LEAVES-THE WAY LEAVES ARE SET ON THE TWIGS ; (a) RIGHT-ANGLED PAIRS ; (1) ARRANGED SINGLY IN TWO ROWS ; (C) IN MORE THAN TWO ROWS ; (CO LEAVES CLUSTERED-THE POSITION OF YOUNG AND OLD LEAVES-LEAF-STALKS AND HOW LEAVES ARE SET ON THE TWIGS-DURATION OF LEAVES
To enable leaves to perform their function of assimilation, it is necessary that each should be placed so that its surface is exposed to the light and air. This is effected by methods that are quite distinct. Certain species may conform to one system, other species may be governed by a different system ; but individual trees habitually conform to the method that governs their species, subject to some modifications that we shall explain presently. The greatest amount of light would fall on a leaf lying in a horizontal position or nearly so (Illus. 98). This is actually the position of leaves on the majority of trees, though different methods of attaining this position are employed. We are acquainted with the various ways in which buds are arranged on the shoots, and the set of the leaves coincides with them. The reason for this is that the buds (destined in the following year to produce new shoots) are formed in the angle between the leaf-stalk and the twig.
(a) Right-angled pairs.--To take first those trees with pairs of buds set at right angles to one another--the Ash, Horse Chestnut, Sycamore, Maple, Guelder-Rose, Spindle, Dogwood, Wayfaring tree, and Privet--it is merely necessary to substitute leaves for buds, when the arrangements already described will answer for those of the leaves also. We
B have this plan which depicts an upright shoot seen from its tip (Fig. 163).
A A--lst and 3rd pair of leaves.
B B-2nd and 4th pair of leaves.
SH Shoot to which the leaf-stalks are attached.
If the alternate pairs of leaves (viz. A A, B B) were exactly under one another, they would not receive the full amount of light and air ; the stalks of the lower, therefore, are successively longer than those of the upper, and then the appearance of a shoot seen from its tip is this (Fig. 164), and the side view of this arrangement would look like this (Fig. 165).
ILLUS. 98. A BOUGH OF AN APPLE TREE SHOWING THE MORE OR LESS HORIZONTAL POSITION OF THE LEAVES
FIG. 166. In the upper diagram the widely-spaced leaves avoid the shadow seen in the lower diagram. The arrow represents the light
The shoots on these trees are more or less vertical ; if they were horizontal, leaves would come directly and close under other leaves, and the arrangement would no longer be effective. It will also be noticed that there are long spaces on the shoot between the points from which each pair of leaf-stalks spring, so that the rays of light play on the lower leaves, and they are not in the shade of those above them (Fig. 166). The Horse Chestnut is conspicuous from carrying the lower boughs in a drooping position, but the branchlets and tips recurve upwards. In this way the leaves are still set on a nearly vertical shoot. On the other hand, the Ash also has many pendent branches, and the leaves are often carried on a horizontal shoot instead of a vertical one. The reason for this probably lies in difference of construction of the leaf ; the leaflets of the Horse Chestnut are heavy, larger, and entire ; Ash leaves are lighter, narrower, and broken up into small leaflets, so that the light is able to pass between the gaps in the leaves, and the same pikicaution against shading is unnecessary. It should be remarked that three of the trees of this group have leaves composed of leaflets, i.e. Horse Chestnut, Ash, Elder, and that five of them carry large leaves, i.e. Horse Chestnut, Ash, Elder, Guelder-Rose, Wayfaring tree. The Field Maple, with its small leaves, is an exception.
The Privet, Spindle, and Cornel have small opposite leaves, but then they hardly exceed the dimensions of a bush. The Buckthorn has its leaves in right-angled pairs, but one leaf of a pair is commonly slightly lower than the other.
Fig. 167.-- The way in which a difficulty is overcome.
We have pointed out the necessity for right-angled pairs of leaves to be carried on a (nearly) vertical shoot ; but the lower branches of these trees are arranged at some angle with the stem that varies from 90° to 45°. So if the branchlets, to which they give rise, followed the system that governs the tree (i.e. pairs of branches set at right angles to one another , only one branchlet out of every alternate pair would be pointing upwards, and its fellow would be actually pointing downwards ; the rest would be in the same plane as the branch itself (Fig. 167). This difficulty is in some cases overcome (to take a Field Maple as an example) by these nearly horizontal branches carrying a number of very short branchlets all pointing upwards--a position attained by those which are on the under side of the branch curving round it till they become vertical. Another obvious advantage of the shortened shoots. is that they do not become cramped by the branches above them, but allow the leaves they carry to obtain their share of the light. Occasionally, however, this manoeuvre is unsuitable, and a totally different one (which we illustrate) is enacted. The twig remains horizontal, but the leaf-stalks twist until they have arranged the leaves in a flat layer, after the fashion of Beech leaves (Fig. 168). In our illustration leaf B has moved across the twig as no vacant space existed on its proper side. Fig. 169 shows the normal arrangement of Maple leaves on a vertical twig.
