The Transparency of Leaves and the Off-Centre Pith of a Bough
Grades of light and shade; leaves translucent against eastern light; a forked branch shown in section
Continuing the scheme of light on foliage, this leaf places the middle-illuminated and middle-shadowed grades between the luster and the principal shadows, and notes that transparency belongs only to transparent things such as leaves — a treatment the author claims none before him had given. A chapter on the transparency of leaves describes how, with the light in the east and the eye looking up from the north, the tree's eastern side appears largely translucent while its western side falls dark. A chapter then argues that the pith-centre of a bough never lies at the middle of its thickness, because branches are not round and hold more moisture on the inner side; a small drawing of a forked branch shown in section carries the letters a, b, c, d, e used in the text.
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Leaves seen translucent against eastern light
When the light is in the east and the eye looks up at the plant from the north, the tree's eastern side appears largely transparent, except for leaves shadowed by other leaves. The western side of the tree stays dark, because it receives the shadow of half the branching — the half turned toward the east.
The pith-centre lies off the middle of a bough (a, b, c, d, e)
The centre of a plant, at the division of its branchings, never sits at the middle of the branch's thickness, because branches are never round and hold more moisture on their inner side. At c, the junction of branches a-e and c-e, the wood grows more toward the centres b and d than from those centres out to the extremities a and e.
Forked branch drawn in cross-section
A small pen drawing set into the lower text column shows a forked bough cut across, its two severed limbs presented as open round sections above a shared stem. Its letter labels correspond to the a, b, c, d, e cited in the passage on the off-centre pith.