i 9 o TROPIC MOVEMENTS 



stronger and causes them to curve upwards. This is due to the fact that 

 the intensity of the hydrotropic excitation diminishes rapidly as the distance 

 from the source of moisture increases. The young sporangiophores are 

 also geotropically excitable, and if they are subjected to strong centrifugal 

 action they curve outwards almost as soon as they emerge from the 

 substratum. Similarly, an upward curvature is at once shown if they 

 develop in saturated air so that they are geotropically but not hydrotro- 

 pically excited. 



Orienting actions of this character may arise from living as well as 

 from dead parts, if these evolve moisture. Pollen-tubes and parasitic fungi 

 are attracted in this way into living tissues, and it is largely owing to 

 stimuli of this kind that the stem of the Mistletoe and the sporophores of 

 parasitic fungi set themselves in a definite position, which is usually nearly 

 at right angles to the surface of the stem upon which they are growing. 



In all symbiotic associations not only formative but also directive 

 interactions are exercised by the symbionts upon each other. These 

 relationships are of the utmost complexity in the higher plants, for by 

 them are determined not only the development and point of origin of 

 shoots, roots, hairs and leaves, but also the tendency to a particular 

 direction of growth of each organ in regard to the main axis. This 

 autotropism l naturally only finds full expression in the absence of all 

 external directive factors, but even when these are in play the position 

 assumed is the result of their co-operation with the autotropic tendencies. 

 Organs may be either auto-orthotropic as in the case of the primary root and 

 stem, or auto-campylotropic as in the case of the leaves and other lateral 

 appendages. The term autotropism may be used in the general sense to 

 correspond with that of automorphosis, and this terminology renders the 

 use of the words rectipetality and curvipetality unnecessary. These terms 

 were indeed used by Vochting 2 more especially in connexion with flowers. 



Every disturbance of equilibrium excites reactions which tend to its 

 restoration, and it is in this way that an organ is brought back into its 

 original position after temporary stimulation has induced movement. 

 Experiments illustrating this fact have been carried out by various investi- 

 gators 3 , and more especially Baranetzsky has shown that the return 



1 The term ' Eigenrichtung ' was suggested by Pfeffer (Pflanzenphysiol., i. Aufl., iSSi, Bd. II, 

 p. 286; Die Reizbarkeit der Pflanzen, 1893, p. 19), and may be translated by ' autotropism.' 



a Vochting, Bewegnngen der Bliithen und Friichte, 1882, pp. 31, 192. Cf. also Czapek, Jahrb. 

 f. wiss. Bot., 1895, Bd. XXVII, p. 313. The terms 'autonasty,' ' autoepinasty,' and the like are less 

 suitable, since the positions assumed are to be regarded as the result of the action of internal 

 directive stimuli. Noll used the word ' morphaesthesia ' to indicate the tendency to assume definite 

 relations of symmetry (Sitzungsb. der Niederrhein. Ges. fur Natur- und Heilkunde, 15. Jan. 1900), 

 but the term is a quite unnecessary one. 



5 Vochting, 1. c., 1882, pp. 31, 182, 192 ; F. Darwin and Pertz, Annals of Botany, 1892, Vol. VI, 

 p. 247 ; Czapek, 1. c., 1895, p. 308 ; Kohl, Ber. d. bot. Ges., 1898, p. 169 ; Baranetzsky, Flora, 1901, 



