114 BOTANY 
right angles to that immediately above and below it. 
This arrangement is called decussate. Where leaves 
are placed in a ring round the stem, as in cleavers, 
the arrangement is said to be whorled, and where 
neither whorled nor opposite it is alternate. 
Compare the number of rows of leaves on a willow 
branch with that on the stem of a tobacco plant. These 
two plants will illustrate another principle coneerned 
with shade prevention. Where the leaves are long, as 
in the tobacco, the internodes of the stem are also long 
so that there shall be no overlapping, whereas short 
leaves, as in the willow, are set at short intervals. 
By an adjustment in the length of the leaf petioles, 
shading may be avoided. The petioles of the lowest 
leaves are often long, carrying them outside the shade 
of the foliage on the upper part of the stem. A similar 
arrangement is seen in many rosette plants such as 
the dandelion and ecat’sear (Fig. 78). In other cases 
the lowest leaves grow outwards horizontally from the 
stem while those above grow more vertically. This 
is the case with the buttercup. The leaves springing 
from the base (the radical leaves) have long petioles 
and spread horizontally. Shading of these is prevented 
in two ways: by the reduction in size of the upper 
leaves, and by the fact that these latter spring more ver- 
tically from the stem. The woolly mullein furnishes ar 
even better example. 
In the case of compound leaves (7.e., leaves, in which, 
like the bean, the blade is divided into several parts) 
there is not generally reduction of the upper leaves 
or lengthening of the lower petioles, since, at some 
time or other during the day, the sunlight is able to 
make its way through the broken blades as they move 
to and fro in the breeze. 
In creeping stems, as a rule, every leaf assumes 
the horizontal position. In the periwinkle (Figs. 76- 
(7) for instance, the arrangement of the leaves is, 
