Light and Shadow on Foliage; Proportion of Yearly Growth
How massed leaves make a luminous sheet at distance yet show separate highlights up close
Continuing the Treatise on Painting's account of trees, Leonardo describes how leaves, pulled down by their weight, overlap into a luminous sheet visible at a distance, while from close by the eye catches a separate highlight (lustro) on each leaf. Upper leaves stay near their origin and lie exposed to light; lower leaves hang obliquely and shadow one another, so they read less clearly to the eye, and this play of lustre belongs to particular light, not to universal light. He names the plane, linden and fig among broad-leaved trees that behave this way, then opens a new section (827) on the proportion of same-year branch thicknesses. A leafy sprig and a cut, ring-marked trunk labelled "old plant" are drawn in the text.
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A luminous sheet at distance, separate highlights up close
Overlapping leaves compose a rather luminous plate that shows itself at long range, but from near one sees the highlights (lustri) on each leaf. This lustre belongs to particular light, for in universal light the leaves have light but no lustre.
Upper leaves exposed, lower leaves lost in shadow
The upper leaves are freer and less covered, so they lie nearer the mass of foliage; the lower ones, hanging obliquely and shadowed by those above, are not wholly grasped by the eye. Trees with compound leaves, and broad-leaved ones such as the plane, linden and fig, all behave this way.
Proportion of same-year branch thicknesses (§827)
A new heading on the proportion the ramifications of plants keep among themselves. The thicknesses of a plant's branching born in one year with its first stem hold the same proportion as those of the years before and after.