7 
110 <A TEXTBOOK OF GENERAL BOTANY 4 

of phototropism are fre- 
quently pronounced in 
plants growing in windows 
or on covered porches. In 
such cases the plants usu- 
ally lean toward the source 
of light (Fig. 108). 
The tips of herbaceous 
stems frequently follow 
the course of the sun dur- 
ing the day, pointing to 
the east in the morning 
and to the west in the 
afternoon. This is true of 
the sunflower. 
Diaphototropism. Leaves 
Fig. 108. A mungo bean seedling which usually turn to face the 
grew upright when illuminated from al] SOUTFCE of greatest illumi- 
sides and then bent toward the light when nation, and also bend in 
illuminated only from the left. (x 4) such a way that they are 
not greatly shaded by the 
leaves above them. The turning and bending of leaves to face 
the light is called diapho- 
totropism. 
When leaves have pet- 
loles, the bending takes 
place in the petioles. If 
the petioles on pendent 
branches were straight, 
the morphologically upper 
side of the leaves would 
face downward, but in 
such cases the petioles 
bend and twist so as to 
expose AREER. Haug s The petioles have twisted so that the mor- 
; phologically upper sides of the leaves are 
strongest light (Fig. 109). uppermost. (x 1) 


Fic. 109. Leaves of Thunbergia grandiflora 
on pendent branch 
