AUTOGENIC AND AITIOGENIC CHANGES OF IRRITABILITY 205 



The pronounced geotropic reaction of a node on the stem of Tradescantia 

 virginica is somewhat decreased when the internode between it and the next younger 

 node is severed, and is entirely suppressed in the case of Tradescantia fluminensis and 

 T. zebrina. According to Kohl \ this is due to the fact that, as in roots and the 

 cotyledons of grasses, the perceptive and reacting zones are separately localized, the 

 young node perceiving the geotropic stimulus and transmitting it to the next older 

 one. Miehe 2 has shown that a geotropic curvature is produced in the third horizontal 

 node of an intact plant when the next younger second node is placed vertically by 

 bending the internode. If the third node is placed vertically, no curvature results 

 in it, but instead a geotropic reaction is awakened in the horizontal younger second 

 node. Furthermore, the geotropic irritability of the third node is diminished when the 

 second node is placed in carbon dioxide or in a plaster cast, so that its growth and 

 functional activity are depressed or stopped. It follows, therefore, that the removal of 

 the younger node awakens positive geotropism in the next older node, and that the 

 same effect can be produced by placing the node in a vertical position ; and to produce 

 a complete change of tone the entire younger node with its bud must be removed. 

 The remaining portion of the internode then dies and is thrown off. The older node 

 loses its geotropic irritability when the continuity of the internodal vascular bundles 

 is broken, so that these must serve for the transmission of the correlative interactions 

 concerned. It is possible that the correlative stimulatory actions in question are 

 derived from the distribution and diffusion of certain of the products of metabolism. 



Vochting s found that after removing the flower of a poppy, or the capitulum of 

 Tussilago Farfara, the temporary positively geotropic power of reaction of the peduncle 

 was arrested, whereas the negative geotropism and the autotropism were unaffected. 

 Since the same effect is produced by the removal of the ovary only of the poppy, the 

 correlative influences which modify the geotropic tone seem to have their origin in 

 this part of the flower. According to Wiesner *, moreover, the upward curvature of 

 a horizontally-placed inflorescence axis of Digitalis and other plants no longer occurs 

 when the flowers have been fertilized. The decapitated peduncle of a Poppy still 

 remains capable of growth, whereas, according to Scholtz 5 , the removal of the flower 

 of Clematis cylindrica or of Dahlia variabilis causes the peduncle to lose the power of 

 growth, and hence also of geotropic reaction. 



Changes of geotropic tone may also be responsible for the absence of torsion in 

 the internodes of Philadelphus and Deutzia when the pair of leaves at the upper end 

 of the internode are removed 6 . Similarly, Noll 7 observed that the removal of the 

 apex of the inflorescence of an orchid resulted in the neighbouring ovaries undergoing 

 no torsion. 



1 Kohl, Bot. Ztg., 1900, p. i. 



2 Miehe, Jahrb. f. wiss. Bot., 1902, Bd. xxxvn, p. 527. 



3 Vochting, Beweg. d. Bliithen vu Friichte, 1882, pp. 107, 126; Scholtz, Cohn's Beitrage z. 

 Biologic, 1892, Bd. v, p. 371. 



4 Wiesner, Biol. Centralbl., 1901, Bd. xxi, p. 803. 



5 Scholtz, 1. c., p. 387. 



6 De Vries, Arb. d. bot. Inst. in Wurzburg, 1872, Bd. II, p. 273; Schwendener u.Krabbe, 1892, 

 Ges. bot. Mitth., Bd. II, p. 309. 



T Noll, Arb. d. bot. Inst. in Wiirzburg, 1887, Bd. Ill, p. 368. 



