Never paint leaves transparent to the sun, because they are
confused; and this is because on the transparency of one leaf will
be seen the shadow of another leaf which is above it. This shadow
has a distinct outline and a certain depth of shade and sometimes is
[as much as] half or a third of the leaf which is shaded; and
consequently such an arrangement is very confused and the imitation
of it should be avoided.

The light shines least through a leaf when it falls upon it at an
acute angle.

The gradations of shade and colour in leaves (430-434).