442 TRANSFORMATION OF ENERGY 



pressure on the cell-wall always results from the osmotic activity of the cell- 

 contents, we can scarcely assume that the limited alteration in pressure resulting 

 from the inclining of the cells can be perceived by the plant. If, like Noll 

 (1902), we make the very modest assumption that the turgor pressure amounts 

 only to three atmospheres, there rests on the ectoplasm in every position a water 

 column of 30 m. ; if we assume the diameter of the cell to be o-i mm., the under 

 side of the organ in the horizontal position must bear an excess of o-i mm. of 

 water over the upper side ; the plant must be able to appreciate the difference 

 between a pressure of 30,000 and 30,000-1 mm., and if the inclination of the 

 cells be less or the turgor be higher it must be able to appreciate even less 

 marked differences of pressure. 



Apart altogether from this conception there are two hypotheses which have 

 recently been advanced to account for the power plants have of appreciating the 

 stimulus of gravity. Noll (1900) imagined that a sensitive apparatus was 

 formed in the ectoplasm, analogous to the statocysts which occur in crayfish, 

 adapted to the appreciation of the direction of gravity but beyond the limits of 

 vision. These must consist of approximately spherical vesicles composed of 

 sensitive plasma filled with sap, and containing a small but relatively heavy body 

 in the fluid. This body would correspond to the statolith of the crayfish, and 

 we may also term it a 'statolith', and it must, according to the position of 

 the plant organ in space, exert a pressure on some definite part of the sensitive 

 plasma, and so induce a ' perception ' in the plant. In order to explain this 

 theory more in detail let us select for study a sporangiophore of Phycomyces^ 

 laid horizontally. The pressure on the outer side of the sensitive plasma would 

 operate in the statocysts of the under side, and a gi'owth acceleration would 

 be induced as a response to this ; on the upper side, however, the insides of the 

 statocysts would be affected by the statoliths and a retardation of growth would 

 result. If the statolith affects the intermediate limit between the regions which 

 induce either an acceleration or retardation of growth, i. e. between the outer and 

 inner hemisphere, then there is no sensation or at least no reaction. In the 

 horizontal position this would be true of the side walls, but in the vertical posi- 

 tion the whole of the statocysts come to lie in neutral regions above or below in 

 the cyst ; in the former case only the statocysts of the sides are unaffected, in 

 the latter all are unaffected. The application of this hypothesis to a positively 

 geotropic cell presents no difficulty and so it need not be discussed here, but 

 certain criticisms may be briefly advanced against this conception of Noll's. 



We are right in opposing the assumption of special relationships which lie 

 beyond the limits of vision until the hypothesis in question presents us with an 

 explanation on broader grounds so comprehensive that we can no longer do with- 

 out it. As an example we may refer to the atomic theory. But we cannot 

 compare Noll's hypothesis in any way with that theory, because the former at- 

 tempts merely to explain the phenomena of geotropism and has no further and 

 wider application. Apart from that, one difficulty presents itself on passing from 

 a consideration of the uni- to that of the multi- cellular organism. Since this latter 

 organism exhibits geotropic curvatures not merely in the uninjured condition 

 but also when cleft longitudinally, we must assume that every individual cell in 

 it is supphed with statocysts. In each individual cell, at all events in the cells of 

 a median vertical lamella of the horizontally placed stem, growth acceleration on 

 the under side and growth retardation on the upper side should be opposed to 

 each other ; in reality, the cells near the under edge show accelerated growth on 

 both sides, those near the upper edge show reduced growth on both sides, and the 

 median zone remains unaffected. Hence we cannot assume the close relationship 

 between perception and reaction that Noll does. Response is regulated by the 

 co-operative action of all cells, correlation plays a part which Noll's hypothesis 

 does not explain. The hypothesis, too, has a certain one-sidedness in the close 



