Tree Art
CHAPTER XXI
LESSER DETAILS
Stipules.--In a Beech wood in Spring, when the limp young leaves are losing their silver fringe, the ground beneath is strewn with the brown, shrivelled, leaf-like forms that have protected them as they came out of the bud. These are the stipules ; they fall as soon as the leaves develop. Those of the Beech are not remarkable for beautiful colour or interesting form ; but in some other cases they are of larger size and are brightly coloured ; in fact, they play a very ornamental part in the Spring-time gala. The Norway Maple plays first fiddle with its scarlet stipules, and is well supported by the Sycamore and Field Maple with theirs of pink and rose, and the Hornbeam and Wych Elm with shades of purple ; those of the Beech, Elm, and Lime are brown in colour and membranous. On some trees, entertaining shapes replace gay colouring, and we see the stipules mimicking leaves, it may be only as far as showing a notched tip ; or, in other cases, the stipular forms are really leaf-like in colour, form, and texture. Those on the Walnut tree, Guelder-Rose, Hawthorn, and Plane (Ill us. 158, p. 307) are interesting for the various formations they illustrate. Holly stipules are minute, and take the unusual colour of black. Stipules may be divided roughly into two divisions : (1) Those acting like the scales in protecting the young leaves in the bud ; and either falling when the bud bursts or in early life lengthening with the leaves and falling after they are developed. (2) Leaf-like persistent appendages at the base of the leaf-stalks of matured leaves. These last are usually green in colour ; they differ in shape on individual--and even the same-- trees, though the same forms will be found repeated on other individuals of the same species. The stipules at the base of the leaf-stalk are usually in pairs--one on either side of the stalk--or are united so that they form a cup round the stalk. Stipules formed on the latter plan are conspicuous in the young growth of a Plane tree ; making a sort of vase with a frilled recurved lip, from which the young leaves spring. On some trees there are no stipules.
Bracts.--The dictionary definition of a bract is " any leaf which bears
ILLUS. 156. THE FLOWERS ON A SPECIES OF SYCAMORE
Notice the notched leaf-like scales at the base of the stalks
ILLUS. 157. BUD SCALES OF INFLORESCENCE - MAPLE
in its axil (the angle between itself and the stem from which it arises) a flower, or a branch which terminates in a flower." If we add that leaves in these positions are usually modified leaves (generally smaller than the real leaves and with an entire margin), and sometimes like scales, we get quite a good idea of what a bract is. The circle of bracts which enclose the stalks of the flower-cluster termed an Umbel, is a noticeable feature of the inflorescence. In a compound inflorescence there should be a bract at the base of the main flower axis and also at
ILLUS. 158. VASE-SHAPED STIPULES OF THE PLANE TREE
each junction where other stalks branch. Bracts must not be confused with the leaf-like forms that commonly constitute the outer circle of the flower itself. In catkins--where the flowers are crowded on a short axis--each flower arises from the axil of a bract, just as it would do in an inflorescence built of longer axes ; but unless the construction of the floret is understood, the bracts from their closeness to the florets might be mistaken for a part of them. It follows that the bracts will be in pairs when the branching is in pairs, and single where the flowers are produced alternately. In cones the bracts are usually hidden between the scales (each bract is on the side of the scale farthest from the axis) ; but in some species they protrude beyond the scales, as in the Douglas Fir and some species of the Silver Fir.
Buds and Scales.--We have carefully followed the arrangement of the buds, and have seen that they arise from the axils of the leafstalks, where they lie protected by stipules and leaf-stalks, sometimes even hidden by the swollen or hollowed form of the stalk (Walnut) ; and in the case of the Plane actually enclosed inside the base of the stalk until its fall. It will be also understood that a bud contains the rudimentary stem, leaves, and in some cases flowers. Buds act, as it were, like a greenhouse, keeping all these parts under cover and pushing forward their growth, to save time during the growing season. Early in summer we see the buds getting ready for the following Spring.
