MECHANOTROPISM 



185 



of the growing zone, as well as the next zone which has just ceased to 

 grow, are able to perceive rheotropic stimuli. Among the roots examined 

 by Berg, only those of Soja Jiispida showed no power of rheotropic reaction, 

 whereas, according to Newcombe, a variety of roots are insensitive. 



Rheotropism has also been detected in the hyphae of fungi, those of 

 Phycowyces and Mucor being negatively and those of Botrytis cinerea 

 being mainly positively rheotropic, according to Jonsson. The strip of 

 filter-paper on which the mycelium is growing has each end immersed in 

 a nutrient liquid, one of the vessels being slightly higher than the other. 

 The slow movement of water thus induced is sufficient to act as a rheotropic 

 stimulus to the hyphae. 



FlG. 39. Radicles of Vicia saliva undergoing rheotropic excitation. The arrow shows 

 the direction of rotation, the movement of water producing the curvatures shown at the 

 end of sixteen hours. 



For purposes of demonstration the apparatus shown in Fig. 39 may be used, 

 the glass dish containing water being rotated on a klinostat, so that the speed of the 

 current to which the radicles are exposed will depend upon their distance from the 

 axis of rotation. The same effect is produced when the seedlings are rotated and 

 the vessel kept stationary, and Jonsson placed the radicles in a narrow straight stream 

 of running water. Berg also succeeded in showing that roots show a rheotropic 

 reaction when growing in soil. 



Traiimatropism. Injury causes a wound-reaction which may exercise 

 a correlative effect upon the growth and movement of associated or remote 

 parts. Among these are included certain tropic curvatures which are 

 induced by local injury to the growing-points of aerial and subterranean 

 roots, due to incision or to cauterization by heat, alkali, acid, or lunar 

 caustic 1 . A few hours afterwards a curvature begins in the elongating 



1 Darwin, The Power of Movement in Plants, 1880, p. 528 ; Spalding, Annals of Botany, 1894, 

 Vol. vin, p. 423 ; Pollock, Botanical Gazette, 1900, Vol. xxix, p. i. 



