There is no boss on branches which has not been produced by some
branch which has failed.

The lower shoots on the branches of trees grow more than the upper
ones and this occurs only because the sap that nourishes them, being
heavy, tends downwards more than upwards; and again, because those
[branches] which grow downwards turn away from the shade which
exists towards the centre of the plant. The older the branches are,
the greater is the difference between their upper and their lower
shoots and in those dating from the same year or epoch.

[Footnote: The sketch accompanying this in the MS. is so effaced
that an exact reproduction was impossible.]