AUTOTROPISM AND SOMATOTROPISM 191 



movement may involve a few transitory oscillations. The return move- 

 ment can naturally only be performed when the power of growth or of 

 expansion is retained, but it is worthy of note that the growing apex 

 of an auto-orthotropic shoot or root continues to grow in a straight line 

 even when the parts immediately behind are permanently curved or 

 forcibly bent, and the attempted autogenic straightening prevented. It 

 follows that the autotropic reaction is strictly localized to the part affected, 

 and hence it is not surprising to find that autotropic return curvatures may 

 be performed by decapitated roots l . 



Autotropic stimuli may, however, affect parts a greater or less 

 distance away by the aid of the correlative mechanism, and indeed the 

 removal of an organ such as the terminal shoot of a Conifer may. affect 

 the autotropism as well as the geotropic irritability of neighbouring 

 branches. It is owing to some autotropic action at a distance of this 

 character that the lateral branches and roots assume at first their auto- 

 tropic position, but are more affected by the geotropic stimulus as 

 they increase in length. The lateral roots always ultimately assume the 

 same plagio-geotropic position independently of the angle which they 

 assume in regard to the parent axis. Dutrochet was therefore in error 

 in assuming that the plagiotropic position of the lateral roots was the 

 resultant of their positive geotropism and their tendency to set themselves 

 at right angles to the main root. It is, however, quite possible that the 

 lateral roots may possess a feeble geotropic irritability as soon as they 

 emerge externally. 



A lateral shoot will only return to its original position when capable 

 of an autotropic curvature. So 'long as no mechanical hindrances intervene, 

 this is the case with hairs and with the lateral roots of second, third, 

 and higher orders, for these have no geotropic irritability, and orient 

 themselves in regard to the main root at angles determined by their 

 autotropism. The same applies to the lateral roots of the first order 

 when developed on a rotating klinostat, for they then grow out for the 

 most part at right angles to the main root ; whereas under normal con- 

 ditions they usually form acute downwardly-facing angles with the per- 

 pendicular main root 2 . 



The orienting actions radiating from living and dead substrata 

 were first recognized by Dutrochet 3 , and were studied more fully by 

 Sachs 4 . Dutrochet erroneously concluded that the autotropic angle was 



Erg.-Bd., p. 143. See also Bonnet, Nutzen d.Blatter, 1762, p. 170; Dutrochet, Memoires, &c., 

 Bruxelles, 1837, P- 3 2 > Ann - d. s i- nat -> J ^44> 3 e s ^ r -> T. II, p. 98; Miiller, Flora, 1876, p. 91 ; 

 Darwin, The Power of Movement in Plants. 



1 Czapek, I.e., p. 322. 



2 Sachs, Arb. d. hot. Inst. in Wiirzburg, 1874, Bd. I, pp. 596, 615. 

 s Dutrochet, Rech. anat. et physiol., 1824, p. 101. 



* Sachs, Arb. d. bot. Inst. in Wiirzburg, 1874, Bd. i, p. 598; 1879, Bd. II, p. 217. 



