MOVEMENTS OF IRRITATION. 451 



the epicotyl from executing any geotropic curvature. After 

 thirty to forty-eight hours we cut thin transverse sections from 

 the growing region of the epicotyl, and find on examination that 

 the cortical cells of the upper side are very rich in plasma, have 

 narrow lumina, and membranes strongly thickened in a collen- 

 chymatous manner, while, on the contrary, the cortical cells of 

 the lower side are poor in plasma, have wide lumina, and thin 

 walls. 



In the experiments with the bean epicotyl, the geotropic curva- 

 ture of which was prevented in the manner indicated, we naturally 

 give the plasmic structures of the cells time to execute fully and 

 completely the movements of irritation due to the action of gravity. 

 These movements consist in a migration of the plasma from the 

 lower to the upper side of the stem. The course of migration 

 will obviously be along the plasmic connections between the cells. 

 The significant accumulation of plasma in the cells of the upper 

 side of the stem has here resulted in great thickening of the mem- 

 branes and consequent feeble surface growth. On the under side 

 of the stem is exhibited trifling growth in thickness, but vigorous 

 surface growth of the cell membranes. Thus here the expansion 

 of the cells under osmotic pressure may easily be considerable. 

 According to Wortmann all the irritable movements of growing 

 organs are brought about primarily by the migration of the plasma 

 under the influence of the stimuli, this in turn influencing the 

 growth of the cell membranes. The facts determined by this 

 observer are undoubted, although as regards their interpretation 

 there is no agreement as yet. 8 We cannot, however, pursue the 

 matter further here. 



1 Very beautiful and searching investigations respecting the pressure which 

 may be exerted by growing plant organs, and the work which they can do, 

 have recently been made by Pfeffer. See Abhandlungen der Konigl. siichs. 

 Gesellsch. d. Wiss., 1893, Bd. 20. I am sorry that I could not give more time 

 to this paper. 



-' See G. Kraus, Abhandlungen der Naturforschenden Gesellschaft zu Halle, 

 Bd. 16, and Correns, Flora, 1892. 



3 See H. de Vries, Landwirthscliaftl. Jahrb., Bd. 9, p. 500. 



4 See G. Kraus, Abhandlungen der Naturforschenden Gesellschaft zu Halle, 

 Bd. 15. 



5 See Thate in Pringsheim's Jahrbiicher, Bd. 13. 



6 See Wortmann, Berichte d. Deutschen botan. Gesellschaft, Bd. 5, p. 461. 



7 See Wortmann, Botan. Zeitung, 1887, p. 819, and 1888, p. 488. 



8 See especially Pfeffer, Abhandlungen der Konigl. sachs. Gesellsch. d. 

 Wissensch., Bd. 18, p. 240. 



