FORMATIVE REACTIONS 73 



activity, the external agency simply acting as a stimulus which directs this 

 activity along particular channels. 



Not only different plants, but also the different organs of the same 

 plant, have dissimilar powers of response, and hence have been termed 

 anisotropic by Sachs l . The opposed geotropic irritabilities of the stem 

 and root is one instance of anisotropy, and another is afforded by the 

 dissimilar behaviour of the upper and under surfaces of dorsiventral 

 organs. Here, and in other cases also, the anisotropy is accompanied 

 by differences of structure, but there may be a physiological dissimilarity 

 without any visible morphological one. 



Whenever anisotropy is present, a diffuse stimulus acting with equal 

 intensity on all sides may produce ah unequally distributed reaction, such 

 as a curvature or thickening on a particular side. To produce such an 

 effect on an isotropic and physiologically radial organ the stimulus must 

 either act on one side only or with greater intensity on that side. 



From a physiological point of view the orienting curvatures produced by 

 one-sided stimulation may be termed 'tropistic' (gcotropism, heliotropism}. Cur- 

 vatures produced by diffuse stimuli owing to the physiological dorsiventrality 

 of the organ affected may be termed 'nastic' (photonasty, geonasty, autonasty], 

 while to indicate the side whose growth is accelerated the terms epinasty, 

 hyponasty, paranasty may be used (viz. photoepinasty). Unilateral thickening 

 by internal or external causes may be called ' trophic,' and the term ' auxesis ' 

 (photoauxesis] may be used to denote the predominant formation of leaves, 

 roots, or hairs upon a particular side 2 . 



1 Sachs, Arb. d. Wiirzb. Inst., 1879, Bd. II, p. 226. Cf. Czapek, Jahrb. f. wiss. Bot., 1898, Ed. 

 xxxii, p. 302. 



2 For the morphological terminology cf. Goebel's Organ ography, Clar. Press, 1900, p, 65 seq. 

 Schimper (quoted by Hofmeister, Allg. Morphol., 1868, p. 604) applied the terms 'epinasty' and 

 ' hyponasty ' to the excentric growth in thickness of branches, but following de Vries (Arb. d. Wiirzb. 

 Inst., 1872, Bd. I, p. 252) they have been used to denote the unequal growth in length of two 

 opposed sides. ' Photonasty ' was used by Pfeffer (Pflanzenphysiol., 1880, I. Aufl., Bd. II, p. 287) and 

 'paranasty' by Holl (Arb. d. Wiirzb. Inst., 1885, Bd. in, p. 229) in the same sense. Wiesner (Ber. 

 d. Bot. Ges., 1895, p. 485; Sitzungsb. d. Wien. Akad., 1895, Bd. civ, I, p. 83) denoted excentric 

 growth in thickness by the term * trophism,' while Weisse (Ber. d. Bot. Ges., 1895, p. 385 ; cf. also 

 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 289) proposed the term 'auxesis' to indicate the 

 formation of appendages on dorsiventral organs. 



[This terminology cannot be said to be satisfactory, nor does it even tabulate the observed 

 phenomena into well-defined and separable groups. Gravity, for example, is not a one-sided 

 stimulus, but is more perfectly diffused than any other, since even in lofty trees it has practically the 

 same intensity at all parts, the maximal difference in its intensity at base and apex being 74 \ - g. 

 The primary geotropic curvatures are the result of a diffuse stimulus acting upon the physiologically 

 bipolar embryo; the fact that the force of gravity is directed towards the centre of the earth is 

 immaterial to the point at issue, and if the cell-contents were of uniform density throughout no 

 geotropic response would be possible. A horizontally-placed radicle curves downwards because the 

 same stimulus produces unequal growth-responses in its upper and under surfaces, i. e. because in this 

 position it becomes for the moment physiologically dorsiventral. This is probably due to the fact 

 that the settling of the denser materials on the under but normally lateral sides of the cells acts as 

 a stimulus awakening unequal growth-responses, but in any case the power of discrimination lies in 



