CHANGES OF IRRITABLE TONE 207 



temperature illumination, nutrient and non-nutrient substances, as well as other 

 agencies l . The geotropic position of the lateral roots, and in some cases also of the 

 primary root, may change somewhat according to the cultural conditions, but this 

 result is probably of complex origin. When insufficiently supplied with water, how- 

 ever, certain radicles do not curve vertically downwards, but assume a more or less 

 plagio-geotropic position' 2 . According to Neljubow, the presence of the acetylene 

 and ethylene of coal-gas in the air around a seedling-stem of Pisum sativum causes it 

 also to assume a plagio-geotropic position 3 . 



The response produced by conjoint stimuli is rarely the sum of their 

 actions when applied singly, even when the power of response remains 

 unaltered, and assuming that both sensations are separately excited and 

 remain distinct until movement is excited. As a matter of fact, it is highly 

 probable that any kind of tropic stimulation affects the tone of the plant 

 and its power of response to other tropic stimuli. The power of response 

 to other stimuli naturally need not be suppressed, and in fact geotropically- 

 excited plants remain capable of response to heliotropic stimuli and vice 

 versa. The energetic response to particular stimuli might, however, render 

 the plant temporarily irresponsive to special tropic agencies, either owing to 

 a temporary suppression of excitability or of the responsive mechanism. 

 In other cases the conditions for the production of a particular irrita- 

 bility might involve preceding tropic excitation. This actually applies to 

 Cuscuta, which develops no contact-irritability when rotated on a klinostat, 

 since the required tone needs the inductive action of gravity for its 

 production. 



A complete or nearly complete inhibition of one form of irritability by 

 the functional exercise of another has not hitherto been detected, although 

 intense stimulation usually depresses the excitability more or less. Changes 

 of tone may, however, be produced by the combined tropic action of two 

 dissimilar stimulatory substances. In addition, when a radial tendril is 

 touched on both sides the excitations extinguish each other and no response 

 is produced. Such actions may either affect the intermediate stages between 

 sensation and response, or the primary sensation, as in the case of Cuscuta. 

 Noll 4 considers changes of tone to be due to the former, and Czapek 5 to the 

 latter, but the arguments of both authors are inconclusive. 



1 A few additional instances are given by Massart, Sur rirritabilite' d. plantes superieures, 1903, 

 p. 13; Klebs, Willkiirliche Entwickelungsanderungen bei Pflanzen, 1903, p. 93. 



2 Sachs, Arb. d. bot. Inst. in Wurzburg, 1873, Bd. I, p. 445 ; Elfving, Beitr. z. Kenntniss der 

 Einwirkung der Schwerkraft ant" Pflanzen, 1880, p. 32; Czapek, 1. c., p. 1252; N6mec, Jahrb. f. wiss. 

 Bot, 1896, Bd. xxxvi, p. 91. 



3 Neljubow, Beihefte z. bot. Centralbl., 1901, Bd. X, p. 128; Singer, Ber. d. bot. Ges., 1903, 



P- 175- 



4 Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 495; Ueber heterogene Induction, 1892, p. 56. 



5 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 246; Sitzuugsb. d. Wien. Akad., 1895, 

 Bd. civ, Abth. i, p. 337. 



