184 TROPIC MOVEMENTS 



the sporangiophores diverge from one another when closely crowded. Frequently 

 they may curve towards or away from a rod of metal or other material fixed upright 

 in the culture medium l . According to Errera and Steyer, this is due to the rod 

 either condensing or evolving water vapour, or changing the degree of saturation 

 of the surrounding air by warming it 2 . There is therefore no need to assume the 

 existence of any mysterious action at a distance, although in some circumstances 

 other stimulatory reactions may come into play. 



Even should it be found that the dissimilar rates of transpiration due to the 

 variations in the percentage of moisture act as the stimuli to curvature there would 

 be no need to change the term ' hydrotropism/ and still less need to invent a new one. 



SECTION 41. Mechanotropism. 



Under this head we may include all orienting movements produced 

 in response to mechanical agencies. Thigmotropism, or haptotropism, has 

 already been fully discussed when dealing with the irritability to contact 

 of tendrils and certain other organs. The seismonic irritability shown in 

 response to mechanical disturbances is not made use of for the attainment 

 of any pronounced tropic curvatures. Nevertheless, certain curvatures due 

 to rubbing or striking on one side may be seismonic reactions. 



RHEOTROPISM. This 'special form of irritability by means of which 

 plants are able to perform curvatures in response to the movement of the 

 water in which they are growing was discovered by Jonsson 3 , and is 

 possessed by a variety of roots. The radicle of Vicia sativa responds 

 especially well, according to Juel, in water moving with a rapidity of 

 0-3 mm. per second. For the radicle of Zea Mays> however, a rapidity 

 of about 3 mm. per second is required. The extent and rapidity of the 

 curvature is increased by a further moderate rise in the rate of flow, but is 

 retarded when it becomes too rapid. When the current is as rapid as 

 500 mm. per second, a portion of the roots of Vicia sativa curve in the 

 direction of the stream, probably owing to the mechanical action of the 

 latter. According to Berg, however, at low temperatures so pronounced 

 a change of tone takes place that the roots no longer respond positively 

 but give a negatively rheotropic reaction. Juel found that decapitated 

 roots also showed positive rheotropism, so that the stimulus cannot be 

 perceived solely by the root-tip. Newcombe 4 indeed finds that the whole 



1 Elfving, Ueber physiologische Fernwirkung einiger Korper, Helsingfors, 1890 ; Zur Kenntniss 

 d. pflanzlichen Irritabilitat, 1893 (reprint from Ofversigt af Finska Vet.-Soc. Forhandlingar, xxxvi). 



2 Errera, Annals of Botany, 1892, Vol. vi, p. 373 ; Steyer, I.e., pp. 16, 21. 



3 Jonsson, Ber. d. hot. Ges., 1883, p. 518; Berg, Studien liber Rheotropismus bei den Keim- 

 wurzeln, 1889 (repr. from Lunds Universitets Arsskrift, Bd. xxxv) ; Juel, Jahrb. f. wiss. Bot., 1900, 

 Bd. xxxiv, p. 507. 



* Newcombe, Botanical Gazette, 1902, Vol. xxxm, p. 177; Annals of Botany, 1902, Vol. xvi, 

 p. 429. 



