220 TROPIC MOVEMENTS 



In certain cases at least a tropic excitation may be produced without 

 the direct co-operation of the nucleus, and presumably the ectoplasmic 

 membrane plays a prominent part in the perception of tropic stimuli 1 . 

 Streaming cells in which only the peripheral layer of protoplasm is at rest 

 may be capable of a tropic response, but this is not an entirely satisfactory 

 proof that the perception is solely due to the peripheral membrane, since an 

 altered configuration of the streaming protoplasm may be maintained by 

 the continued action of a tropic stimulus in spite of the regular change. The 

 chloroplastids in a streaming cell of Elodea are, indeed, capable of phototropic 

 orienting movements in spite of their circulation around the cell. Contact- 

 stimuli naturally primarily affect the ectoplasmic membrane, and cells are 

 capable of tropic response when the protoplasm is reduced to a thin layer 

 of ectoplasm, while cilia composed solely of ectoplasm perceive stimuli. 

 None of these facts, however, affords any conclusive proof of the localization 

 of irritability in the peripheral layer, and as a matter of fact the whole 

 of the cytoplasm is irritable and capable of reaction. Probably the 

 different parts, including the nucleus, commonly co-operate in perception 

 and response, or in the former alone. The ectoplasmic membrane is only 

 relatively a permanent structure, and its irritability is undoubtedly not 

 alike in all cases. 



Both plagiotropic and parallelotropic orientation may result from 

 a single tropic perception, in spite of the assumption of Sachs and 

 de Vries that a plagiotropic response to a single orienting agency must 

 always be due to the antagonism of opposing tendencies to movement. 

 The fact that an autogenic or aitiogenic conversion of a positive into a 

 negative tropism is possible does not show that the cells and tissues contain 

 both negatively and positively reacting elements, as was supposed to be 

 the case by Wiesner 1 . Even when the orientation is due to two separate 

 stimuli these may fuse to a single impulse and excite only one tendency to 

 movement. A change in the degree of sensitivity may or may not affect 

 the tropic position assumed in response to the conjoint action of two 

 orienting agencies. Czapek formerly considered the plagiotropism of 

 lateral roots and of diageotropic rhizomes to result from the co-operation 

 of their positive and transversal geotropism 2 , but now considers that their 

 plagio-geotropic position is assumed in response to a single tropic excita- 

 tion. Czapek's arguments are mainly based upon the dissimilar behaviour 

 of roots bent upwards and downwards through equal angles, and upon the 

 increase of the positively geotropic movement on exposure to rising 



1 Wiesner, Die heliotropischen Erscheinungen, 1880, Bd. II, p. 21. Cf. Pfeffer, Osmotische 

 Untersuchungen, 1877, p. 211. 



* Czapek, Sitzungsb. d. Wien. Akad., 1895, Bd. civ, i, p. 1257. Cf. Noll, Sinnesleben d. 

 Pflanzen, 1896, p. 86 (reprint from Ber. d. Senkenberger naturforsch. Ges. in Frankfurt). 



