202 TROPIC MOVEMENTS 



apices, and that this effect spreads from the excitable zone. We are, however, pro- 

 bably dealing with a secondary reaction, resulting from the primary processes of 

 sensation and induction. 



SECTION 45. Instances of Autogenic and Aitiogenic Changes 



of Irritability. 



The special irritabilities of stems, roots, and other organs cannot come 

 into being before the primordial rudiments are developed, and in many cases 

 may only appear when a certain stage of development has been reached. 

 Thus stems and leaves while in the bud, or when just escaping from it, 

 usually show no geotropic or heliotropic irritability. In addition, the nodes 

 of stems do not at first possess any geotropic irritability, while those of 

 DiantJnis bannaticns only develop this irritability when fully grown ] . In 

 the case of Spirogyra, Bacteria, and other asomatophytes only embryonic 

 cells are available, while the geotropic perception and reaction of mould- 

 fungi is restricted to the embryonic growing apex of the hypha. Further- 

 more, the geotropic irritability of the apical meristem of a root is lost 

 in the elongating segment-cells, whereas in other cases a tropic sense may 

 persist after the power of reaction has been lost. Automatic changes of 

 tropic irritabilities are also frequently used to produce curvatures under 

 constant external conditions, and periodic movements may be normally 

 induced by regular autogenic changes of tone. 



In addition, changes of the external conditions may induce changes 

 of tone resulting in modifications in the character or rapidity of tropic 

 reactions. It has already been mentioned that the heliotropic reaction 

 of seedling-stems is suppressed by a partial pressure of oxygen which still 

 permits of geotropic stimulation and curvature, while in air rarefied enough 

 to suppress curvature no perception of a tropic stimulus or after-effect are 

 possible. According to Czapek 2 , however, the root of Lnpinus is able 

 to perceive a geotropic stimulus in the entire absence of free oxygen. A 

 root kept in a horizontal position at o to 2 C. for twenty-four hours in 

 oxygenless air showed on a klinostat a curvature due to the geotropic 

 induction on returning to ordinary air and room temperature. 



Low temperatures retard geotropic reaction sooner than geotropic 

 sensation 3 , so that roots of Lnpinus placed horizontally for eighteen hours 

 at o to 2 C. perform a geotropic curvature when returned to a more 

 favourable temperature as the after-effect of the previous induction. The 

 curvature is, however, not very pronounced, partly owing to the lowered 



1 Earth, Die geotropische Wachsthumskriimmung der Knoten, 1894, pp. 8, 28. The same 

 applies to the development of irritability in tendrils and in the pulvini of Mimosa and other plants. 



2 Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 277. 



3 Czapek, 1. c., p. 272 ; Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 195. 



