The Form of Trunks and the Light on Leaves
Why trunks swell at branch and root, how plants feed, and light on leaves from below or above
Finishing the discussion of distance, Leonardo notes that the shadowed gaps between leaves lose their true darkness far off, both because of the thickness of interposed air and because the tiny lights and darks blur together, leaving the lit part dominant. A botanical chapter then explains that a plant's trunk loses its roundness where branches and roots spring from it, since these upper and lower limbs are the members through which the plant feeds, above by dew and rain through the leaves in summer, below by the earth's contact with the roots in winter. Two further chapters treat the size of lights and shadows on foliage seen from below, above or level, and begin a chapter on the illumination of plants.
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Shadowed gaps between leaves lose their darkness at distance
The shadowed intervals between leaves do not show their due darkness at long range, for two reasons: the thickness of the interposed air, and the blurring of the minute forms at their edges, which confounds their recognition. Because the lit part stays more noticeable than the shadowed, those shadows read as only faintly dark.
Why trunks swell at branch and root, and how plants feed
A plant's trunk does not keep the roundness of its thickness where it meets the springing of branches or roots, because these upper and lower branchings are the members through which the plant is nourished. In summer it feeds above by dew and rain through the leaves; in winter below, through the earth's contact with its roots.
Light and shadow on leaves seen from below, above or level
Branchings of plants are seen from below, from above, or level. Under a universal light, foliage viewed from below shows more shadow than light; from above, more light than shadow; and viewed level, the lit and shadowed parts are equal.