130 INFLUENCE OF THE EXTERNAL CONDITIONS ON GROWTH 



Various authors' have shown that the formation of attaching organs in Botrytis 

 cinerea, Mucor stolonifer, and a few other fungi is induced by contact, and that the 

 production and development of the rhizoids 2 and attaching disks 3 of certain, but 

 not of all algae, are induced or accelerated by the same cause. The formation 

 of root-hairs on aerial roots is determined mainly by the percentage of moisture, 

 and not by contact, for in moist air they appear on all sides of the root 4 . Pfeffer 

 erroneously considered that the stimulus of contact favoured the formation of root- 

 hairs on the gemmae of Marchantia B , and Goebel's observations do not prove that 

 contact induces the development of the rhizoids of Riccia fluitans 6 . Sachs con- 

 cluded that contact is in some cases responsible for the development of the 

 pitchers of Nepenthes, but the experiments are not conclusive 7 . According to 

 Raciborski, certain tropical ferns only form sporophylls when their rhizomes are 

 fixed to a support, but this is probably the result of some indirect action 8 . 



Growth is affected in other ways by mechanical agencies, and since 

 blows, pressure, and tension are all fatal when sufficiently intense, it is only 

 to be expected that when somewhat less intense they should exercise 

 a certain retarding action. Further, single or repeated changes of tension 

 and pressure, such as are produced by bending, torsion, or shaking, may 

 influence the rapidity of growth. At the same time, however, other factors, 

 such as changes in the supply of food and oxygen, or in the rate of 

 transpiration, may co-operate in producing the result observed. 



Both terrestrial and aquatic plants must be able to resist the 

 varying stresses which the movements of wind and water bring to bear 

 upon them if they are to survive 9 . Thus large algae are unable to develop 

 in mountain torrents, because they would at once be torn from the rock, 

 whereas small algae, diatoms, and other organisms may exist directly 

 beneath a waterfall. The power of resistance is increased by minuteness, 

 but nevertheless many microscopic organisms may be injured and finally 

 killed by strong continuous mechanical shocks or vibrations. On the other 

 hand, mechanical stimuli strengthen certain organs, and it is not impossible 

 that some motile organisms may be unable to develop if not permitted 

 to exercise the function of locomotion. 



1 De Bary, Bot. Ztg., 1886, p. 412; Wortmann, Bot. Ztg., 1881, p. 385; Ward, Annals of 

 Botany, 1888, Vol. n, p. 332 ; Busgen, Bot. Ztg., 1893, p. 53. 



2 Borge, Bot. Centralbl., 1895, Bd. LXI, p. 319; Bitter, Jahrb. f. wiss. Bot., 1899, Bd. XXXIV, 

 p. 230; Ber. d. Bot. Ges., 1899, pp. 264, 272. 



s On the attaching disks of Plocamium cf. Goebel, Organography, 1900, I, pp. 40, 269. 



4 Cf. Ewart, Ann. d. Jard. Bot. d. Buitenzorg, 1898, T. XV, p. 237. The disk on the tendril of 

 Ampelopsis Veitchii produces numerous hairs as the result of contact (Darwin, Climbing Plants, 

 1875, p. I, footnote ; Cohn, Bot. Ztg., 1878, p. 27). 



8 Pfeffer, Unters. a. d. Bot. Inst. zu Tubingen, 1885, Bd. I, p. 528. 



' Goebel, Organography, 1900, I, p. 268. 



7 Mentioned by Goebel, Pflanzenbiol. Schilderungen, 1891, II, p. 96. 



8 Raciborski, Flora, 1900, p. 25. 



9 On the mechanical action of rain cf. Wiesner, Bot. Centralbl., 1896, Bd. LXV, p. 42, and Ann. 

 d. Jard. Bot. d. Buitenzorg, 1897, T. XIV, p. 283. 



