278 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



brought about by either a bending or a twisting of the petiole, frequently by 

 both of these processes together. If the plant is inverted and lighted only from 

 below, then the leaves react so as to maintain their normally upper surfaces 

 directed downward, toward the source of illumination. 



What has been stated above con- 

 cerning the phototropism of leaves 

 holds for most plants, but there are 

 a few exceptions. The leaves of 

 some plants growing in hot regions 

 do not find their position of pho- 

 totropic equilibrium when the leaf- 

 blade is perpendicular to the direc- 

 tion of the impinging light, but they 

 bend so as to make the blade assum e 

 an acute angle to the line of the light 

 rays. Finally, there are so-called 

 compass-plants, 1 which more or less 

 regularly bring their leaves into a 

 position so that the two faces of the 

 blade face east and west, the leaf- 

 tips pointing obliquely upward and 

 alternately north and south (Fig. 

 129). This arrangement results in 

 the so-called profile position of the 

 leaves at midday, at which time the 

 leaf surfaces are parallel to the direc- 

 tion of the direct rays of sunlight, 

 an orientation that tends to render 

 them less liable to excessive heat- 

 ing. Such reactions to light are 

 more or less perfectly exhibited 

 in Sylphium lacineatum, Lactuca 

 scar tola (wild lettuce), and others. 

 Many flowers also exhibit the 

 phototropic response. Several 

 species of Tragopogon furnish ex- 

 amples of flower-heads that bend 

 toward the sun. Before sunrise 

 the flower-heads all bend toward 

 the east, though they are still closed. They open as soon as the sun rises. 

 In the morning a meadow of blossoming Tragopogon appears all bright with 

 flowers when viewed from the east, but looks uniformly green when seen from 

 the west; in the latter case only the green involucres of the flower-heads are 

 seen. During the day the flowers change their position as the sun advances 



1 Stahl, E., Ueber sogenannte Compasspflanzen. Jenaische Zeitsch. Naturwiss. 15: 381-389. 1881. 



Fig. 129. — Compass-plant, Sylphium laciniatum 

 as seen from the east or west (left), and as seen 

 from the north or south (right). (After Stahl.) 



