SEPARATE LOCALIZATION OF PERCEPTION AND RESPONSE 199 



expected that the removal of the tip of the root and of the cotyledon of a Grass 

 should produce exactly the same effect, since in one case we are dealing with 

 undifferentiated meristem, and in the other with a nearly adult differentiated tissue. 

 Hence, any incision into the root-apex temporarily inhibits its irritability, whereas the 

 complete removal of the tip of the cotyledon of a Grass is required, according to 

 Rothert, to produce the same effect. 



The traumatic inhibition of the heliotropic and geotropic sensibilities on the one 

 hand, and the retardation of growth on the other, are two distinct reactions to 

 the same external agency. It is only possible to demonstrate the conduction 

 of stimuli leading to both forms of response when the zone of action is directly 

 excitable, but nothing is known as to the inherent character of the phenomenon. 

 Nevertheless, the removal of the apex of the cotyledon of Avena must either entirely 

 inhibit the power of perception of heliotropic stimuli or must prevent the awakened 

 sensation progressing to the induction of movement. According to Rothert 1 , the 

 inductory processes once begun are not stopped by the injury, but progress, and are 

 propagated to the active zones. After only short exposure to unilateral illumination, 

 a heliotropic after-effect is shown in spite of the removal of the tip of the cotyledon, 

 and leads to a curvature. In roots, however, prolonged induction is required before 

 any geotropic after-effect is shown, and in such cases the ductory processes might 

 already have reached and affected the active zones before the sensitive apex was 

 removed. Darwin 2 , for instance, decapitated the roots after they had been kept for 

 one to one-and-a-half hours in a horizontal position. Czapek 3 has shown why this 

 after-effect cannot be used to demonstrate the localization of the geotropic irritability 

 in the root-apex, and has also found that short induction periods may produce 

 perceptible after-effects 4 . It is, however, always possible that the processes of 

 induction themselves may be affected by traumatic agencies, and hence probably 

 arose the fact that Czapek 5 was unable to detect any geotropic after-effect in the 

 roots of Lupinus. Nor is it surprising that a short period of induction may not 

 be able to overcome the existent tendencies and the effects of decapitation, and 

 hence may fail to produce any after-effect. Owing to the fact that Nemec 6 did not 

 consider this possibility, his experiments fail to determine whether the injury entirely 

 suppresses the geotropic excitability of the root, or whether the sensory processes are 

 still excited up to a certain point. Decapitated parts, even when in a condition of 

 traumatonus, are still capable of reaction, and may indeed be capable of certain tropic 

 responses. 



The conduction of stimuli usually occurs over a short distance only, even when 

 the transference is from one organ to another, as from the cotyledon to the hypocotyl 

 of Panicum. Copeland 7 suggests that the positively geotropic curvature of certain 



1 Rothert, 1. c., p. 200. 



3 Darwin, The Power of Movement in Plants, 1880, p. 525. 



3 Czapek, Jahrb. f. wiss. Bot., 1895, Bd. XXVII, p. 252. 



4 Czapek, ibid., 1898, Bd. xxxil, p. 219. 



5 Czapek, 1. c., 1895, p. 252. 



' N6mec, Fiinfstiick's Beitrage z. wiss. Bot., 1901, Bd. IV, p. 186. 

 7 Copeland, Botanical Gazette, 1901, Vol. xxxi, p. 410. 