The protection of the young leaves from frost while in the bud, by wool, hairs, and stipules, has been mentioned, but not the bud scales, that form the outer tiles, as it were. These are arranged in the same order as the leaves--if the leaves are in pairs, so will be the bud scales (Horse Chestnut, Illus. 159), or the scales will follow the leaves in being alternate (Beech). The scales may be considered as undeveloped leaves ; nevertheless they sometimes prove their analogy by producing the leaf-like tips that we occasionally see on the Horse Chestnut and varieties of Walnut. Some scales are covered with gum (Horse Chestnut), others with a sort of resin (Poplar), as a protection against wet. The buds of the Plane have a cap of fur over them. Some scales may also be considered as undeveloped stipules ; the form of a pair of stipules sometimes being represented by a double-tipped edge. Bud scales are often very numerous : Lubbock, in his Buds and Stipules, minutely describes thirteen on an Elm bud. Generally the scales are not long enough to cover the bud, but overlap one another like inverted tiles. The colour of the scales varies, from purple-brown to yellow or green or even black, as in the Ash. The Alder-Buckthorn is remarkable for having no scales to protect the bud. Buds containing flowers can usually be recognised by their larger size, and in most case by their rounder form.
On the Hawthorn, Sloe, Wild Pear, and some other
trees, there are a number of sharp-pointed spines. These are twigs arrested in growth, a fact that is shown by their being produced from one of the usual side buds in the axils of the leaves ; sometimes they bear stunted leaves and rudiments of flowers, and additional buds may spring up at their junction with the shoot. The spines on the Acacia, on the other hand, are transformed stipules. The change horn leaves to spines is not shown by our forest trees, though it occurs in shrubs ; the nearest approach to it being in the difference between the smooth-leaved and prickly-leaved Holly. Prickles such as those on the wild Rose arc additional outgrowths from the bark
ILLUS. 159. BUD SCALES AND REMAINS OF THE PACKING WOOL ON THE NEW SHOOT OF A HORSE CHESTNUT
only, and are easily broken off ; their origin being so distinct, they must not be confused with spines. The metamorphosis of organs is quite outside the scope of this book, but we might mention the curious example of the Butcher's Broom, which in some localities is a common bush in the coverts ; the " leaves " are dark green, sharply pointed, and bear half-way up on their " central rib," or the under side of the " leaf," the blossom and subsequent fruit. These so-called " leaves " are actually transformed flattened branches, each springing from the axil of a real leaf which has the appearance of a bract. At the junction of the flower and the " leaf," there is also a bract, just as there would have been if the " leaf " were a branch.
Leaves of seedlings.--While in the woods, we come across seedlings of most of the trees growing there, and of others whose seed has been dropped by birds or carried by the wind. In my book British Trees, I have described the leaves of the Hornbeam seedling and some others. " The seedling of the Hornbeam is a striking example of the dissimilarity which so often exists between the seed-leaves of a tree and the true leaves which are afterwards produced. In this case the seed-leaves have a disc-like form, a flat surface, an even, uncut edge ; while the blades of the true leaves are fluted and their outlines are notched. In making the comparison many points must be taken into consideration. For example, some cotyledons (seed-leaves) have no footstalks, while the true leaves possess long ones ; or, again, the cotyledons may be arranged in a pair, the one opposite the other on either side of the stem, while their successors show no such arrangement, and spring from different points on the stein. Seed-leaves also differ from the true in colour, in texture, and in size. Sometimes the transition from the pattern of the cotyledon to that of the leaf is a gradual process, and the leaves that immediately follow the cotyledon rarely show the type of its completion. Often there are many stages to be passed through, and the disguise in each is scarcely less complete than it was in the case of the first seed-leaves. But little by little the type emerges as each leaf, or pair of leaves, follows in the procession of development, until perfection is attained. Even in individual seedlings from trees of the same species, there is variety in the stages of transition."
The seed leaves of the Beech are fan-shaped ; the first pair of true leaves are correct in form, but appear (from being on a short axis) to be a pair instead of set alternately. Ash seed-leaves are long, narrow, and tapering at both ends. The first pair of leaves have serrated edges, but consist of two single leaves instead of a series of leaflets --occasionally they bear three leaflets each. Sycamore seed-leaves resemble two pieces of narrow, green, pointed ribbon. The first leaves are heart-shaped, with a long tapering point and serrated edges, and possess only the rudiments of the lobes of the perfect leaf. The seal-leaves of the Scots Pine resemble true leaves. They are arranged in a bundle, and radiate from the stein after the fashion of a vase. The cotyledons of the Oak have little resemblance to leaves ; and still less those of the Chestnut. In the former, the acorn is split in half, and carried on either side of the growing stem ; so the position at least of leaves is assumed. In the latter, the nut entire has not outwardly the faintest resemblance to a leaf.
