IV ON PLANT LIFE 144 
which the twig can safely support, the length 
also is determined. This argument is well 
illustrated by comparing the leaves of the 
Beech with those of the Spanish Chestnut. 
The arrangement is similar, and the distance 
between the buds being about the same, so is 
the width of the leaves. But the terminal 
branches of the Spanish Chestnut being much 
stronger, the leaves can safely be heavier ; 
hence the width being fixed, they grow in 
length and assume the well-known and 
peculiar sword-blade shape. 
In the Sycamores, Maples (Fig. 16), and 
Horse-Chestnuts the arrangement is altogether 
different. The shoots are stiff and upright 
with leaves placed at right angles to the 
branches instead of beimg parallel to them. 
The leaves are in pairs and decussate with 
one another ; while the lower ones have long 
petioles which bring them almost to the level 
of the upper pairs, the whole thus forming a 
beautiful dome. 
For leaves arranged as in the Beech the 
gentle swell at the base is admirably suited ; 
but in a crown of leaves such as those of the 
