vertical position). 



If some such theory is not adopted we must imagine with 

 Haberlandt that the difference between positions i. and ii. 

 depends on the weight of the statoliths in i. being on the 

 basal half of the lateral wall, and on the apical half in ii. 

 It seems to me that the difference of sensitiveness in the 

 two regions would have to be very great, considering that 

 in the horizontal position, in which the gravistimulus is 

 less than in position ii., the full pressure of a considerable 

 fraction of the total starch acts on the supposed extra- 

 sensitive region of the cell-wall. 



But when all has been said there remains a difficulty with 

 which I do not know how to deal. It is clear that, accord- 

 ing to either theory, the critical position should be the hori- 



t In the whole of this discussion the organs are supposed to be >upported 

 by the morphological base. 



- Czapek (95, i). As doubt has been expressed z 

 worth while mentioning that Miss Pertz (99) hasco 

 haulms of grasses. 



3 The fact that at angles above 135° the stimulus 

 the organ is horizontal seems to point to the conclu 

 end wall in graviperception is relatively great. 



