362 



Mr. F. Darwin. 



[Mar. 6, 



The Statolith-theory of Geotropism." By Francis DAKWiisr r 

 M.A., M.B., F.K.S. Eeceived March 6 — Read March 12,. 

 1903. 



To make clear the point of view from which the experiments here 

 recorded were undertaken, a few words on the modern theory of 

 gravitational sensitiveness are necessary. For fuller details the 

 reader is referred to the papers of Noll, Nemec and Haberlandt,* to 

 whom the theory in question is due. 



AVhen a geotropic organ suffers a change of position it responds by 

 an appropriate curvature, and ultimately reassumes vertically. With 

 the mechanism of curvature I am not concerned, the problem to 

 be solved is the source of the stimulus to which the curvature is a 

 response. In other words : when a plant, which normally grows verti- 

 cally upwards, is laid horizontally, it obviously perceives the change ; 

 by what mechanism is the perception rendered possible ? 



The view of Nemec and Haberlandt is that the stimulus is due to 

 the presence of* bodies of greater specific gravity than the cell sap, 

 which in consequence of their weight always fall to the physically 

 lower regions of the cells. These bodies are usually movable starch 

 grains, and may conveniently be termed statoliths. As long as 

 an organ is vertical, i.e., in its normal position, the statoliths, in conse- 

 quence of their weight, collect in a heap or layer on the basal walls 

 of the cells. When the organ is placed horizontal^ the heap of stato- 

 liths topples over and gradually spreads out in a layer on the lateral 

 wails, which are now horizontal. It is clear that in this circumstance- 

 is to be found the possibility of an appropriate stimulus. We can 

 conceive that the pressure of the statoliths on the protoplasm of the 

 lateral instead of the basal walls, might become associated with certain 

 definite curvatures. Thus in a typical stem-structure the reflex action 

 correlated with pressure on the lateral cell walls would be an upward 

 curve, in a typical root the same stimulus would result in a downward 

 curve. 



The starch grains or other heavy bodies have been named stato- 

 liths in harmony with the nomenclature of the zoologists, who 

 have given the name to the heavy particles which supply certain 

 animals (e.g., the Crustacea) with the means of orienting themselves 

 spatially. The beautiful experiment of Kreidlf has definitely proved 



* Noll, ' Heterogene Induction,' Leipzig, 1892. 



Haberlandt's and Kemec's papers were published simultaneously in vol. 18 (1900) 

 of the ' Berichte dcr Deutschen Botanischen Gesellschaft.' See also papers by 

 both authors in vol. 20 of the same Journal ; also Nemec, in ' Pringsheini's 

 Jahrbucher,' vol. 36, 1901, and Haberlandt in the same Journal, vol. 38, 1903. 



f 'Sitzb. Wiener Ak. d. Wiss.,' vol. 102, 1893, Abth. 3. 



