IRRITABILITY. 45 1 



Ranunculus Ficaria, when in the zenith position, curved so as 

 to place the upper surface of the blade at right angles to the 

 direction of the incident rays : in the Cherry and in the Bean, 

 under similar circumstances, the leaves curved epinastically 

 backwards towards the stem, so that the angle which their 

 upper surfaces made with the direction of the incident rays 

 was much greater than a right angle. It seems probable that 

 strongly marked photo-epinasty is peculiar to a certain period 

 in the development of the leaf, the diaheliotropism asserting 

 itself later, so that ultimately the fixed light-position, in which 

 the dorsal surface is perpendicular to the direction of the 

 incident rays, is assumed. This may also apply to dorsi- 

 ventral shoots, but a fuller investigation of the whole subject 

 is necessary before we can arrive at such definite conclusions 

 as will enable us to give a completely satisfactory explanation 

 of these complicated phenomena. 



With regard to the mechanism by which the heliotropic 

 curvature of dorsiventral organs is effected, it will suffice for 

 the present to say that it is essentially the same as in the case 

 of orthotropic organs. 



BIBLIOGRAPHY, 



Sachs ; Arb. d. hot. Inst. in Wurzburg, u, 1879. 



F. Darwin ; Journ. Linn. Soc., XVin, 1881. 



Vochting; Die Bewegungen der Bluthen und Friichte, Bonn, 1882. 



Dutrochet ; Recherches sur la structure intime des Animaux et des 



Ve'ge'taux, 1824. 



Sachs ; Arb. d. bot. Inst. in Wurzburg, I, 1874. 

 Sachs ; Arb. d. bot. Inst. in Wurzburg, II, 1879. 

 Pfeffer; Arb. d. bot. Inst. in Wurzburg, i, 1871. 

 Zimmermann ; Arb. d. bot. Inst. in Wiirzburg, u, 1882. 

 Frank ; Die natiirliche wagerechte Richtung von Pflanzentheilen, 



Leipzig, 1870. 

 de Vries ; Arb. d. bot. Inst. in Wurzburg, I, 1872. 



292 



