MECHANOTROPISM 187 



time the general wound-reaction had mainly ceased, whereas the regeneration 

 of the injured region was prevented by the plaster cast. 



The traumatropic curvature was discovered by Darwin, and was further in- 

 vestigated by Spalding, who showed that certain authors were incorrect in denying 

 the existence of any such curvature 1 . Naturally there is no question of a tropic 

 curvature when the injury is so pronounced as to lead to the partial or complete 

 death of the whole of the tissues on one side of the growing zone of a root or other 

 organ, for in this case the retardation or cessation of growth on one side, and its 

 continuance on the other, unavoidably results in a curvature. Nor is any trau- 

 matropic irritability in play when an injurious agency retards the growth of that side 

 of the organ to which it is applied. It was in this way that the curvatures of roots 

 were produced which Newcombe considered to be thigmotropic in character, and 

 possibly similar curvatures may be produced by the unilateral action of poisonous 

 gases. The true traumatropic curvatures, however, are shown by roots even when the 

 zones of perception and response are some distance apart. The tip of the seedling 

 leaf of Avena, however, which is sensitive to heliotropic stimuli does not appear to 

 have any traumatropic irritability. 



The traumatropic curvature is independent of whether the defect to 

 which it is a response has been produced by mechanical, chemical, or 

 electrical means. The other two mechanotropic reactions differ in that 

 the rheotropic response is excited by a current of water, but the thigmo- 

 tropic only by contact with solid bodies. It is not impossible that rheo- 

 tropism, hydrotropism, and osmotropism may all be forms of the same 

 irritability, and that the primary processes of perception may be alike in 

 all three cases 2 . In the case of osmotropism and hydrotropism, the 

 stimulation might arise from differences of turgor on the opposed sides 

 of the irritable organ, produced in the first case by the differences in the 

 concentration of the surrounding medium, and in the second by the 

 different rates of transpiration in unequally moist air. No such differences 

 of turgor can be responsible for- the rheotropic excitation, although the 

 unequal pressure of the water on the front and back of the root might 

 lead to a movement of water through the tissues which might operate as 

 a stimulus. 



As far as is known, however, these three forms of irritability by no 

 means always occur together, but are in most cases separately developed, 

 and hence it is more probable that they are integrally distinct manifestations 

 of irritability. Roots which are strongly hydrotropic do not appear to 

 be osmotropic, while the osmotropic hyphae of certain fungi also show 

 rheotropism but have no hydrotropic irritability 3 . Roots are, it is true, both 



1 See Spalding, Annals of Botany, 1894, Vol. vin, p. 440 ; Bot. Centralbl., 1883, Bd. xm, p. 180. 



2 Cf. Juel, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, pp. 507, 533; Rothert, Flora, 1901, p. 415. 



8 Steyer, Reizkriimmungen von Phy corny ces, 1901, p. 28. The sporangiophore of Phycomyces is, 

 however, strongly hydrotropic. 



