HYDROTROPISM 183 



lateral roots 1 are positively hydrotropic, and hence curve towards moister 

 soil or moister regions of the surrounding air. In this way the roots of 

 plants growing on the sides of cliffs keep themselves buried in the soil 

 or curve back towards it. The positive hydrotropism of the rhizoids of 

 Marchantia 2 is of equal importance when the plant is growing on the sides 

 of rocks, and the possession by the pollen-tube of this form of irritability 

 aids it in applying itself closely to the stigma 3 . 



On the other hand, the sporangiophores of Phycomyces and of other 

 Mucoriniae 4 , as well as the stipe of Coprinus velaris, according to Molisch, 

 are negatively hydrotropic. According to Steyer, however, the sporangio- 

 phore of Phycomyces assumes a diatropic direction of growth at a certain 

 distance from a wet surface, whereas when further away it performs a slight 

 positive curvature towards the region where the percentage of moisture is 

 most to its liking. When the young sporangiophore first rises above the 

 medium, it is strongly negatively hydrotropic (hydrophobic), and hence 

 grows at right angles to the surface of the substance, since the moistness 

 of the subjacent air decreases regularly in successive upward layers. 



The aerial organs appear to be devoid of any hydrotropic irritability, 

 for it is only in the case of the hypocotyl of Linum usitatissimum that 

 feeble negative hydrotropism is shown 5 . 



For demonstration purposes seeds may be germinated in sawdust on an 

 obliquely inclined sieve, or on the porous clay niters recommended by Molisch. 

 Since the roots do not curve to the moist surface when the air is saturated with 

 moisture, it is evident that differences in the percentage of moisture form the external 

 causes inducing curvature. In the case of Phycomyces the culture medium, such as 

 a slice of bread, may be covered with a sheet of mica having small holes bored 

 through it. The sporangiophores which grow through these holes may be used for 

 experimentation. 



It is owing to their negative hydrotropism coupled with their transpiration that 



in Trans. Liverpool Biol. Soc., 1896, Vol. x, p. 191, on a klinostat, I was unable to obtain any 

 constant and definite curvatures of the radicles away from the deoxygenated portion of the medium. 

 The whole subject, however, well merits further investigation.] 



* Knight (Phil. Trans., 1811, p. 212) first made it certain that the curvature of the roots to 

 moister substrata was due to their hydrotropic irritability, which at a later date was studied in detail 

 by Sachs, Arb. d. bot. Inst. in Wiirzburg, 1872, Bd. I, p. 209; and Molisch, Sitzungsb. d. Wien. 

 Akad., 1883, Bd. LXXXVIII, Abth. I, p. 897. Further research is required on the influence of the 

 irregular distribution of moisture upon the development of roots in soil. 



2 Molisch, 1. c., p. 932. 



3 Miyoshi, Flora, 1894, p. 84. 



4 Wortmann, Bot. Ztg., 1881, p. 368; Molisch, I.e., p. 935 ; Dietz, Unters. a. d. bot. Inst. zu 

 Tubingen, 1888, Bd. n, p. 478 ; Steyer, Reizkriimmungen bei Phycomyces^ 1901, p. 14. The negative 

 hydrotropism observed by Klebs (Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 55) in the sporangiophore 

 of Sporodinia is disputed by von Falck (Cohn's Beitrage z. Biologic, 1901, Bd. vm, p. 237). On 

 the fruit-stalk of Dictyostdium cf. Potts, Flora, 1902, Ergzbd., p. 319. 



5 Molisch, I.e., p. 937; Dietz, I.e., p. 480. According to Vochting, Bot. Ztg., 1902, p. 98, 

 the shoots of potatoes are hydrotropic. Cf. Singer, Ber. d. bot. Ges., 1903, p. 175. 