The Bark.--The bark on a Beech or a Plane is as noticeable for its smoothness as that of an Oak or Scots Pine is for its roughness. This difference in texture is accounted for by three distinct methods of growth, examples of which are seen in the former and latter trees respectively. The rind of the younger parts of a tree is made up of three layers--an inner layer or " cambium," a middle layer, the " bast," and an outer green layer or " epidermis." Each of these is provided with cells, and grows by the addition of new cells on its inner surface. The epidermis or outer skin does not increase after a year or perhaps two years, when it is replaced by cork formed by its underlying tissue ; while each year the cambium forms a new ring of wood to add to the girth of the trunk. This cork or bark is made up of cells for the passage of air and moisture. The outer cells live only a short time, and lose their elasticity ; consequently the bark cracks, and the dead pieces become separated by grooves from the increase in diameter of the trunk within. The pieces of dead bark, thickened by new layers formed inside it, in some species remain on the trees many years (Cork-Oak) ; in other species, they merely form thin papery layers (Birch, Plane), and scale off. The fibrous layer of bast continues to grow by additions every year from the cambium layer. The bast is in some cases strongly
developed--as in the Lime ; or it
may be produced only in the first year--as in the Birch, Beech, and Maple. Young shoots are covered with smooth green epidermis ; but during the season the green colour disappears, being usually replaced by some shade of ash-grey or brown- grey of the newly formed bark. As the twigs get older, the colour is less conspicuous and a corresponding difference in loss of smoothness occurs. The delicate gradations in the colour of the twigs to the greys of the older branches adds much to the harmony of the woods in winter.
ILLUS. 160. TRUNK OF SPANISH CHESTNUT
Notice the spiral twist of the lurk
ILLUS. 161. ANOTHER TRUNK OR A SPANISH CHESTNUT
ILLUS. 162 BARK OF LARCH
ILLUS. 163. THE BARK ON A WALNUT TREE
ILLUS. 164. THE BARK ON A LIME TREE
ILLUS. 165. BOLE OF AN OAK TREE
There are shining silver-greys on young Oak trees and Rowans : varied grey tints on Ash saplings and the fully grown tree : purple-grey on Alder twigs : chestnut and red-brown on Birch : grey and nut-brown on Hazel : red and purple on Maple and Cherry : yellow tints on the Elder : and mealy-white stems on the Wayfaring tree. The Cornel takes crimson, as also do some species of Withies, while others assume yellow or orange or red ; some are smooth and shining, others are woolly. The shoots from the Spanish Chestnut stools are purple ; the Spindle alone is green. The change in colouring may be observed on the Plane, where the twigs of one year's growth are olive-green, the older ones dark brown tinged with red, and the branches dark grey ; the thin bark of the trunk flakes off annually, exposing a smooth surface where the grey is tinged with yellow.
The ultimate bark on the tree trunks varies in construction, as in colour, in the different species. In the Spanish Chestnut (Illus. 160) it forms vertical spirally twisted keels, hardly at all cross-sectioned ; on the Acacia these keels have an interlacing appearance ; and on the Ash they form a sort of elongated diamond pattern. The Rowan has a smooth rind split by lines that form broken rings round the stern. The Sycamore is covered with slight patternless fissures. The projecting bark of the Hornbeam is smooth, and lighter in colour than the wide gaps between it. The bark of the Pear divides into little blocks. There is much in common in the Oak and Elm in the rough covering of their trunks. A still more cork-like bark, split in vertical layers, covers the base of a Birch stern ; in the upper part there is the extreme of silky smoothness, gleaming silver white, with papery layers unwinding in circular strips and disclosing new shades of pinks and greys of the layers underneath. The bark of a young Larch is smooth and greenish-brown with vertical stripes of a lighter shade ; later in life it is fissured and becomes scaly, bearing some resemblance in colour to the red- grey of the Scots Pine, though usually more mauve in colour (Illus. 162). The bark of the Black Poplar is rough and dark, while that of the White Poplar is smooth, pale, and grey, pitted with lozenge-shaped marks that are divided vertically by a slight red-brown incision. These marks are grouped in patches and horizontal rows, and form rings, which accentuate the roundness of the trunk. At the lower part of the bole the diamond-shaped marks disappear, the incisions become deeper, and the bark forms vertical cork-like ribs between them. The Grey Poplar has similar scars ; the bark is smooth and stone-coloured, with shades of green and yellow.
The difference in texture, colour, and arrangement of the bark is well-marked on each species, and plays its part in the variety in which nature delights.
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