AUTOGENIC AND AITIOGENIC CHANGES OF IRRITABILITY 203 



irritability due to the low temperature, and partly owing to the fact that 

 the power of reaction may be temporarily depressed. Similar results were 

 obtained by Czapek by the sufficiently intense action of chloroform, carbon 

 dioxide and caffeine. 



According to Czapek 1 , the receptivity of geotropically-sensitive organs 

 continually rises as the temperature does, so that the relationship between 

 receptivity and temperature is represented by an ascending curve, as is that 

 between respiration and temperature, whereas the growth-curve falls beyond 

 a certain optimum temperature. The injury of the cotyledon of Avena^ 

 and of the root-apices of a variety of plants, produces a transitory inhibi- 

 tion of the power of perception, but not always of the power of conducting 

 stimuli, while the power of perceiving tropic stimuli is still retained when 

 growth is mechanically prevented by embedding in a plaster cast. 



The inhibition of the power of perception or reaction is an instance of 

 the aitiogenic modification of tropic properties ; but, in addition, changes 

 of tone may be induced which cause alterations in the position of equilibrium 

 under the same constant stimulus. Both diffuse and unilateral stimuli 

 may directly and indirectly produce changes of tone, and a particular 

 tone may either rapidly appear owing to the changed conditions or may 

 gradually result from the conditions prevailing during development. These 

 considerations apply to existent organs, although external influences may 

 also induce a formation of organs with specific powers of reaction. No 

 sharp distinction can, however, be drawn, since the modification of tone may 

 only appear in the portions of the organ developed under the new con- 

 ditions, or, in the case of a Bacterium, in the new individuals. Cultivated 

 plants often show varied powers of reaction under different conditions, and 

 Vochting 2 found that the flowers of Itnpatiens parviflora and the cleisto- 

 gamic flowers of Linaria spuria possessed no power of geotropic orientation 

 when developed in feeble light. The cultural conditions also apparently 

 exercise a pronounced effect upon the power of reaction of Bacteria and 

 other micro-organisms 3 , while in certain cases races may be developed with 

 particular tactic or tropic properties. 



Although injuries may cause a transitory depression or inhibition of 

 the geotropic and heliotropic irritabilities, pieces of stems and roots are 

 usually capable of tropic reaction. Nevertheless, the injury probably 

 may either affect the rapidity of reaction, or produce a correlative modifica- 

 tion or suppression of the position of equilibrium or of the power of tropic 

 reaction. In certain cases, however, the removal or prevention of growth 

 of an organ may produce profound changes of irritability in neighbouring 



1 Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxxn, pp. 198, 303. 

 a Vochting, ibid., 1893, Bd. xxv, pp. 179, 189. 

 9 Cf. Rothert, Flora, 1901, p. 416. 



