VIII. Botany for Painters and Elements of Landscape Painting: 407.
407.
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.