248 



PHYSIOLOGY OF GROWTH AND CONFIGURATION 



brought about by either a bending or a t^vi&ting of the petiole, frequently by 

 both of these processes together. If the plant is inverted and Hghted 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 equihbrium when the leaf- 

 blade is perpendicular to the direc- 

 tion of the impinging hght, but they 

 bend so as to make the blade assume 

 an acute angle to the Hne of the light 

 rays. Finally, there are so-called 

 compass-plants,! 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 obHquely 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 developed 

 m Sylphium lacineaium, Lactuca 

 scariola (wild lettuce), and others. 

 Many flowers also exhibit the 

 phototropic response. Several 

 species of Tragopogon furnish ex- 

 amples of flower-heads that bend 

 «.^Z'r, 'f?;~rtT^T"^^''"*' f ^,'f ^fr '""•»«'«'». toward the sun. Before sunrise the 



as seen trom the east or west (left), and as seen „ 



from the north or south (right). (After stahl.) nower-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 



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



