FORMS OF YOUNG LEAVES--TEXTURE OF LEAVES -- THEIR COLOUR--LEAF PATTERNS --T HE MARGIN -- THE VEINS -- THE PLANES OF A LEAF
Forms of young leaves.--As might be expected, the method by which leaves are folded up while in the bud is discernible for a long time after expansion. Thus in Spring-time we see quite large leaves of the Cherry still folded in half (Illus. 109). Other examples of leaves that in the bud are folded in half from the central rib after the fashion of a sheet of writing paper are afforded by those of the Ash, Hazel, and Lime ; others are folded like a fan (Fig. 178), Such as the Maple (Illus. 116), Sycamore (Illus. 156), Hornbeam, Beech, Birch, and Wayfaring tree. Some leaves are neither folded nor crumpled up, but are curved to fit the dome of the bud (Fig. 179), i.e. those of the Pine, Fir (Illus. 73), Larch, Yew, Holly, Privet, and Spindle.
Others, again, have the two halves of the leaf-blade rolled up from the outer margin inwards towards
the central rib, so that each leaf resembles a pair of scrolls of equal size, united by the rib, the outsides
of the scrolls being the back of the leaf-blade (Fig. 180). This happens with the Apple, Pear (Illus. 110), and Poplars. The leaf of the Plane is also rolled like this ; but in its case the outside of the scrolls is the upper surface of the blade (Fig. 181). A single scroll (Fig. 182) is formed by leaves of the Buckthorn, Plum, and some Willows. These are rolled up as a spill might be, from one margin of the blade only, with the under surface of the leaf outside.
It is quite usual to see large Poplar leaves still in scroll form. The limpness of young leaves of big build is conspicuous in those trying to obtain their first horizontal position, such as the Sycamore, Horse Chestnut, and Plane (Illus. 112).
ILLUS. 109. THE LEAVES OF THE BIRD CHERRY REMAIN FOLDED IN HALF AFTER GAINING THEIR LIBERTY FROM THE BUDS
ILLUS. 110. ON A PEAR TREE THE LEAVES ARE PACKED IN SCROLLS WHILE IN THE BUD
ILLUS. 111. OAK LEAVES BREAKING OUT FROM THE BUD
Texture of leave.--The texture of the leaves plays an important part when the trees are seen close at hand. The transparency of a young
112. YOUNG LEAVES OF THE FIELD MAPLE
Compare their limp attitude with that of the mature leaves illustrated Chap. XX, Illus. 136
Beech leaf in one position through which the sun shines--so sweet and gay in colour--looks nearly white in another position, from its silky surface. The glitter from the polished Holly leaves takes many tints in response to the changing sky, to which the gritty surface of an Elm or Hazel leaf is less responsive. The thick leathery texture of the Privet, Box, Holly, and Holm Oak, the furry or downy-coated under sides of the Whitebeam, White Poplar, and Wayfaring tree are all worth observation.
Colour of leaves.--The local colour of leaves seems to defy description, for the hairs on one, the gloss on another, gather so much of the sky colour as to lose the individual colour that might apply to them when seen indoors ; but since the leaves of each species are consistently dull, glossy, granulated, or hairy, they never lose their identity. It must be remembered, however, that in some species young leaves have a gloss that is lost as they mature (example, Sycamore, Beech), and that others start life with a coat of fur and become smooth (example Plane). The light can pass through some leaves when they are young only, as is the case with Holly leaves. It seems superfluous to add that the local colour of leaves is different in spring, summer, and autumn.
Leaf Patterns : (a) Of conifers.--Most of the conifers agree in having leaves remarkably narrow for their length. The leaves of the Spruce (Fir tribe) are needle-shaped and angular, those of the Silver Fir and the Yew are flat, while those of the Scots and Austrian Pines have one side convex and the other flat. The species vary between a blunt leaf and one sharply pointed. Yew leaves are curved, Larch leaves are straight. Deciduous trees (with the exception of the Larch) have their leaves in the form of a more or less thin blade, instead of the needle form of the conifers.
