OTHER STIMULI 129 



Ray ' found that Sterigmatocystis becomes more firmly attached when the culture- 

 fluid is kept in motion, and very striking responses to stress and strain are given 

 by many other attaching organs, such as hooks, tendrils, and aerial roots 2 . 



SECTION 37. Other Stimuli. 



When an organ grows against an immovable object, the mechanical 

 retardation of growth induces certain secondary stimulatory effects upon 

 it, and upon other organs as well 3 . The changes of tension act in 

 this way, and growth may also be affected by contact, and by mechanical 

 vibrations in certain organs which have acquired the power of responding 

 to such stimuli. The leaves of Mimosa pudica, for example, respond to all 

 mechanical vibrations, however produced, whereas tendrils respond only to 

 contact with solid bodies, and not to contact with liquids, such as water 

 or mercury. Tendrils are thus able to discriminate between liquids and 

 solids by means of their sense of touch, and they are unaffected by contact 

 with wet gelatine if it contains sufficient water. 



The stimulus of contact causes an acceleration of growth in tendrils, 

 and also in the sporangiophore of Phycomyces, and this leads to a curvature 

 if the stimulus is applied to one side only, or if the organ is more sensitive 

 on one side than on the other. In addition the same stimulus often 

 produces a pronounced secondary thickening in tendrils, twining petioles, 

 or the hooks of climbers. Contact is also requisite to induce the production 

 of attaching disks on the tendrils of Ampelopsis and certain Bignoniaceae, 

 and also the formation of the parasitic emergences of Cuscuta^. Similarly 

 the attaching organs of certain algae and fungi are only formed in contact 

 with solid bodies. 



In some cases contact retards growth, as when the edge of a mush- 

 room comes into contact with a grass-stem, which it grows round 

 and encloses 5 , owing to growth being retarded at the point of contact. 

 According to Schwarz the elongation of the root-hairs is retarded by 

 contact with solid bodies 6 , and disturbances of growth due to contact are 

 largely responsible for the peculiar shapes assumed by root-hairs and 

 fungal hyphae T . 



1 J. Ray, Rev. G<5n. d. Bot., 1897, T. ix, p. 353. 



a Ewart, Ann. du Jard. Bot. de Buitenzorg, 1898, T. XV, pp. 208, 218, 222, 231, 234. 



* Pfeffer, Druck- u. Arbeitsleistungen, 1893, pp. 333, 437. 



4 On the Haptera of Podostemonaceae cf. Warming, Bot. Ztg., 1883, p. 193. 



5 Macaire, Mem. d. 1. Soc. d. Geneve, II, Pt. II, p. 124, quoted by Treviranus, Physiologic, 

 Bd. II, p. 194 ; J. Schmitz, Linnaea, 1843, Bd. xiv, p. 448. Frequently it is the grass-haulm which 

 bores through the mushroom. 



6 Fr. Schwarz, Unters. a. d. Bot. Inst. zu Tubingen, 1883, Bd. I, p. 179. 



7 Id., 1. c. ; Reinhardt, Jahrb. f. wiss. Bot., 1892, Ed. XXIII, p. 495; C. Sokolowa, Wachsthum 

 d. Wurzelhaare u. Rhizoiden, 1897. 



FFEFFER. I! 



