494 



PLANT RESPONSE 



and necessary result of such changes ; ' or in other words that 

 ' light and gravitation could be classed together as external 

 agencies acting, not directly, but in some unknown indirect 

 manner.' ^ 



There are two distinct points to be borne in mind in 

 connection with the effect of gravity : first, the question as 

 to how gravity exercises stimulation ; and secondly, that of 

 how, in answer to this stimulus, a definite responsive curvature 

 is induced. 



Mode in which stimulation is brought about. — Now it 

 is clear that, as regards the former of these points, the only 

 conceivable way in which gravity could produce stimulation 



is by the effect of weight ; 

 not, that is to say, by the 

 weight of the plant as a whole, 

 but by the differential effect 

 of weight in the cells. Two 

 important theories have been 

 advanced which offer very 

 rational explanations of the 

 means by which gravi-percep- 

 tion may be induced. Accord- 

 ing to these, the necessary 

 differential weight-effect may 

 be due to the weight of the 

 cell-contents, whether of the 

 sap itself, or of those heavy particles like starch-grains which 

 are contained in it. When, therefore, the cell is laid horizon- 

 tally, it is the lower tangential wall which has to support the 

 relatively greater weight (fig. 203). This theory of hydrostatic 

 or radial pressure was suggested by Pfeffer, and supported by 

 Czapek. The other theory, of statoliths — advocated by Noll, 

 Haberlandt, and Nemec— substitutes for the weight of the 



' A luminous resume of our present state of knowledge on this subject, with 

 all its difficulties and obscurities, is contained in the addresses of Francis Darwin 

 delivered before the British Association in 1891 and 1904. From these I have 

 made the quotations which appear in the text. Figs. 203, 204, and 209 appear 

 as illustrafions of the latter address in Nature of September 8, 1904. 



Fig. 203. Diagrammatic Representa- 

 tion showing Differential Effect 

 of Weight on Lateral Walls of 

 Cells 



In the figure to the right the cell is 

 laid horizontally, and the lateral 

 wall, D, has to bear greater weight 

 than c (after Francis Darwin). 



