i82 TROPIC MOVEMENTS 



is brought about in the following way. The first penetration of the stigma by the 

 pollen-tube is induced by chemotropic stimulation aided by the hydrotropism of 

 the pollen-tube, and possibly also by aerotropic and other stimuli. The growth of the 

 tube down the conducting tissue appears to take place independently of any chemo- 

 tropic action. The actual entry at the micropyle appears to be brought about by the 

 exudation of a stimulating material from the ovule, for the pollen-tubes penetrate 

 the micropyles of isolated ovules injected with sugar, but not when injected with 

 non-chemotropic solutions, or when the ovules and pollen-tubes are placed in a solu- 

 tion of sugar so that the action of the sugar exuding from the micropyle is masked. 



Aerotropism. According to Celakovsky l , the hyphae of Dictyuchus monosporus 

 curve towards water richer in oxygen, but pollen-tubes towards water poorer in 

 oxygen, according to Molisch 2 . Roots, on the other hand, were found by Molisch 3 

 to be positively oxytropic and to curve from air deficient in oxygen to air where it 

 was more abundant. According to the same author, the one-sided accumulation of 

 carbon dioxide, as well as the unilateral action of ether and camphor vapours, produces 

 a negatively tropic curvature both in normal and in decapitated roots 4 . The reactions 

 are, however, feeble, and it has yet to be shown that they take a prominent part in 

 the orientation of roots in water and soil. It is also uncertain whether the upward 

 growth of roots in mud or in soil whose pores are clogged with water 5 is due to 

 oxytropism or to an alteration of the geotropic irritability produced by the deficiency 

 of oxygen 6 . 



SECTION 40. Hydrotropism. 



Many plants show tropic curvatures either towards moisture (positive 

 hydrotropism), or away from it (negative hydrotropism). Both the main and 



cf. Dalmer, Jenaische Zeitschr. f. Naturw., 1880, Bd. xiv, p. 39; Strasburger, Jahrb. f. wiss. Bot., 

 1886, Bd. xvn, p. 50; Busse, Bot. Centralbl., 1900, Bd. LXXXIV, p. 209; Murbeck, Verhalten des 

 Pollenschlauchs bei Alchemilla u. d. Chalazogamie, 1901, p. 7 (reprint from Lunds Universitets 

 Arsskrift, Bd. xxxvin). 



1 Celakovsky, Ueber d. Aerotropismus von Dictyuchus monosporus. Reprint, 1897, p. 8. 



2 Molisch, Sitzungsb. d. Wien. Akad., 1893, Bd. en, Abth. I, p. 432 ; Miyoshi, Flora, 1894, p. 87. 



3 Molisch, I.e., 1884, Bd. xc, I, p. 194. According to Steyer (Reizkriimmungen bei 

 Phy corny ces nitens, 1901) the unilateral accumulation of carbon dioxide induces no tropic curvature 

 in the sporangiophore of Phycomyces nitens. [Bennett (Botanical Gazette, 1904, Vol. xxxvn, 

 p. 241) has conclusively shown that the roots of Zea, Cucurbita, Rafhanus, Vicia, Pisum, and 

 Lupinus have no aerotropic irritability, and that the curvatures observed by Molisch were hydro- 

 tropic in character.] 



* Molisch, 1. c., Vol. xc, pp. 172, 194. Cf. also Rothert, Flora, 1894, Ergzbd., p. 216. 



5 Cf. Jost, Bot. Ztg., 1887, p. 169; Goebel, ibid., p. 717; Schenck, Jahrb. f. wiss. Bot., 1889, 

 Bd. XX, pp. 534, 564, 569; Wieler, ibid., 1898, Bd. xxxil, p. 503. On the curvatures of roots 

 produced by deoxygenated water cf. Ewart, Trans. Liverpool Biol. Soc., 1894, v l' VIII P- 2 4- 



[ 6 The absence of oxygen, or the presence of poisonous gases, produces disturbances of growth 

 often resulting in irregular curvatures, which are not always traumatropic in character. When the 

 curvature is towards the region less deficient in oxygen, growth will be more rapid, and in this way 

 a certain biological advantage may be gained by parts of the root system, or by some of the seedlings. 

 It appears, however, as though the avoidance by the roots of regions poor in oxygen is in part aided 

 by the suppression or reversal of the geotropic irritability, for on repeating the experiments described 



