Vol. I: VIII. Botany for Painters and Elements of Landscape Painting: 456.
The Da Vinci Notebooks
That part of a tree which has shadow for background, is all of one tone, and wherever the trees or branches are thickest they will be darkest, because there are no little intervals of air. But where the boughs lie against a background of other boughs, the brighter parts are seen lightest and the leaves lustrous from the sunlight falling on them.