Leaf patterns of deciduous trees.--On some deciduous trees the leaf-blade is long and narrow ; conspicuous examples occur in several of the Willows, the Privet, the Almond, and less noticeably in the leaflets of the Ash, Mountain Ash, and some leaves of the Blackthorn and the Spindle. The Cherry and the Sweet Chestnut take a more elliptic form. The chief character of many of the foregoing depends upon whether the base or the tip, or both, tapers or has a more or less blunt ending. Some leaves might almost be enclosed by a circle ; this is seen chiefly in those of the Aspen and often of the Grey Poplar, and in some leaves of Hazel and Lime. A triangle (set with the leaf-stalk joining the centre of its base) would be the guiding form for leaves of several species, i.e. the Birch, Black Poplar, Lombardy Poplar, and some leaves of the Maple, Planes, White Poplar, and Hawthorn. Some of the leaves of the Lime and of the Black and Lombardy Poplar bear the shape of a conventional heart. The outline of an egg with the leaf-stalk joining the stoutest end is seen in the Pear, Apple, Goat Willow, Elm, Cornel, Beech, and Buckthorn. The same form--but usually with a more elongated tip--is found in the Wayfaring tree and the Whitebeam. The egg reversed would describe the leaf of the Wych Elm and the Spindle,
ILLUS. 113. WOOL-COVERED LEAVES OF GREY POPLAR
(See Chap. XVI, Illus. 102, for the position and texture of the mature leaves. For shape of leaf see page 211, (C) of " Leaf Patterns," in this chapter)
though the latter has a sharp tip added. An oval describes the leaflets of the Walnut and the broadest leaves of the Blackthorn. A more unusual form is found in the leaf which has a blunt tip and tapers to the leaf-stalk like a tennis racket. This form distinguishes the Alder and many leaves of the Alder-Buckthorn. The Hazel leaf has this shape with a short-pointed tip added, and the base often ends in small lobes ; the leaves on an individual tree vary so much in form that an oval (or egg form) set either way might equally well describe them. However, the foregoing descriptions, with the leaf plans illustrated before one, should explain the chief difference of the species. (See " Leaf Patterns," Figs. 200-250.)
If a leaf is folded at the midrib, the two halves are usually found to be similar ; but in some trees the leaves are lop-sided (Fig. 183), as is seen in the Elm, Lime, and Oak. Some Willow leaves, instead of having the central rib straight from tip to base, have it bowed to one side ; consequently the outline of the leaf is convex on one side and concave on the other (Fig. 184).
The Margin.--The outline of a leaf may be smooth and unbroken, as is seen in the Privet, Fig. 183 Fig. 184 Scots Pine, Yew, Larch, Medlar, Cornel, Box, and others (Fig. 185 (1)). More often, projections are formed, where the secondary ribs (those given off by the central rib) reach the margin. The points on the margin may be very sharp, and the spaces
between them deeply fluted like a fish's fin, as in the Sweet Chestnut and in some leaves of the Plane ; or less sharp and the fluting shallower--like a fan, as happens in a Beech leaf. In the Grey Poplar and the Aspen, the projections and hollows are sometimes rounded like a cog-wheel, and more often like a toothed wheel. In most leaves the margin is toothed like a saw (Fig. 185 (2)) (the handle of the saw being at the leaf-stalk), as in the Black Poplar, Mountain Ash, Ash, Apple, Alder, and some Willows. The teeth themselves may be furnished with small teeth, as are those of the Hornbeam, Birch, Elm, Lime, Horse Chestnut, Hazel, Cherry. The teeth, instead of being set saw-like, may point outwards, as in the Wayfaring tree. The leaf may be deeply scalloped between the secondary ribs, thus forming the characteristic lobes of the Oak (Fig. 185 (5)) ; or, if the incision reaches the midrib, separate leaflets result, as in the Ash. The incisions may be between several main ribs that radiate from the junction of leaf and stalk (Fig. 186) ; there is then, as a rule, a terminal point on either side of the main lobe formed as in the Guelder-Rose. The side lobes may give off a pair of basal lobes, as in the Oriental Plane. Again, these lobes at the base may be distinct, as in the Maple and Sycamore. All or some of the lobes may be equally or unequally notched, toothed (Sycamore, Maple), or rounded. This form of leaf, when incised to the leaf-stalk, produces radiating leaflets (example, the Horse Chestnut). The leaves of the Planes and Maples are remarkable for their diversity of form and proportion. The leaves of the Holly usually bristle with spines ; but those at the top of the trees are often smooth-edged. Some Holly trees are composed of entirely smooth-edged leaves ; this is also the case with the Holm Oak. (See Figs. 200-250.)
