287. The principal errors which had been committed by artists in drawing trees had arisen from their regarding the bough as ramifying irregularly, and somewhat losing in energy towards the extremity; whereas the real boughs threw their whole energy, and multiplied their substance, towards the extremities, ranking themselves in more or less cup-shaped tiers round the trunk, and forming a compact united surface at the exterior of the tree. 288. In the course of arrival at this form, the bough, throughout its whole length, showed itself to be influenced by a force like that of an animal's instinct. Its minor curves and angles were all subjected to one strong ruling tendency and law of advance, dependent partly on the aim of every shoot to raise itself upright, partly on the necessity which each was under to yield due place to the neighboring leaves, and obtain for itself as much light and air as possible. It had indeed been ascertained that vegetable tissue was liable to contractions and expansion (under fixed mechanical conditions) by light, heat, moisture, etc. But vegetable tissue in the living branch did not contract nor expand under external influence alone. The principle of life manifested itself either by contention with, or felicitous recognition of, external force. It accepted with a visible, active, and apparently joyful concurrence, the influences which led the bough towards its due place in the economy of the tree; and it obeyed reluctantly, partially, and with distorted curvatures, those which forced it to violate the typical organic form. The attention of painters of foliage had seldom been drawn with sufficient accuracy to the lines either of branch curvature, or leaf contour, as expressing these subtle laws of incipient volition; but the relative merit of the great schools of figure design might, in absence of all other evidence, be determined, almost without error, by observing the precision of their treatment of leaf curvature. The leaf-painting round the head of Ariosto by Titian, in the National Gallery, might be instanced. 289. The leaf thus differed from the flower in forming and protecting behind it, not only the bud in which was the form of a new shoot like itself, but a piece of permanent work, and produced substance, by which every following shoot could be placed under different circumstances from its predecessor. Every leaf labored to solidify this substance during its own life; but the seed left by the flower matured only as the flower perished.