216 TROPIC MOVEMENTS 



containing this acid or maleic acid, so that both probably effect the same sensation '. 

 The same conclusion applies in the case of bacteria, when the attractive action 

 of dextrin is equally lowered in solutions of dextrin and of meat-extract <i . Various 

 bacteria are attracted by potassium-salts, meat-extract, and other substances in 

 a similar manner, whereas the attractive action of oxygen is shown only in the case 

 of certain forms, and appears to depend upon the development of a special sensory 

 excitability. When different stimuli excite the same response, we must, in the first 

 instance, presuppose the existence of dissimilar sensory perceptions, which in other 

 cases may be singly developed. 



SECTION 49. The Conditions for Stimulation and its Progress. 



In parallelotropic and plagiotropic organs the conditions for stimulation 

 are given when the organ is displaced from its normal position. When 

 a pafallelotropic organ is inverted, however, slight autotropic curvatures 

 cause one side to be more stimulated than the other, and the organ curves 

 more and more rapidly out of the labile inverted position of equilibrium 

 into a normal stable one. In all cases the tropic stimulation results from 

 the unequal application of the external agency, and none is exercised 

 when the latter is uniformly distributed or acts equally in all directions. 

 Hence a plant placed between and equidistant from two equal sources 

 of illumination would show no heliotropic curvature, and the same would 

 be the case in a geotropic root placed between two planets exercising 

 the same mass-attraction upon it. 



Tropic irritability, therefore, depends upon a power of differential 

 sensation, that is a power of detecting differences in the intensity of the 

 exciting agency 3 or in its direction of application, although the detailed 

 mode of response may vary according to the irritability affected, and, 

 in fact, unilateral illumination may exercise more than one kind of orienting 

 action. Indeed, certain organisms may respond to differences in the 

 intensity of the illumination, others to the direction of the incidental rays, 

 while the action of gravity can only be of the latter character, since its 

 intensity is the same at all points inside and outside an organ. 



Although the conditions are simpler in radial organs than in dorsi- 

 ventral ones, Loeb is incorrect in supposing that symmetrically disposed 

 points are exposed to equal intensities of the orienting agency when 

 a radial organ has assumed its proper orientation 4 . The assumption 

 of a new tropic position by an organ in response to displacement always 



1 Pfeffer, Unters. a. d. hot. Inst. zu Tubingen, 1884, Bd. I, p. 397. * Id., 1888, p. 635. 



3 Cf. Pfeffer, Pflanzenphysiol., i. Aufl., Bd. II, p. 329, u. Unters. a. d. hot. Inst. zu Tubingen, 

 1884, Bd. I, p. 477. Nagel (Bot. Ztg., Ref., 1901, p. 297) has no grounds for supposing that only 

 phobotactic organisms possess a discriminatory sense. 



4 Loeb, Pfliiger's Archiv f. Physiologic, 1897, Bd. LXVI, p. 441 ; Vergleichende Gehirn- 

 physiologie, 1899, p. 4. 



