164 TROPIC MOVEMENTS 



In accordance with their special function, the attaching roots of the Ivy, 

 of Aroids, and of Orchids show usually little or no geotropism, but are com- 

 monly provided with a distinct heliotropic irritability. The erect growth of 

 the breathing- roots (pneumatophores) of certain Mangrove -trees, on the other 

 hand, appears to be due to their negative geotropism 1 . The roots of many 

 plants such as Palms, Sugar-canes and others appear, however, above the 

 soil when the latter is kept wet 2 , and it requires to be determined whether 

 this is due to aerotropism, to negative geotropism induced by the peculiar 

 conditions, or to other causes. 



Most horizontally-growing rhizomes maintain their position by the 

 aid of their strong diageotropism, and the growing zones curve back to 

 the normal position when the rhizome is disturbed. This applies not only 

 to dorsiventral rhizomes but also to physiologically radial and more erect 

 ones, including the root-stocks of Heliocharis palustris> Sparganium 

 ramosum^ and Scirpus maritimus*. The subterranean runners of Adoxa 

 moschatellina, Trientalis europaea^ and Circaea lutetiana are physiologically 

 radial, but nevertheless assume a more or less horizontal position in darkness 

 or in the soil. Exposure to diffuse light, however, induces such an altera- 

 tion in their geotropic irritability as to cause them to assume a positively 

 klinotropic, or even under special circumstances a positively parallelotropic 

 direction of growth*. 



The downwardly-growing rhizomes of Yucca and Cordyline seem to 

 possess positive geotropism 5 , which appears also to be responsible for 

 the downward curvature of the peduncle of Papaver, which later becomes 

 negatively geotropic and straightens as the flower expands 6 . An alteration 

 of irritability is sometimes, but not always, employed to produce the upward 

 growth of the foliage-bearing portion of a sympodial rhizome, and to 

 induce changes in the position of flower-buds, flowers, fruits, and even of 



1 Karsten, Bibl. hot., 1891, Heft 22, pp. 49, 55; Schimper, Bot. Mitth. a. d. Tropen, 1891, 

 Heft 3, p. 37; Went, Ann. d. Jard. bot. de Buitenzorg, 1894, Vol. XII, p. 26; Goebel, Organo- 

 graphy, Part II. On the radicle of Trapa cf. Kerner, Pflanzenleben, 1887, Bd. i, p. 83. On 

 negatively geotropic aerial roots cf. Wiesner, Die heliotropischen Erscheinungen, 1880, II, p. 77. 



2 Kerner, Natural History of Plants, 1895, Vol. I, p. 88. See also Sachs, Flora, 1893, p. 4. 

 According to Eriksson, Bot. Centralbl., 1895, Bd. LXI, p. 273, Carex arenaria and other sand-plants 

 possess upwardly-growing roots. 



3 Elfving, Arb. d. bot. Inst. in Wiirzburg, 1880, Bd. II, p. 489; Czapek, Sitzungsb. d. Wien. 

 Akad., 1895, Bd. Civ, Abth. i, p. 1218. According to Barth (Die geotrop. Wachsthumskrummung 

 d. Knoten, 1894, p. 35), the subterranean runners of Triticum refiens show no perceptible geotropic 

 irritability. 



* Stahl, Ber. d. bot. Ges., 1884, p. 385 ; Goebel, Bot. Ztg., 1880, p. 790; Czapek, 1. c., p. 1230 ; 

 Rimbach, Fiinfstiick's Beitr. z. wiss. Bot., 1899, Bd. in, p. 201. 



5 See the literature given in Vol. II, p. 194. 



6 The literature will be given later, and it will be shown that we are dealing with a true 

 geotropic curvature, and not with a mere mechanical drooping produced by the weight of the flower- 

 bud. Wiesner (Sitzungsb. d. Wien. Akad., 1902, Bd. cxi, Abth. i, p. 747) does not, however, now 

 consider the downward curvature of the peduncle of a Poppy to be geotropic in character. 



