no RESEARCHES ON FUNGI 



for, in general principle, it may be similar to that of the leaves of 

 many flowering plants. 



As is well known, there are many shade-plants of which the 

 laminae are in a condition of heliotropic equiUbrium only when 

 they are placed at right angles to the direction of the most intense 

 diffuse illumination. These diaheliotropic leaves assume their 

 heliotropically fixed positions by means of appropriate curvatures 

 or torsions of the whole petiole or of a pulvinoid portion of the 

 petiole or of a typically developed pulvinus. It was suggested by 

 Dutrochet 1 in 1837 that the leaf -blade of the leaves in question 

 perceives the direction of the hght and exerts a directive influence 

 upon the petiole ; and Vochting,^ in 1888, by means of carefully 

 thought-out experiments, proved that this is so for Malva verticillata. 

 Haberlandt,3 in 1905, succeeded in demonstrating a similar directive 

 influence of the lamina in several other plants. Thus he found 

 that : in Begonia discolor the petiole, even when completely darkened 

 by a sheath of tin-foil, turns the lamina into its heliotropically 

 fixed position ; and that in Monstera deliciosa the upper region of 

 the petiole, which is developed as a pulvinus, even when completely 

 darkened by a sheath of tin-foil, executes the curvature or the 

 torsion necessary to restore the lamina to the position of heliotropic 

 equilibrium with the greatest possible precision. 



To explain the diaheliotropism of the leaves of shade-plants, 

 etc., Haberlandt^ has put forward his ocellar theory, according to 

 which : (1) the power of perceiving photic stimuli is vested in the 

 upper epidermis ; (2) the epidermis sends a stimulus to the petiole ; 

 (3) the stimulus causes the petiole or its pulvinus to curve or tmst 

 until the lamina comes to be at right angles to the direction of the 

 strongest incident rays of light ; and (4) each cell of the epidermis 

 acts as a light-perceiving organ or ocellus and is only in a position 



1 H. J. Dutrochet, Memoires pour servir a Vhistoire anatomique et physiologique 

 des Vegekmx et des Animaux, Paris, 1837, T. II, p. 107. 



2 H. Vochting, " Ueber die Lichtstellung der Laubblatter," Botanische Zeitung, 

 Jahrg. XLVI, 1888, pp. 519-523. Vochting showed that the lamina, when obliquely 

 illuminated, forces the petiole to execute movements, when the petiole is kept in 

 the dark or, if it is lighted, in opposition to its own heliotropic tendencies. 



3 G. Haberlandt, Die Lichtsinnesorgane der Laubblatter, Leipzig, 1905, pp. 17-19. 

 * G. Haberlandt, ibid., pp. 120-122 ; also Physiological Plant Anatomy, trans- 

 lated by M. Drummond, London, 1914, pp. 613-630. 



