
THE STEM 111 
In many cases the bending and twisting of the petioles or the 
twisting of the stem is much more important than the place 
of attachment of the leaves in determining the. position of the 
leaf blades. This is often very clearly seen in the different 
positions of leaves 
on vertical and hori- 
zontal stems of the 
same plant. On ver- 
tical - stems, which 
are not shaded on 
one side, the leaves 
extend straight out 
from the point of 
attachment, while if 
the illumination is 
one-sided, as upon 
unshaded horizontal 
branches, the stems 
or petioles frequently 
bend and twist so as 
to bring the blades 
into such a_ posi- 
tion that they face 
the strongest light. 
Compare Fig. 91 with 
Fig. 96, and Fig. 111 
with Fig. 110. On 
horizontal branches 
the leaves are often 
o 
3 NE 
I 
ia 

Fic. 110. Vertical branch of Cestrum nocturnum 
illuminated from all sides 
Compare with Fig. 111. (x 4) 
arranged in an approximately horizontal position. As an example 
take the case of a plant on which the leaves are arranged in five 
vertical rows. On a vertical stem the leaves will extend from the 
stem in five directions (Fig. 110). On a horizontal branch of 
the same plant the petioles will bend and twist so as to bring the 
blades into an approximately horizontal position, and the leaves 
will appear to grow from only two sides of the stem (Fig. 111). 
