IRRITABILITY. 481 



the root, being surrounded by the wet paper, cannot be sup- 

 posed to be at all affected by a moist surface at some little 

 distance from it. Molisch found, as a matter of fact, that 

 roots thus arranged, with tips projecting to the extent of 

 I 1*5 mm., curved towards the moist surface in six hours. 

 These facts, it must be admitted, go far to prove that the 

 hydrotropic irritability of roots is localised in their tips. 



With regard to hydrotropism also, we shall defer a full 

 discussion of the mechanism of the curvatures to a subsequent 

 lecture. But it must be urged in this case also that the 

 curvature is not due to the stimulation of one side or other of 

 the organ, to the stimulation, for instance, of the concave side 

 by the moist surface, or, as Molisch insists, to the stimulation 

 by the relative dryness of the air, of the side which becomes 

 convex. The whole organ is stimulated, and the curvature is 

 merely a mechanical necessity for the expression of the 

 response of the whole organ to the stimulus. 



BIBLIOGRAPHY. 



Gravity (Geotropism). 



Frank; Beitr. zur Pflanzenphysiologie, Leipzig, 1868. 



Hofmeister ; Pringsheim's Jahrb. f. wiss. Bot., ill, 1863. 



Pinot; Ann. d. sci. nat, s6r. I, t. xvm, 1829, (in the revue biblio- 



graphique, p. 94). 

 Mulder; Ann. d. sci. nat., sdr. i, t. xxn, 1831, (in the revue biblio- 



graphique, p. 129). 

 Payer; Comptes rendus, xvm, 1844. 

 Johnson ; Edin. new philos. Journal, 1828. 

 Vochting; Bewegungen der Bliithen und Friichte, Bonn, 1882. 

 Fiinfstiick; Ber. d. deutsch. bot. Gesellschaft, I, 1883. 

 Dodart ; Hist, de PAcad. roy. des Sciences, 1700. 

 Bonnet ; Recherches sur 1'Usage des Feuilles, 1754. 

 Duhamel; Physique des Arbres, 1758, II, p. 137. 

 Knight ; Phil. Trans., 1806. 



Darwin ; The Power of Movement in Plants, 1880. 

 Sachs ; Arb. d. bot. Inst. in Wiirzburg, II, 1880. 

 Duhamel ; (quoted by Bonnet, Usage des Feuilles, p. 91). 

 Schimper ; Die Epiphyten Westindiens, Bot. Centralblatt, xvn, 1884. 



v. 31 



