INSTANCES OF SPECIFIC TROPIC IRRITABILITY 225 



free from starch after prolonged exposure to low temperatures also lost 

 their geotropic irritability, but regained it at favourable temperatures 

 simultaneously with the reappearance of the starch. Haberlandt supposes 

 that these observations afford definite proof of the function of starch-grains 

 as the agents for geotropic excitation, but it is quite possible that the 

 solution and regeneration of the starch might merely form accidental 

 accompaniments of the disappearance and restoration of the geotropic 

 irritability l . It has yet to be found whether the geotropic irritability of 

 starchless organs is similarly affected by low temperatures. The geotropic 

 irritability is modified by many factors, and Darwin found that the helio- 

 tropic reaction is also weakened at low temperatures, though to a less extent 

 than the geotropic one 2 . Irritability in general seems to be affected by 

 low temperatures, and it is quite possible that in certain cases a tropic 

 sensibility may only be fully restored some time after growth has been 

 resumed under renewed favourable conditions. 



Haberlandt 3 found that the nodes of Trade scantia virginica lost their 

 power of geotropic response when the cortex included the endodermis or 

 starch-layer, and concludes that the latter is the seat of geotropic perception. 

 The effect might, however, be the direct result of the injury inhibiting the 

 geotropic irritability, or removing tissue essential for the production of 

 a curvature 4 . On the other hand, the fact that weak centrifugal action 

 incapable of producing any displacement of the starch-grains may act as 

 an excitation to curvature does not disprove Haberlandt's views, for the 

 starch may exert local pressure without being displaced 5 . The short 

 period of presentation required during intermittent excitation to produce 

 a response affords no argument one way or the other. Gentle shaking, 

 which might be supposed to cause the starch-grains to exert a greater 

 contact stimulus, does actually accelerate the geotropic reaction 6 , but here 

 also other actions may be involved besides the apparent one. Naturally 

 also the ascent of air-bubbles or of oil-globules in the cell might act as an 

 excitation as well as the descent of the denser starch-grains. 



A local accumulation of protoplasm such as might be responsible for 

 the geotropic excitation does not appear to be produced by the usual 

 intensity of gravity, or at least not in all plants. Nemec 7 observed that in 



1 Additional arguments against Haberlandt's conclusions are given by Noll, Ber. d. bot. Ges., 

 1902, p. 423. 



2 F. Darwin, Proceedings of the Royal Society, 1903, Vol. LXXI, p. 362. 



3 Haberlandt, Ber. d. bot. Ges., 1900, p. 269. 



* See Jost, Biol. Centralbl., 1902, Bd. XXII, p. 174. 



5 Cf. Jost, 1. c., 1902, Bd. XXII, p. 176 ; Haberlandt, Ber. d. bot. Ges., 1902, p. 191. 



6 Haberlandt, 1. c., 1903, p. 489 ; Darwin, 1. c., 1903, p. 366. 



7 Nemec, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 147: cf. Jost, 1. c., p. 177. On the appear- 

 ance of certain minute bodies at the tips of the rhizoids of Chara cf. Giesenhagen, Ber. d. bot. Ges., 

 1901, p. 227 ; Jost, 1. c., p. 173 ; Nemec, Ber. d. bot. Ges., 1902, p. 351. 



PFEFFER. Ill Q 



