CONJOINT EFFECTS 127 



All aitionastic reactions dependent upon physiological reactions need 

 not, however, result in rapid or pronounced movement, for slow movements 

 may be of the utmost value in ensuring appropriate positions of the sub- 

 aerial organs more especially in regard to light. The rhizomes of Adoxa 

 moschatellina, of Circaea^ and of a few other plants show no power of 

 photonastic reaction when rotated on a klinostat, but do so when exposed 

 to the inductive action of gravity. When the rhizome has assumed 

 a transversely geotropic position in darkness, exposure to diffuse light 

 excites a downward curvature which increases to a certain maximum as 

 the illumination increases. Renewed darkening results in the assumption 

 of the original diageotropic position. The subaerial runners of certain 

 plants behave in the same way, for they become erect in darkness, and 

 curve to a horizontal position when exposed to sufficiently strong diffuse 

 illumination. Geotropic induction may indeed take place in several photo- 

 nastic responses, especially when the organ possesses a strong geotropic 

 irritability. According to Lidforss, the thermonastic reaction of the shoots 

 of Holosteum ttmbellattim, Lamium purpureum^ Veronica chamaedrys, and 

 Mimulus Tilingii depends upon geotropic induction, but not that of the 

 peduncles of Anemone nemorosa' 1 . 



These curvatures are to be classed as photonastic, since under this 

 head we include all reactions due to changes in the intensity of the 

 diffuse illumination without specifying the detailed mode of perception 

 and response. The same would still be the case when the illumination 

 merely modified the geotropic irritability, and hence produced varying geo- 

 tropic curvatures according to its intensity. Indeed, if primary importance is 

 attached to the geotropic irritability, the illumination and temperature may 

 be regarded as modifying the geotropic tone, for, apart from all considera- 

 tions as to the internal physiological reactions, it remains the fact that 

 the same tropic action of gravity may produce varying degrees of 

 curvature according to whether the plant is strongly or feebly illuminated, 

 that is according to its phototonic condition. 



The knowledge that a particular curvature is due to the co-operation 

 of light and gravity, the former altering while the latter remains constant, 

 does not reveal all that is to be learnt about the phenomenon. The 

 geotropic irritability might alter according to the intensity of the illumina- 

 tion ; or, the former remaining unaltered, the dorsiventrality induced by the 

 constant stimulus of gravity might co-operate with the variable photonastic 

 response. Other factors might also come into play, but it is clear that 

 in all cases the geotropic stimulus is as directive in character as when 

 the photonastic irritability is based upon an inherent dorsiventrality, and 



1 Lidforss, Bot. Centralbl., 1901, Bd. LXXXVIII, p. 169; Jahrb. f. wiss. Bot., 1902, Bd. xxxvm, 

 P- 343- 





