198 TROPIC MOVEMENTS 



to be localized in the root-apex. Molisch l was, however, able to obtain a curvature 

 when the root was enveloped right up to the tip in moist tissue-paper, while Pfeffer 2 

 found that, if only the extreme tip was clothed in moist paper while the rest of the 

 root was exposed to hydrotropic stimulation no curvature followed. Hence the 

 power of perceiving hydrotropic stimuli is developed in the root-apex alone. 



The heliotropic and geotropic irritabilities are not equally distributed throughout 

 the sensitive apex of the cotyledon of Gramineae, and presumably the geotropic 

 irritability of the root-apex gradually disappears in the differentiating tissues. 

 Czapek 3 found that the length of the geotropically irritable zone in the roots of 

 Ltipimis and Faba was about 1-5 millimetres. If a less zone than this is included 

 in the terminal limb of the glass cap, a curvature takes place when the apex is placed 

 vertically, since the horizontal region just behind is geotropically excitable. Hence 

 the power of perception cannot be restricted to the extreme tip of the growing-point 

 or to the calyptrogen layer. The conclusion of Fritsch and Nemec, that perception 

 is localized in the root-cap, is based partly upon faulty experiments and partly upon 

 incorrect ideas as to the process of stimulation 4 . Czapek concludes that the whole 

 of the meristem and of the young tissues abutting upon it is capable of perceiving 

 geotropic stimuli. According to Wachtel ', the geotropic irritability returns to 

 decapitated roots where the apical meristem is regenerated 6 . The removal of the 

 epidermis from the cotyledons of Gramineae 7 does not prevent them from perceiving 

 geotropic stimuli. 



It is difficult to determine by operation whether certain tissues are more highly 

 excitable than others, since the removal of the other tissues may not only affect the 

 power of response, but may also result in traumatropic curvature. Rothert 8 found the 

 removal of the tip of the cotyledon of Panicum or Avena caused the geotropic 

 and heliotropic irritabilities to be entirely suspended for a few hours. At the same 

 time, growth is retarded, but not to such an extent as to cause the cessation of 

 a curvature which had already begun or which had just been induced. Similarly, 

 transverse or longitudinal incisions or punctures in the root-apex inhibit the geotropic 

 irritability for a few hours or even a couple of days, although the percipient organ 

 is neither removed nor destroyed. It is not surprising that the irritability should 

 return sooner after such an incision has been made than when the root-apex is 

 entirely removed 9 . Owing to the fact that the injury excites an energetic process 

 of regeneration, it is difficult or impossible by operative experiments to determine the 

 part played by different tissues in the perception of stimuli. It is hardly to be 



1 Molisch, Sitzungsb. d. Wien. Akad., 1883, Bd. LXXXVIII, Abth. I, p. 897. 



2 Cf. Rothert, 1. c., p. 212 ; Czapek, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 316. 



3 Czapek, loc. cit., 1895, Bd. xxvn, p. 262 ; Ber. d. bot. Ges., 1901, Generalvers., p. 117. 



* Czapek, Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 230; Ber. d. bot. Ges., 1901, pp. 117, 119. 

 5 Cf. Czapek, 1. c., 1901, p. 118. 



' Cf. also N6mec, Jahrb. f. wiss. Bot., 1901, Bd. XXXVI, p. 98 ; Fiinfstiick's Beitrage z. wiss. 

 Botanik, 1901, Bd. IV, p. 193. 



7 Czapek,!. c., 1898, p. 255. 



8 Rothert, Cohn's Beitrage z. Biol., 1896, Bd. VII, pp. 191, 211. 



9 Czapek, 1. c., 1898, p. 202 ; 1. c., 1901, p. 118; N6mec, 1. c., p. 97. 



