WHY, VERY FREQUENTLY, TIMBER HAS VEINS THAT ARE NOT STRAIGHT.

When the branches which grow the second year above the branch of the
preceding year, are not of equal thickness above the antecedent
branches, but are on one side, then the vigour of the lower branch
is diverted to nourish the one above it, although it may be somewhat
on one side.

But if the ramifications are equal in their growth, the veins of the
main stem will be straight [parallel] and equidistant at every
degree of the height of the plant.

Wherefore, O Painter! you, who do not know these laws! in order to
escape the blame of those who understand them, it will be well that
you should represent every thing from nature, and not despise such
study as those do who work [only] for money.

The direction of growth (403-407).