114 Botany




Status: Complete

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 concerned
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, wheras 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 cat'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 an
even better example.
In the case of compound leaves (i.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-
77) for instance, the arrangement of the leaves is,

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