(b) Leaves arranged singly in two rows.--Taking, next, the trees with alternate leaves set singly in two rows, we find a totally different arrangement. The twigs form flat, more or less horizontal, layers ; and in the case of the lowest boughs they droop. The leaves are arranged all in one plane with these twigs, as if they had been pressed under a weight (Fig. 170). This is conspicuously the case with the Beech and the Hornbeam (Fig. 171, Illus. 99), and hardly less so with the Hazel and the Lime. It also occurs on vigorous young shoots of the Elm when it is young ; but it is less noticeable on the shorter twigs that are formed when the tree is older. It will be seen that the leaves of this group have short stalks, and are rounded or egg-shaped in form ; also that the nodes on the shoot from which they rise are set near together--in fact, just near enough to allow each leaf to be next its neighbour without overlapping, but with no wasted space between. Elm leaves are borne on a nearly straight shoot, Beech leaves on one
ILLUS. 99. FOLIAGE OF HORNBEAM, SHOWING HOW EACH LEAF IS EXPOSED TO THE LIGHT
that zigzags from one side to the other. Consequently, an economy of space is effected by the stalks of the Beech leaves being set obliquely and those of the Elm leaves at nearly a right angle with the shoot. The upper shoots of these trees, on the other hand, are often upright ; and the leaves, accommodating themselves to the position, are commonly set with the blade at right angles with the shoot,so as to remain horizontal). Though the leaves of the Sweet Chestnut,
Birch (Illus. 101), and Yaw (Illus. 24, p. 75) are arranged on quite another plan ; they sometimes form flat layers (in one plane) on the horizontal twigs.
The chief point to remember about this group (h) is that the leaves and twigs all lie in one plane, while group (a) was distinguished by the blades of the leaf being set at right angles to the twig.
(c) Scattered leaves arranged in more than two rows.--A diagram plan of leaf-stalks, when they are arranged singly, but in more than two rows on the shoot, would represent the spokes of a wheel (Fig. 172)-- the number of spokes corresponding with a set of leaf-stalks, counted spirally in succession until one is found springing from the same side of the shoot as the first one counted (i.e. on an imaginary line directly above the first one). In these cases the leaves do not lie in the same
Fig. 170.--The way in which Beech leaves are spaced
plane with the twig (as happens when the leaves are in two rows). They generally are borne on twigs tending upwards (Illus. 89), the blade of the leaf lying more or less at right angles with the axis of the twig, and the leaf-stalk set at less than a right angle with the twig, so that
Fig. 171.--Example of economy of space--leaves of Hornbeam
the leaf is pointing in the direction of the apex of the twig rather than lying horizontally (Illus. 100). This plan is not consistently followed, however ; for in the Yew and some other conifers the leaf-stalks bend round the twigs (when they are lying horizontally), and so the leaves are in one plane with the twig. The difference between an upright shoot of Yew--with its single Bic) scattered leaves jutting out in several directions--and these flat layers of leafage on the same tree is conspicuous. The leaves of the Birch and Alder form three rows down the stalk, though the leaves of the former sometimes appear to be in pairs. The Poplar, Ash, and Apple form five rows. On some of these trees the leaves droop, either from a curved leaf-stalk or from pendent twigs. The Willows, the Birch (Illus. 101), and the Cherry (Illus. 58, p. 142), are examples ; and a more conspicuous one is the Black Poplar (Illus. 25, p. 77), where a large number of the leaves hang vertically.
(d) Leaves clustered.--When the leaves are borne so close together on the shoot as to appear in tufts, rosettes are formed. The star-shaped bunches of the Oak, the rosettes on the arrested twigs of the Hawthorn (Illus. 70, p. 167), and the shuttlecock form of the Scots Pine and Larch (Illus. 104) are easy to recognise. It should be noted that single leaves, spaced clearly apart, are also produced on Larch and Hawthorn shoots, and that the spiral lines formed by the leaf bases on the Spruce are characteristic of it and of members of the Pine tribe. The leaves of the Scots Pine are sheathed at the base, two leaves to a sheath (Illus. 141, p. 287). The Austrian and Stone Pines, like the
Scots Pine, have two leaves in a sheath ; some other Pines have bundles of three and five to a sheath. The Firs, on the other hand, all carry the leaves singly (Illus. 73).
The position of young and old leaves.--Young leaves, after breaking out from the bud, assume a temporary position that is not retained in maturity. This is worth noting, since the general appearance of the same trees in Spring-time and Summer is so at variance ; and much that gives the key-note to the season is due to this small fact about the position of the leaves. As an example, bring to mind a Horse Chestnut tree in Spring, with its stout shoots draped in young leaves that hang limp and nerveless after the fashion of a damp half-closed umbrella (Illus. 159, p. 309), and then recall it in Summer with radiating leaves that stretch out from the shoots and form the horizontal tiers of leafage, so easy of recognition even at a distance. Again, those quaint white leaves stiffly upright (whether on upturned
* 1 See illustrations of leaves, Illus. 24, 25, 26, Chap. IV
ILLUS. 100. LEAVES ON AN UPRIGHT SHOOT OF A THORN TREE
(See also drawing of the rosettes of leaves on arrested twigs, Chap. XII, Illus. 70.)
