200 TROPIC MOVEMENTS 



hypocotyls and cotyledons is due to processes of induction transmitted from the 

 sensory region of the root-apex. 



Tropic stimuli are only slowly conducted, as are most stimuli in plants. 

 Under favourable conditions a heliotropic stimulus may travel at a rate 

 of i mm. to 1 mm. in five minutes in the case of Avena and Brodiaea l , 

 while the geotropic excitation may pass from the root-apex at a rate of 

 i mm. in five minutes 2 . Stimuli must travel in sensitive tendrils over at 

 least 1 8 mm. in five minutes, as measured by the difference in time between 

 the application of a stimulus to the concave side and the commencement 

 of the acceleration of growth on the convex side and resultant curvature 3 . 

 Presumably the stimulus may spread in all directions where conducting 

 tissue is available ; but, according to Rothert 4 , heliotropic stimuli travel 

 mainly in the basipetal direction in the cotyledon of Avena. Since the 

 latter has only two longitudinal vascular bundles, it is easy to cut these 

 and show that the heliotropic stimulus is able to travel through the 

 fundamental parenchyma 5 . According to Czapek , the same is true for 

 geotropic stimuli, although it does not follow that here and in other cases 

 the vascular bundles are devoid of all power of conducting stimuli. In 

 addition, the cortical tissue of roots is able to transmit geotropic 7 and 

 traumatropic 8 stimuli, for curvatures can still be produced in the active 

 zone when only a strip of living cortex is left between the stimulated 

 apex and the growing zones behind. No geotropic reactions can, however, 

 be excited in a node of Tradescantia fluminensis by stimulation of the 

 next younger node if the continuity of the vascular bundles is broken 9 . 

 Probably also the stimuli involved in the regulation of translocation mainly 

 travel through the vascular bundles. 



Geotropic 10 and traumatropic n stimuli are still able to travel from the 

 apex of the root to the active zone and to produce a normal curvature 

 when a pair of incisions are made in the path of the stimulus on opposite 

 sides one above the other and past the median line. This shows that the 

 stimulus may have followed a curved path and may be capable of lateral 



1 Rothert, 1. c., pp. 137, 209. 



2 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 219. 



3 H. Fitting, Jahrb. f. wiss. Bot., 1903, Bd. xxxvin, p. 610. 



4 Rothert, 1. c., p. 52. 5 Rothert, 1. c., pp. 63, 209. 



6 Czapek, 1. c., 1898, Bd. XXXII, p. 255. 



7 Czapek, 1. c., p. 220. 



8 Pollock, Botanical Gazette, 1900, Vol. xxix, p. 24. 



9 Miehe, Jahrb. f. wiss. Bot., 1902, Bd. xxxvn, p. 527. 



10 Czapek, 1. c., 1898, p. 220. Cf. also N6mec, Jahrb. f. wiss. Bot., 1901, Bd. xxxvi, p. 96. 

 N6mec states in another paper (Fiinfstiick's Beitriige z. wiss. Bot., 1901, Bd. iv, p. 207) that the 

 stimulus does not travel beyond an incision in the active zone of the root. See also N&mec, Die 

 Reizleitung u. die reizleitenden Structuren, 1901, p. 134. 



11 Pollock, 1. c., p. 24. 



