The lowest branches, after they have formed the angle of their
separation from the parent stem, always bend downwards so as not to
crowd against the other branches which follow them on the same stem
and to be better able to take the air which nourishes them. As is
shown by the angle _b a c_; the branch _a c_ after it has made the
corner of the angle _a c_ bends downwards to _c d_ and the lesser
shoot _c_ dries up, being too thin.

The main branch always goes below, as is shown by the branch _f n
m_, which does not go to _f n o_.

The forms of trees (408--411).