ILLUS. 101. BIRCH LEAVES
ILLUS. 102. THE VERTICAL LEAF ARRANGEMENT OF THE GREY POPLAR
ILLUS. 103. LEAF ARRANGEMENT OF THE OAK
ILLUS. 104. THE TUFTED (SHUTTLECOCK) FORM OF LEAVES ON THE LARCH
or horizontal twigs) mark the Whitebeam in the copse, by their shape and the exposure of the white under sides, that in their Summer pose are inconspicuous unless ruffled by the wind (Illus. 107). Examples of
Fig. 173 - Oak
young leaves that droop during expansion are afforded by the Wych Elm (Illus. 96, 106), Lime, Maple (Illus. 112, p. 232), Beech, Hornbeam, and Hazel, and the partly developed leaves of ; the Horse Chestnut and Sycamore. On the other hand, we find upward-pointing leaves in the
ILLUS. 105. MATURE LEAVES OF ENGLISH ELM
ILLUS. 106. LIMP YOUNG LEAVES OF WYCH ELM
Ash, Whitebeam (Illus. 107), Holly, Wayfaring tree (Illus. 93, p. 203), the Guelder-Rose (Illus. 92, p. 202), Oak (Illus. 111, p. 231).
Leaf-stalks and how leaves are set on the twigs.--The way a leaf is attached to the twig is worth consideration. We find an Oak leaf, some four or five inches long, attached by a petiole one-eighth of an inch or less in length ; and an Aspen borne by a petiole longer than the leaf itself. To take less extreme cases, we have the Poplars, Lime, Pear, Buckthorn, Birch, Sycamore, Maple, Guelder Rose, Horse Chestnut, Crab Apple, with proportionately long stalks ; while the Oak, Holly, Privet,' Wayfaring tree, Hornbeam, Beech, Hazel, Sweet Chestnut, some Willows, Whitebeam, and Elm have proportionately short ones. The Larch and the Conifers, since their leaves take the form of a more or less thickened leaf-stalk, need not be considered. The flexibility or rigidness of the stalk often accounts for the lie of the leaves. Some arch with the weight of the leaf, such as the Sweet Chestnut, Wild Cherry, and many others ; they may even hang,
in their limpness, as do some of the Poplar leaves. The contrast to these are the Horse Chestnut, Sycamore, Maple, Beech, Hornbeam, and Hazel, where the leaf is held by a stiffer petiole. The twist on the leaf-stalks of some of the Willows adds to the decorative pattern the leaves make, while the upward curve of a Holly petiole seems in keeping with the branch formation. These matters and the actual shape of the petioles--their round, rectangular, flattened or keeled surface, and their swollen bases where they hide the young buds for next year--may be worth the study of the designer. On most trees the leaves lie in a sat position--horizontally or slightly drooping--such as those of the Maple, Horse Chestnut, Beech, and Hornbeam.
This is also the case with the Ash, Walnut, Hazel, Elm, and Spanish Chestnut, except that in these they incline upwards on the young
* 1 See " Leaf Patterns," Chap. XVII.
* 2 See illustrations, Chap. IV, et seq.
ILLUS. 107. YOUNG LEAVES OF WHITE BEAM STANDING UPRIGHT THE UPPER TWIG WITH THE BAT BUDS CONTAINS FLOWERS
vertical shoots (Illus. 108). Oak and Holly leaves, and also many others, are set on the twig at rather less than a right angle, and preserve that position in whatever direction it may take. Larch leaves
ILLUS. 108. --ASH LEAVES
This branchlet of an Ash shows the normal position suitable for leaves that are arranged in decussate pairs. The plan that is follow ed by the buds is described Chap. XII, p. 162. The leaves on a pendent branch are drawn, Chap. VI, p. 106.
seem to follow no rule ; they point upwards, downwards, and sideways, or each leaf of a group may point in a different direction.
Duration of leaves.--Trees are divided roughly into two classes--(1) Deciduous, (2) Evergreen.
(1) Deciduous trees are those whose leaves fall before new ones are formed, so that the branches are for a time naked. This is not strictly true of the Oak and Beech, as they occasionally retain the brown shrivelled leaves during the winter.
(2) Evergreen trees are those which retain their leaves until new ones have been matured--as in the Holly, where the leaves of one season remain till the next. Some of the Pines retain their leaves for several years, and on one species--the monkey puzzle (araucaria imbricata)-- They are persistent for about twelve to fifteen years.