206 TROPIC MOVEMENTS 



SECTION 46. Changes of Irritable Tone (continued}. 

 The thermonastic and photonastic curvatures produced by changes 

 of illumination or temperature are either the result of indirect changes in 

 the geotropic tone or are due to the action of gravity in producing physio- 

 logical dorsiventrality in the responding organ. When the latter is the 

 case a response may be shown at first on the klinostat, but when none 

 is shown it still remains to be determined whether the actual curvature 

 involves a labile ephemeral induction or a modification of the geotropic 

 tone. Definite results may be obtained in the future, but it is worthy of note 

 that an increased reaction following a rise in the intensity of the directive 

 agency might merely be the result of its enhanced dorsiventral inductive 

 action. Probably both changes of tone and inductive actions are utilized 

 separately and in various combinations by different plants for special 

 purposes. The increase in the intensity of a diffuse stimulus may modify 

 the tropic action of the same agency. This occurs whenever an increase 

 in the intensity of diffuse illumination or in concentration so alters or weakens 

 the tropic sensitivity to unilateral illumination or to the unequal distribu- 

 tion of a chemical substance that a change of position results. 



Instances of the influence of illumination upon the geotropic irritability are 

 afforded by the subterranean runners of Adoxa moschatellina, Trientalis curopaea, and 

 Circaea lutetiana, which are diageotropic in darkness, but curve downwards when 

 illuminated, even if already embedded in the soil. The curvature is accelerated in 

 Adoxa by the fact that illumination hastens or awakens the growth of the previously 

 darkened runner \ It is also owing to a change of their geotropic irritability that the 

 runners and other shoots of a variety of plants become approximately vertical in 

 darkness, but assume plagiotropic to horizontal positions under diffuse illumination of 

 increasing intensity ~. Illumination also causes a certain geotropic downward curva- 

 ture of the lateral roots, causing the angle between them and the main root to 

 diminish by about 20 to 30 s . Czapek found that this reaction was no longer 

 shown when the apex was covered with tinfoil, so that the tonic stimulus of light is 

 only perceived by the growing apex. 



The geotropic angle of the lateral roots is somewhat lessened by a rise of tem- 

 perature 4 , which also affects the geotropic position of certain shoots and leaves. 

 In dorsiventral organs, however, aitionastic curvatures may complicate matters, and 

 it is always possible that changes of the heliotropic tone may be induced by alterations 

 in the diffuse external conditions. No researches have, however, been performed in 

 this direction, although it is certain that not only the phototropic, but also other tropic 

 positions of equilibrium may be more or less modified by the diffuse action of 



1 Stahl, Ber. d. hot. Ges., 1884, p. 391. 



2 Czapek, Sitzungsb. d. Wien. Akad., 1895, Bd. civ, Abth. i, p. 1234; Oltmanns, Flora, 1897, 

 p. 24 ; Goebel, Organography, Vol. I, 1900, p. 93 ; Maige, Ann. sci. nat., 1900, 8 e sei., T. xi, p. 248. 



3 Czapek, 1. c., 1895, Bd. civ, Abth. i, p. 1245; Stahl, 1. c., 1884, p. 393. 



* Czapek, 1. c., p. 1251 ; Sachs, Arb. d. bot. Inst. in \Viirzburg, 1874, Bd. I, p. 624. 



