1903.] 



Th e Statoliih-tkeory of Geotropism. 



363 



that the Crustacean Palcemon is guided in keeping its dorso-ventral 

 plane vertical, and dorsal surface upwards, by the stimulus of the 

 statoliths resting on the internal surface of its otocysts. 



In favour of their theory, Nemec and Haberlanclt have stated 

 fully and well the general argument based on the distribution of stato- 

 liths. Thus, broadly speaking, statoliths are found to occur (in 

 Phanerogams at least) in organs and parts of organs capable of gravi- 

 tational stimulation. On the other hand, speaking generally, stato- 

 liths are absent where there is no power of curving geotropically. 

 Thus in the older parts of stems, which have lost the power of curva- 

 ture, the statoliths disappear from the endodermis.* 



The presence of statoliths in the tip, and only in the tip of the 

 root, is a striking fact, and one that is in general accordance with the 

 theory set forth in the Power of Movement in Plants, and confirmed 

 since the appearance of that work by more than one observer.! The 

 same thing is true of the cotyledon of Setaria and Sorghum, the 

 statoliths are practically confined to this part of the seedling in 

 which the gravitational sensitiveness has been shown to reside.^ For 

 further details of the general argument from distribution we must 

 refer to the writings of Nemec and Haberlandt. Personally, I con- 

 sider the general argument to be highly convincing, but this opinion 

 seems not to be shared by others, and in any case it does not permit us 

 to neglect experimental tests. 



Experimental Evidence. 



Nemec lays some stress on the loss of geotropism following the 

 destruction of the statoliths. He employs Pfeffer's method of 

 embedding a seedling bean or pea in liquid plaster of Paris. In this 

 way, when the gypsum sets, the seedling is prevented from growing, 

 and for some unknown reason the treatment leads to the disappearance 

 of the starch from the group of specialised cells at the tip of the 

 root. According to Nemec seedlings so treated fail to respond to 

 gravitational stimulus. But the argument is not complete, it should 

 be shown that the starchless roots are not also inhibited in their 

 reaction to other stimuli. For instance, supposing the experiment 

 was performed on seedlings of Sinapis, it should be ascertained 

 whether the starchless radicles would or would not curve apohelio- 

 tropically, while remaining incapable of geotropic curvature. 



I have tried a similar experiment on Setaria and Sorghum. It was 

 discovered accidentally that the cotyledons lose a great part of their 



* Haberlandt, ' Pringslieiin's Jahrbiiclier,' vol. 38 (1903), p. 451 ; and £ Ber. 

 der Deutschen Bofc. Ges.,' vol. 18 (1900), p. 264. 



f Pfeffer. ' Annals of Botany,' vol. 8 (1891), p. 317 ; Czapek, 1 Pringslieim's 

 Jahrbiicher,' 1895; F. Darwin, 'Linn. Soc. Journal,' vol. 35, 1902, p. 266. 



X V. Darwin, ' Annals of Botany,' vol. 13, 1899, p. 567. 



VOL. LXXI. 2 D 



