Why plants feel no pain when they are broken
A prose note arguing that pain exists to protect moving creatures, so immobile plants need none
The leaf (inverted, in red chalk) carries a single block of reflective prose with no diagram. Leonardo reasons that nature gave pain to sentient creatures endowed with motion in order to preserve the organs that motion could damage; since plants have no motion and cannot strike against opposing objects, pain would be useless to them, and so breaking a plant causes it no suffering as it would an animal.
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Pain guards moving bodies, so plants have none
If nature ordained pain in sentient souls endowed with motion, to preserve the organs that motion might diminish or spoil, then souls without motion have nothing to strike against opposing objects; therefore pain is not necessary in plants, and breaking them they feel no pain as animals do.