Veins of the leaf.--The manner in which the blade of a leaf is sectioned by the veins will be sufficiently understood by the diagrams of feat forms (Figs. 200-2Z0) ; though, in most cases, only the noticeable divisions are there represented. The mid - vein running from the stalk to the tip (or in lobed leaves, such as the Plane and the Sycamore, to the tip of each lobe) is the stoutest, and is termed a rib ; the ribs that rise from it are smaller ; while others that rise from the secondary ribs are still less, and give rise in turn to numerous small veins intersecting the space (reticulated leaves) between the ribs. Generally, the ribs are conspicuous as raised cords on the under surface of the leaf and as furrows on the upper surface. In some the midrib only is noticeable (Fir, Privet) (Fig. 188) ; in others the whole network is clear (Goat Willow). Leaves in which the principal ribs start from the base (or from points on the midrib), and tend towards the tip of the leaf, have usually an unnotched margin (Cornel) (Fig. 187). Leaves with a toothed or indented edge have a central main rib, from points along which arise secondary ribs running into each principal tooth (Spanish Chestnut, Chestnut, Beech, Oak, Hornbeam, Alder) (Fig. 190) ; if the leaf-edge is broken up into a number of teeth, the secondary ribs branch, so that many of the teeth are reached by the secondary ribs or others to which they give rise (Wayfaring tree, Lime, Poplar, Elder) (Fig. 191). When leaves are compound, each leaflet follows one of the above plans ; as also happens with each lobe of a deeply sectioned leaf. It should be observed that in many leaves the main rib does not take a straight course from base to tip, but zigzags from side to side
ILLUS. 114. THE BLADE OF AN ALDER LEAF LIES IN ONE PLANE
from each point from which it gives off a secondary rib (example, Beech).
Planes of a leaf.--In many trees the whole leaf blade (if we except the undulations of its surface) lies in one plane, as, for instance, in the Alder (Illus. 114), Beech, Hornbeam (Illus. 99), Wayfaring tree, Whitebeam, and others. Often the blade forms more than one plane (see drawing of Elm leaves, Chap. XVI, Illus. 105) ; it may be because the two halves from the midrib form a wide V--this is usually the case with the Sweet Chestnut, Buckthorn, Goat Willow, and many young leaves. Sometimes the leaf is curved from base to tip or twisted on its axis--noticeably in some Willows. The Holly leaf attracts attention by its many diverse planes with the consequent power of reflecting different lights on the same leaf. The leaf surface itself is drawn into various forms by the ribs. These forms are very diverse, but are characteristic of each species. In some the surface between the ribs is deeply puckered or bulging, in others merely wavy or wrinkled. On suckers (shoots produced from adventitious buds on the roots), and vigorous shoots from pollard stools, the leaves are often abnormal in size, shape, and colouring.
Fig. 246 Fig. 247
THE WAY FLOWERS ARE ARRANGED
THE mode in which flowers are arranged is called the inflorescence. There are two distinct kinds of inflorescence, and to one or the other of these belong the various arrangements of flowers to which distinctive names have been given :
(1) Indefinite inflorescence (Fig. 251). This term is used to explain that the main axis of the flower system is not terminated by a flower ; but that the main axis gives rise to lateral stalks, each terminated by a flower.
(2) Definite inflorescence (Fig. 252). This term, as its name implies, is used when the main axis (as well as the lateral stalks) is terminated by a flower.
