SEPARATE LOCALIZATION OF PERCEPTION AND RESPONSE 193 



through the percipient zone, as well as when the stimulus merely spreads 

 more or less from the directly excited responding zone to surrounding 

 responsive regions. If in the latter case a particular zone lose the power 

 of growth and response, it may still remain capable of receiving stimuli 

 and transmitting them to neighbouring active zones. A separation of 

 perception and response also occurs when a portion of a growing zone 

 loses the power of perception, or when the meristem-cells at a growing 

 apex develop a special irritability before their rapid stretching growth 

 begins, and lose it as soon as this rapid growth commences. This is 

 actually the .case in the growing apex of the root, for the power of 

 receiving geotropic stimuli is lost as soon as the tissue- differentiation begins. 

 In other cases, however, a special irritability is absent from the primary 

 meristem, and only appears as the tissues differentiate. 



The importance of these relationships was pointed out by Pfeffer 1 , 

 but many observers denied the accuracy of 



A c Darwin's investigations 2 . Although certain of 



the experiments were not altogether satisfactory, 

 the correctness of Darwin's conclusions was estab- 

 lished by Czapek, and our knowledge of the 

 localization of the phototropic irritability was 

 considerably amplified and extended by Rothert 8 . 

 As in other cases, the division of labour is not 

 always complete, so that one zone may be more 

 perceptive, the other more responsive. In such 

 generalized organs direct and indirect tropic 

 stimuli may co-operate in producing a particular 

 response. 



The heliotropic curvature of grass seedlings 

 is especially instructive, and was studied in detail 



by Rothert. In the cases of Setaria viridis, Panicum miliaceum, and a few 

 other Paniceae only the cotyledons are perceptive, whereas the pronounced 

 curvature is produced in the hypocotyl which is not directly excitable. The 

 hypocotyl of Sorghum vulgare, however, possesses a feeble phototropic 

 irritability. The same applies to the subapical portion of the cotyledon 4 of 

 Avena sativa^ which performs the heliotropic curvature in this plant, mainly 

 in response to the indirect excitation arising from the highly irritable tip of 



1 Pfeffer, Pflanzenphysiologie, 1881, Bd. n, p. 327. 



* Cf. the literature given by Rothert, Flora, Ergzbd., 1894, p. 179; Czapek, Jahrb. f. wiss. Bot., 

 1 895, Bd. xxvil, p. 244. 



8 Rothert (Cohn's Beitrage zur Biologic, 1896, Bd. vil, p. 3). 



4 The same terms are used as by Rothert, without expressing any view as to the still doubtful 

 morphological nature of these organs. The term coleoptile, or cotyledonary sheath, may be used 

 instead of cotyledon, and mesocotyl instead of hypocotyl. Cf. Goebel, Organography, Vol. II, 1905, 

 p. 408. 



FIG. 41. Seedlings of Panicum 

 miliaceum. A unstimulated. B, 

 after shorter, C, after longer helio- 

 tropic stimulation from the right, 

 r, cotyledon. A, hypocotyl. 



