i88 TROPIC MOVEMENTS 



rheotropic and hydrotropic, but hydrotropic stimuli are only perceived 

 at the root-apex, whereas rheotropic stimuli are also perceived in the zone 

 of stretching growth. Probably further researches, especially in connexion 

 with freely motile organisms, will reveal additional instances of the occur- 

 rence of these forms of irritability single or in combination. Nothing is 

 known as to the mode in which rheotropic, osmotropic, and hydrotropic 

 stimuli are perceived, but it is quite possible that osmotropic stimulation 

 may be the result of a very different form of excitation to that involved 

 in the production of rheotropic or hydrotropic responses. On this basis it 

 is easy to understand why transpiration is able to excite an increased 

 development of cuticle but not the withdrawal of water due to the osmotic 

 action of a saline solution. Elfving l found that no curvature was induced 

 in the strongly hydrotropic sporangiophore of Phycomyces by the impact 

 of a stream of saturated air, but this empirical fact permits of no conclusions 

 as to the nature of a hydrotropic excitation. 



SECTION 42. Galvanotropism. 



Since many freely motile organisms are strongly galvanotactic, it 

 might be expected that the organs of fixed plants would often be capable 

 of galvanotropic curvature. Hitherto, however, this form of irritability has 

 only been detected in the radicles of seedlings, which according to some 

 authors are positively, and according to others negatively galvanotropic 2 . 

 Brunchhorst considers that these contradictory results are due to the fact 

 that when the current is weak the curvature is towards the kathode, but 

 when strong towards the anode. This latter positive curvature is, according 

 to Brunchhorst, traumatropic in character, being due to the injury of the 

 anodal side of the root by the strong current. Further researches are, 

 however, required to determine whether this is actually the case, and also to 

 elucidate more thoroughly the observed phenomena. 



Additional investigation is also needed concerning the negatively 

 directed curvatures produced on the sporangiophore of Pkycomyces, accord- 

 ing to Hegler 3 , by the action of the Hertzian electrical waves, also 

 concerning the negative curvatures observed by Lepellier 4 away from 



* 



1 Elfving, Zur Kenntniss d. pflanzlichen Irritabilitat, 1893, p. 4. 



a Elfving, Bot. Ztg., 1882, p. 257; Miiller-Hettlingen, Pflliger's Archiv f. Physiol., 1883, 

 Bd. xxxi, p. 201 ; Branchhorst, Ber. d. hot. Ges., 1884, p. 204; Notizen iiber d. Galvanotropismns, 

 1889 (reprint from Bergens Museums Aarsberetning) ; Rischawi, Bot. Centralbl., 1885, Bd. xxu, 

 p. 121. [None of the methods used, even by Brunchhorst, is wholly satisfactory. See Ewart and 

 Bayliss, Proceedings of the Royal Society, Nov., 1905.] 



3 Hegler, Ueber die physiologische Wirkung der Hertzischen Elektricitatswellen auf Pflanzen, 

 1891 (reprint from Verhandlg. d. Ges. deutscher Naturf. u. Aerzte in Halle). 



* Letellier, Bull, de la Soc. bot. de France, 1899, T. VI, p. n. Steyer (Reizkriimmungen bei 

 Phycomyces, 1901, p. 17) obtained negative results with Phycomyces. On the action of statical 

 electricity cf. also Danilewsky, Die physiolog. Fernwirkungen der Elektricitat, 1902 ; Loeb, Pfliiger's 

 Archiv f. Physiol., 1897, Bd. LXVII, p. 483; Bd. LXIX, p. 99. 



