230 TROPIC MOVEMENTS 



tion, while ' photocliny ' might be used to indicate responses due to the 

 direction of the incident rays, but the terms are premature at present. 



CHEMOTROPISM. Chemotropic stimulation is dependent upon the 

 direction of diffusion in so far as the latter produces the differences of 

 concentration to which the organism responds 1 . It is, however, uncertain 

 whether the stimulatory substance must actually penetrate, or whether 

 the mere contact with the ectoplasmic membrane produces the chemical 

 action, or modification of surface-tension, which forms the first stage of 

 perception. Many strong excitants do not appear to penetrate the proto- 

 plast, or at least do so with difficulty, but an apparent impermeability 

 may allow of the penetration of traces of the substance sufficient to excite 

 internal stimulation. Even when the substance readily penetrates, the 

 stimulation may occur either during or after absorption. 



OSMOTROPISM. The maximal osmotic action is exercised by imper- 

 meable substances, which may also be expected to exert the greatest 

 osmotropic action. It is, however, uncertain whether the tropic stimulus 

 is due to the unequal withdrawal of water, to the movement of water 

 through the cell, to the osmotic pressures, or to surface-tension. It is 

 also possible that a readily penetrating substance might exercise a tropic 

 excitation, for any unequal distribution in the external medium will also 

 be produced in the cell. A variety of observations upon freely motile 

 organisms seem, however, to show that readily penetrating substances 

 exercise little or no osmotropic action. Since, however, osmotaxis may 

 arise in more than one way, it is possible that organisms may exist which 

 are especially responsible to readily penetrating substances. 



PART IV 



THE MECHANISM OF TROPIC MOVEMENT 

 SECTION 52. The Progress and Mode of Movement. 



All tropic curvatures produced by the aid of growth naturally cease 

 to be performed when the power of growth is lost, whereas the presence 

 of pulvini capable of variation movements renders possible various tropic 

 responses in adult organs 2 . It is, however, not known whether pulvini 

 may possess other tropic irritabilities in addition to those of geotropism and 

 heliotropism, although no heliotropic variation curvatures appear to occur 



1 For details see Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1888, Bd. II, p. 650; 1884, Bd. i, 



P- 475- 



2 Pfeffer, Periodische Bewegungen, 1875, p. 63. On the pulvini of Marantaceae cf. Schwen- 

 dener, 1896, Gesammelte Abhandlungen, Bd. II, pp. 203, 210; Debski, Anzeiger d. Akad. d. 

 Wiss. in Krakau, Juli 1895. 



