THE ORIENTATION OF FOLIAGE-LEAVES 261 



solely upon the intensity of the light rays independently of their direction 

 or heating effect, and hence the leaflets fold together when the sunlight is 

 reflected upon the pulvini from beneath, but expand when the pulvini are 

 shaded and the laminas fully exposed l . When the leaf is strongly illu- 

 minated from the side the main pulvinus twists into a more or less diahelio- 

 tropic position and the leaflets perform the same closure as before in 

 response to the intense light. We have, therefore, here an instance in which 

 the irritability in the pulvini of the same leaf varies according to their position 

 and the task they have to perform. All leaves provided with pulvini seem 

 able to respond to intense illumination, although it is not in all cases 

 certain whether the response is photonastic or heliotropic in character. 



Photometric leaves which respond by growth-curvatures may, however, 

 also place themselves at varying angles with the direction of intense 

 illumination. It is, however, only rarely that they attain a profile position 

 as in Lactuca virosa, Silphium laciniatum^ and a few other plants, in which 

 the position is assumed by a torsion at the base of the leaf. Since this 

 orientation is mainly due to the intense midday sun, the leaves of these 

 so-called compass-plants set their laminas in exposed localities, mainly in 

 a perpendicular plane running north and south, whereas in shady situations 

 the leaves show neither this orientation nor do they assume the profile 

 position 2 . 



1 Cf. Ewart, Annals of Botany, 1897, Vol. XI, p. 448. 



3 Stahl, Ueber sogenannte Compasspflanzen, 1881 (reprint from the Zeitschr. f. Naturwiss., 

 Bd. XV); Oltmanns, Flora, 1892, p. 248; Bay, Botanical Gazette, 1894, Vol. XIX, p. 251. On the 

 branching system of Biota see Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 268. 



