The Natural Philosophy of Plant Form, published in 1950 has been considered the most important of Arber's books. Arber discusses the processes behind forming a concept from research and examines the philosophy of plant morphology. Arber uses this to examine the structure of flowering plants, and proposes the partial-shoot theory of the leaf. According to this theory, each element of the plant is a shoot or a partial shoot. Leaves are partial shoots that show reduced growth capacity. She mentions: “the leaf is a partial-shoot, revealing an inherent urge towards becoming a whole shoot, but never actually attaining this goal, since radial symmetry and the capacity for apical growth suffer inhibition”. The parallelism of leaf and shoot dates back to Goethe, who first described compound leaves as in "reality branches, the buds of which cannot develop, since the Common stalk is too frail". For Arber, compound leaves are clusters of united partial-shoots. Recent developmental genetic evidence has supported aspects of the partial shoot-theory of the leaf, especially in the case of compound leaves.