Chapter 33
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..h.i.t by the luminous ray coming between right angles.
[Footnote: See Pl. XXVIII, No. 1.]
421.
Young plants have more transparent leaves and a more l.u.s.trous 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, l.u.s.tre [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, l.u.s.tre [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.
A leaf with a concave surface seen from the under side and up-side-down will sometimes show itself as half in shade, and
425.
Although those leaves which have a polished surface are to a great extent of the same colour on the right side and on the reverse, it may happen that the side which is turned towards the atmosphere will have something of the colour of the atmosphere; and it will seem to have more of this colour of the atmosphere in proportion as the eye is nearer to it and sees it more foreshortened. And, without exception the shadows show as darker on the upper side than on the lower, from the contrast offered by the high lights which limit the shadows.
The under side of the leaf, although its colour may be in itself the same as that of the upper side, shows a still finer colour-a colour that is green verging on yellow-and this happens when the leaf is placed between
426.
the eye and the light which falls upon it from the opposite side.
And its shadows are in the same positions as those were of the opposite side. Therefore, O Painter! when you do trees close at hand, remember that if the eye is almost under the tree you will see its leaves [some] on the upper and [some] on the under side, and the upper side will be bluer in proportion as they are seen more foreshortened, and the same leaf sometimes shows part of the right side and part of the under side, whence you must make it of two colours.
Of the transparency of leaves (427-429).
427.
The shadows in transparent leaves seen from the under side are the same shadows as there are on the right side of this leaf, they will show through to the underside together with lights, but the l.u.s.tre [reflected light] can never show through.
428.
When one green has another [green] behind it, the l.u.s.tre on the leaves and their transparent [lights] show more strongly than in those which are [seen] against the brightness of the atmosphere.
And if the sun illuminates the leaves without their coming between it and the eye and without the eye facing the sun, then the reflected lights and the transparent lights are very strong.
It is very effective to show some branches which are low down and dark and so set off the illuminated greens which are at some distance from the dark greens seen below. That part is darkest which is nearest to the eye or which is farthest from the luminous atmosphere.
429.
Never paint leaves transparent to the sun, because they are confused; and this is because on the transparency of one leaf will be seen the shadow of another leaf which is above it. This shadow has a distinct outline and a certain depth of shade and sometimes is [as much as] half or a third of the leaf which is shaded; and consequently such an arrangement is very confused and the imitation of it should be avoided.
The light s.h.i.+nes least through a leaf when it falls upon it at an acute angle.
The gradations of shade and colour in leaves (430-434).
430.
The shadows of plants are never black, for where the atmosphere penetrates there can never be utter darkness.
431.
If the light comes from m and the eye is at n the eye will see the colour of the leaves a b all affected by the colour of m -that is of the atmosphere; and b c will be seen from the under side as transparent, with a beautiful green colour verging on yellow.
If m is the luminous body lighting up the leaf s all the eyes that see the under side of this leaf will see it of a beautiful light green, being transparent.
In very many cases the positions of the leaves will be without shadow [or in full light], and their under side will be transparent and the right side l.u.s.trous [reflecting light].
432.
The willow and other similar trees, which have their boughs lopped every 3 or 4 years, put forth very straight branches, and their shadow is about the middle where these boughs spring; and towards the extreme ends they cast but little shade from having small leaves and few and slender branches. Hence the boughs which rise towards the sky will have but little shade and little relief; and the branches which are at an angle from the horizon, downwards, spring from the dark part of the shadow and grow thinner by degrees up to their ends, and these will be in strong relief, being in gradations of light against a background of shadow.
That tree will have the least shadow which has the fewest branches and few leaves.
433.