Physical constraints on growth produce continuous variations in the shape of biological objects that correspond to their sizes. The author investigated whether 2 such properties of tree form could be visually discriminated and used to evaluate the height of trees. Observers judged simulated tree silhouettes of constant image size. Trees were placed appropriately within a ground texture gradient, as were 6 cylinders. Observers judged trees, then cylinders. Tree form was shown to confer a metric on ground texture gradients. Different observers judged cylinders without seeing trees. The horizon ratio was shown to be ineffective as an alternative source of scale. The largest trees were systematically underestimated. Comparison was made to judgments of real trees viewed binocularly, monocularly through a tube, or in pictures. Underestimation of larger trees with restricted viewing was comparable to that obtained using simulated trees.