  Trees in works of art have stirred emotions in viewers for millennia.
Leonardo da Vinci described geometric proportions in trees to provide both
guidelines for painting and insights into tree form and function. Da Vinci's
Rule of trees further implies fractal branching with a particular scaling
exponent $\alpha = 2$ governing both proportions between the diameters of
adjoining boughs and the number of boughs of a given diameter. Contemporary
biology increasingly supports an analogous rule with $\alpha = 3$ known as
Murray's Law. Here we relate trees in art to a theory of proportion inspired by
both da Vinci and modern tree physiology. We measure $\alpha$ in 16th century
Islamic architecture, Edo period Japanese painting and 20th century European
art, finding $\alpha$ in the range 1.5 to 2.5. We find that both conformity and
deviations from ideal branching create stylistic effect and accommodate
constraints on design and implementation. Finally, we analyze an abstract tree
by Piet Mondrian which forgoes explicit branching but accurately captures the
modern scaling exponent $\alpha = 3$, anticipating Murray's Law by 15 years.
This perspective extends classical mathematical, biological and artistic ways
to understand, recreate and appreciate the beauty of trees.
