10 A TEXT-BOOK OF BOTANY 
light; but if it be turned around so as to bring the other 
side of the plant to the light, the leaves will become adjusted 
gradually to the new direction. Many plants have more 
or less power to direct their leaves, and it would be in- 
teresting to observe what common plants of any region 
possess it. 
(2) Problem of shading.—It is evident that leaves of the 
same plant are in danger of shading one another; and while 
it cannot always be pre- 
vented, there are ways by 
which the danger is dimin- 
ished. The problem of the 
plant is to develop as much 
leaf surface as possible and 
to place it in the most fa- 
vorable position for work. 
The spiral arrangement of 
leaves prevents two suc- 
cessive leaves standing in 
the same plane, and results 
in vertical rows of leaves 
distributed about the stem. 
The narrower the leaves, 
the more numerous may 
be the vertical rows; and 
the broader the leaves, the 
fewer the vertical rows 
(Tig. 5). In many herbs 
whose leaves are rather 
large and close together, 
Fic. 5.—A_ broad-leaved plant, showing the petioles of the lower 
[Cee eE BDL ae EER iebled leaves are usually longer 
than those above, and 
thus their blades are thrust beyond the shadow. The 
same result is obtained when the lowest leaves of a plant 
