I 9 4 TROPIC MOVEMENTS 



the cotyledon. The tip of the hypocotyl of many cruciferous seedlings, or 

 that of the epicotyl of Vicia sativa, is more irritable than the basal 

 regions ; but in other seedlings, such as those of Tropaeolum, Solanum, 

 and Coriandrum, and the organs of very many adult plants, the helio- 

 tropic sensibility is fairly evenly distributed. 



The above examples of localized perception are also instances of the 

 transmission of tropic stimuli, but the same is shown in the peduncle of 

 Brodiaea congesta, one of the Liliaceae, although the perceptive and 

 responsive zones are not separately localized. Thus a phototropic stimulus 

 radiates in three hours to a distance 6 cms. from a directly illuminated area. 

 A somewhat less pronounced transmission is shown by the stems of Linum 

 usitatissimum and Coleus, whereas most plant-organs have only a feeble 

 power of conducting heliotropic stimuli. The stem of Galium purpureum, 

 however, not only affords an instance of the ready transmission of stimuli, 

 but is also able to receive and transmit the latter even when the power 

 of response is lost. Thus the basal parts of the internodes which remain 

 longer capable of growth and curvature may be excited indirectly by 

 stimuli applied to the apical non-growing region which has lost the power 

 of curvature l . 



Similarly, geotropic stimuli perceived by the root-tip are transmitted 

 to the actively growing zones behind, which are not directly excitable. 

 The tip of the root itself is, however, able to perform slight geotropic curva- 

 ture 2 , and forms the percipient organ for hydrotropic, and possibly also for 

 negatively galvanotropic 3 and heliotropic stimuli. As regards the latter, 

 however, Rothert 4 was unable to obtain sure results, nor do the experi- 

 ments of Darwin 5 and of Kohl 6 form sure proof of the localization 

 of the heliotropic irritability in the root-tip. Traumatropic curvatures 

 are also usually directed from the root-apex, although the parts behind 

 may be directly excited as well, and indeed all tropic irritability need 

 not of necessity be localized in the root-tip. Thermotropic, aerotropic, 

 rheotropic, and thigmotropic stimuli may, in fact, be perceived by the 

 curving regions, and these may often be the only parts capable of direct 

 excitation. The localization of the heliotropic irritability to the tip of the 

 cotyledon of certain Grasses does not, therefore, necessarily indicate that 

 the geotropic irritability will be similarly localized, although experiment 

 has shown that this is the case. The power of perception is retained by 

 the tip of the cotyledon after it has ceased to grow, whereas in the primary 

 meristem of roots the geotropic irritability disappears when stretching 

 growth commences. 



1 Rothert, 1. c., p. 139. 2 Czapek, Jahrb. f. wiss. Bot., 1900, Bd. xxv, p. 361. 



3 [On the true nature of this irritability see Ewart and Bayliss, Proc. Roy. Soc., Sep., 1905.] 



4 Rothert, 1. c., p. 140 ; Flora, 1894, Ergzbd., p. 207. 



5 Darwin, 1. c., p. 413. 6 Die Mechanik d. Reizkriimmungen, 1894, p. 26. 



