Other ramifications are spherical, as those of such trees as put forth their shoots and leaves in the order of the sixth being placed above the first. Others are thin and light like the willow and others. 417. You will see in the lower branches of the elder, which puts forth leaves two and two placed crosswise [at right angles] one above another, that if the stem rises straight up towards the sky this order never fails; and its largest leaves are on the thickest part of the stem and the smallest on the slenderest part, that is towards the top. But, to return to the lower branches, I say that the leaves on these are placed on them crosswise like [those on] the upper branches; and as, by the law of all leaves, they are compelled to turn their upper surface towards the sky to catch the dew at night, it is necessary that those so placed should twist round and no longer form a cross. [Footnote: See Pl. XXVII, No. 5.] 418. A leaf always turns its upper side towards the sky so that it may the better receive, on all its surface, the dew which drops gently from the atmosphere. And these leaves are so distributed on the plant as that one shall cover the other as little as possible, but shall lie alternately one above another as may be seen in the ivy which covers the walls. And this alternation serves two ends; that is, to leave intervals by which the air and sun may penetrate between them. The 2nd reason is that the drops which fall from the first leaf may fall onto the fourth or--in other trees--onto the sixth. 419. Every shoot and every fruit is produced above the insertion [in the axil] of its leaf which serves it as a mother, giving it water from the rain and moisture from the dew which falls at night from above, and often it protects them against the too great heat of the rays of the sun. LIGHT ON BRANCHES AND LEAVES (420--422). 420. That part of the body will be most illuminated which is hit by the luminous ray coming between right angles. [Footnote: See Pl. XXVIII, No. 1.] 421. Young plants have more transparent leaves and a more lustrous bark than old ones; and particularly the walnut is lighter coloured in May than in September. 422. OF THE ACCIDENTS OF COLOURING IN TREES. The accidents of colour in the foliage of trees are 4. That is: shadow, light, lustre [reflected light] and transparency. OF THE VISIBILITY OF THESE ACCIDENTS. These accidents of colour in the foliage of trees become confused at a great distance and that which has most breadth [whether light or shade, &c.] will be most conspicuous. The proportions of light and shade in a leaf (423-426). 423. OF THE SHADOWS OF A LEAF. Sometimes a leaf has three accidents [of light] that is: shade, lustre [reflected light] and transparency [transmitted light]. Thus, if the light were at _n_ as regards the leaf _s_, and the eye at _m_, it would see _a_ in full light, _b_ in shadow and _c_ transparent. 424.