THE CONDITIONS FOR STIMULATION AND ITS PROGRESS 217 



involves a certain change of tone, which, however slight, must inevitably 

 result from the altered conditions. Noll's objections to this conclusion 

 are based upon a one-sided consideration of the external relationships 1 . 

 It is, in fact, true that a lowered heliotropic sensibility results either from 

 a general increase of illumination or from an increase of illumination 

 parallel to the long axis of a parallelotropic organ. As an instance of 

 such action it may be mentioned that Hering found a general retardation 

 of growth in length to occur in inverted plants or organs, and similarly 

 the growth excited in the node of a grass by the diffuse horizontal action 

 of gravity is inhibited by the parallelotropic action of gravity. Tropic 

 stimuli often exercise more than one effect, so that the resultant position 

 may be due to the co-operation of two or more activities, as, for instance, 

 in the plagiotropic prothallia of Ferns, where the continuance of the labile 

 dorsiventral induction affords at the same time an instance of the main- 

 tenance of a special tone appropriate to the position assumed. The 

 tropic excitation due to a change of position usually rapidly increases to 

 a certain limit, as the angle of divergence from the normal position increases. 

 In the case of parallelotropic organs the maximal angle of divergence 

 from the normal position is 180, but in that of plagiotropic organs 

 not more than 90. The maximal excitation in the case of certain 

 parallelotropic organs, and possibly of all, is not reached until the divergence 

 is greater than 90 C., but it is quite possible that in some cases the 

 tropic excitation may be greater when the organ is at right angles to 

 the orienting agency, as was, in fact, concluded to be the case by Sachs, 

 and also by Bateson and Darwin 2 . This view is supported by Massart 3 , 

 but Elfving, on the other hand, supposed that the maximal geotropic 

 excitation is exercised when the main root is inverted 4 . It is, however, 

 quite certain that the geotropic stimulus is not directly proportional to 

 the sine of the angle of divergence, i.e. to the component of the force of 

 gravity acting at right angles to the stem, although an approximate 

 correspondence may be shown when the divergences are small. Czapek 6 

 found, however, that the maximal geotropic action was exercised when 

 all the parallelotropic organs examined by him were diverted from their 

 normal positions through angles of 140 to 160 C. The increase of 

 excitation was evidenced in the first instance by the rapidity of reaction. 



1 Noll, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 487. Cf. Pfeffer, Die Reizbarkeit d. Pflanzen, 

 1893, p. 19 ; Unters. a. d. hot. Inst. zu Tiibingen, 1888, Bd. I, p. 476; Czapek, Jahrb. f. wiss. Bot., 

 1898, Bd. xxxn, p. 195 ; G. Haberlandt, Jahrb. f. wiss. Bot., 1903, Bd. XXXVIII, p. 468 ; Noll, Ber.. 

 d. bot. Ges., 1902, p. 416. 



2 Sachs, Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. II, p. 240; Flora, 1873, p. 325; Bateson and 

 F. Darwin, Annals of Botany, 1888, Vol. II, p. 65. 



3 Massart, Sur I'lrritabilite" d. plantes superieures, 1902, p. 28. 



4 Elfving, Beitrage z. Kenntniss der Wirkung d. Schwerkraft auf Pflanzen, 1880, p. 32. 



5 Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvil, pp. 283, 297 ; 1898, Bd. XXXII, p. 193. 



