224 TROPIC MOVEMENTS 



perceptible excitation, since they merely add to or subtract from the 

 enormously greater internal osmotic pressure l . Noll has recently sug- 

 gested that the centrosphere with its centrosome may act as the percipient 

 organ for detecting the direction of gravitational stimuli, but the fact that 

 the centrosphere and centrosome do not appear to be permanent organs of 

 the cells of flowering plants suffices to show the danger of putting forward 

 hypotheses unsupported by experimental evidence 2 . 



Any local discrete pressure produced by the accumulation of the starch- 

 grains or other bodies might act as an internal stimulus, and the deforma- 

 tions and changes of configuration due to the rearrangement of the denser 

 and lighter particles might be equally effective. In the former case the 

 cell-mechanism of the plant would resemble that of the equilibratory organs 

 of certain animals 3 . These possess statocysts or otocysts in which lie 

 dense particles, statoliths or otoliths, and the latter pressing on the under 

 inner surface of the otocyst excite sensory reactions directed towards the 

 maintenance of the normal position of the otocysts and of the organism. 

 Kreidl even found that the insertion of particles of iron in place of the usual 

 otoliths caused the organisms to orient themselves in regard to a magnet 4 . 

 It is, however, not known whether the side walls of the otocyst are sensitive, 

 but not the ventral wall, or whether only the latter is irritable. In the former 

 case movement would follow until the otocyst experienced no excitation, 

 whereas in the latter case it would be directed towards the renewal of the 

 normal constant tonic stimulus radiating from the otocyst. 



According to Nemec and Haberlandt, the excitation in plant-cells is 

 usually due to the starch-grains, although other bodies may become 

 effective in fungal hyphae and other organs possessing geotropic irritability 

 but devoid of starch-grains. No sure proof of such action has, however, 

 been brought forward as yet, for the fact that when a cell is reversed the 

 starch-grains fall from one end to the other with the required rapidity 

 merely shows that the supposed stimulatory action is not an impossible one. 

 Jost has, indeed, shown that none of the arguments put forward by Nemec 

 is conclusive, while Nemec's conclusion that the power of perception of 

 geotropic stimuli is restricted to the starch-bearing columella of the root-cap 

 is negatived by the fact that the excitable apical region is usually about 

 1-5 mm. long. 



Haberlandt 5 found that the stems of certain plants which had become 



1 Noll, Ber. d. bot. Ges., 1902, p. 425. 



a Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 502 ; Ber. d. bot. Ges., 1902, p. 403. 



3 Cf. Noll, Das Sinnesleben d. Pflanzen, 1896, p. 71 ; Bethe, Biol. Centralbl., 1894, Bd. XIV, 

 p. 95 ; Stciner, Centralbl. f. Physiol., 1898, Bd. xil, p. 775 ; Laudenbach, ibid., 1900, Bd. xill, 

 p. 586; Ilyin, ibid., 1901, Bd. xiv, p. 361 ; Haberlandt, Ber. d. bot. Ges., 1902, p. 448. 



* Kreidl, Sitzungsb. d. Wien. Akad., 1892, Bd. ci ; 1893, Bd. Cii. 



5 Haberlandt, 1. c., 1902, p. 193 ; Jahrb. f. wiss. Bot., 1903, Bd. xxxvni, p. 447. 



