INSTANCES OF SPECIFIC TROPIC IRRITABILITY 223 



view of Knight's, and considered that the positive geotropism of the root 

 was due to the less dense nutrient materials collecting on the upper side of 

 a horizontally-placed main root and favouring the growth of this side. On 

 this assumption it is difficult to see how the nutrient materials would reach 

 the apex of the root when vertical. 



The theories of Traube and of Cisielski J were mainly based upon 

 observations made on precipitation membranes. Their general trend was 

 that the tensions due to mass-attraction, and the thickening of the walls 

 due to more favourable nutrition, were responsible for both negative and 

 positive geotropism. Dutrochet ' 2 endeavoured to explain the phenomena 

 as being due to the co-operation of endosmotic actions with the tissue-strains, 

 and with the distribution of nutrient materials resulting from anatomical 

 considerations and their relative densities. Mohl and Hofmeister 3 showed, 

 however, that anatomical structure has nothing to do with geotropic 

 irritability, but all these authors failed to recognize that gravity and also 

 light acted merely as exciting stimuli. 



It is only necessary to clothe these mechanical views of Knight, 

 Dutrochet, and Hofmeister in a modern dress 4 by supposing that the 

 moving materials act as stimuli instead of nutritively to arrive at the recent 

 hypotheses of Berthold, Noll, Nemec, and Haberlandt 5 . These authors 

 agree in supposing that the physical sinking of the denser bodies in the 

 cells, and the changes of pressure thereby produced, act as the immediate 

 causes of the tropic excitation. If analogy is any guide, it seems, however, 

 more probable that the excitation is the result of an internal contact- 

 stimulus. Possibly the strong thigmotropic excitability of the ectoplasmic 

 membrane in the epidermal cells of tendrils is transferred to the endoplasmic 

 membrane on the side walls of the cells in parallelotropic organs, and to 

 the membrane on the end walls in plagiotropic ones. In the vertical and 

 horizontal positions the hydrostatic pressures on the end and side walls of 

 an elongated cell alter slightly, but it is not easy to see how these changes 

 could act as the stimulating actions regulating geotropic curvatures, nor how 

 they could mechanically affect growth as Sachs suggested 6 . The maximal 

 differences of hydrostatic pressure in the longest root-cells are extremely 

 small, and in virtue of Weber's law they cannot possibly produce any 



1 Traube, Bot. Ztg., 1875, p. 67: cf. Pfeffer, Osmot. Unters., 1877, p. 215; Cisielski, Cohn's 

 Beitrage z. Biologic, 1872, Bd. n, Heft 2, p. 23. 



2 Dutrochet, Ann. sc. nat., 1833, i re sen, T. xxix, p. 413; Memoires, etc., Bruxelles, 1837, 

 p. 292. 



3 Hofmeister, Jahrb. f. wiss. Bot., 1863, Bd. in, p. 178. 



4 Pfeffer, Period. Bewegungen, 1875, p. 147. 



'" Berthold, Protoplasmamechanik, 1886, p. 73; Noll, Heterogene Induction, 1892; Nemec, 

 Ber. d. hot. Ges., 1900, p. 241 ; 1901, p. 310 ; Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. So; Ber. d. 

 bot. Ges., 1902, p. 339 ; Haberlandt, Ber. d. bot. Ges., 1900, p. 261 ; 1902, p. 189 ; Jahrb. f. wiss. Bot., 

 1903, Bd. xxxvili, p. 447. A summary is given by Jost, Biol. Centralbl., 1902, Bd. xxil, p. 161. 



6 Pfeffer, Period. Bewegungen, 1875, p. 149. 