One of the most important differences in these two systems is seen in the comparative succession in which the flower-buds open out into flowers ; and it will save confusion if we confine ourselves first to an explanation of these different effects.
(1) Indefinite Inflorescence.--In this system, the main axis gives off flowers in succession from the side of the growing point of the axis until it ceases growing. If the main axis is so short that the lateral stalks bearing flowers seem to radiate from one point (Fig. 253), the buds on the outer stalks will be the first to open into flowers ; while the buds in the centre of the mass (because they are the newest formed) will be the last to open. The same order in the expansion of the flowers takes place when the main axis is not shortened ; but the flower stalks are successively shorter towards its tip, so that they and those below on longer stalks are brought up to the same level (Fig. 254). If the main axis is lengthened (and the lower flower stalks are not elongated as in the last example), the flowers at the base of the axis will be the first to open, followed successively by others to the tip of the axis, where buds are newly formed (Fig. :255),
(2) Definite Inflorescence.--ln this system (Fig. 256), the flower-bud terminating the main axis is the first to develop, and is followed by others formed below it. These spring from the axils on the main axis, or from the axils on the secondary stalks. In the latter case, each secondary stalk--with its branchings--follows the
same plan of the central flower, developing before the outer ones (Fig. 257).
Various arrangements are the result of one or the other of these indefinite or definite types.
(1) Indefinite Inflorescence.--Taking first the (1) Indefinite Inflorescence, we find it broadly divided into
arrangements known as Raceme, Catkin, Corymb, Umbel, and Capitum.
The term Raceme is applied when a central growing stalk bears on its sides axillary stalks tipped with flowers. These stalks are produced successively, and carry a bract at their base (Fig. 258). The Bird Cherry inflorescence (Illus. 109, p. 229), with its long central stalk and its stalked flowers developing successively from the base of the main stalk to its tip, where buds are still forming, is a good example ; the Sycamore (Illus. 115), Acacia, Laburnum, and Field Maple are others. The axillary stalks on a raceme are of nearly equal length. When a raceme is compound-by some of the secondary axes being branched--it is called a Panicle.
The Catkin consists of a main stalk that is deciduous, having attached to it, without stalks of their own, the unisexual flowers. Catkins are carried by the Poplars, Willows, Hazel, Sweet Chestnut, Birch, Alder, Oak, Walnut, in length and Plane (Illus. 66, 90 ; 117-119 ; 129-140, pp. 163, 196 ; 252-254 ; 272-277 ; 281, 286). Poplar catkins are pendent, while in most of the Willows they are erect. The catkins of the Poplar consist entirely of male or female organs. The Sweet Chestnut catkins carry the male flowers on the base portion, and from there to the tip the female flowers. On the Hazel, Oak, and Walnut, it is only the male flowers that are produced as catkins.
The Capitum,--The Plane has the male and female flowers compressed to a ball, suspended by the catkin stalk ; like beads threaded at intervals on a string, the arrangement is known as a " Capitum " (Illus. 120).
ILLUS. 115. EXAMPLE OF RACEME-SYCAMORE FLOWERS
ILLUS. 116. EXAMPLE OF RACEME--MAPLE FLOWERS
(See larger drawing Chap. XXI)
ILLUS. 117. EXAMPLE OF CATKIN--THE HAZEL
The Corymb.--If you make a drawing of a raceme, and then alter the length of the axillary stalks--making those at the base the longest and the rest successively shorter to the tip, so that they carry their
ILLUS. 119. EXAMPLE OF HANGING CATKINS. THE FEMALE FLOWERS OF THE BLACK POPLAR
ILLUS. 120. EXAMPLE OF A CAPITUM ON CATKIN STALK. THE CATKINS OF THE PLANE TREE
(For earlier stage of development see Chap. XXI)
ILLUS. 121. EXAMPLE OF CORYMB-HAWTHORN BLOSSOM
ILLUS. 122. EXAMPLE OF UMBEL-FLOWERS OF THE CRAB APPLE
ILLUS. 123. EXAMPLE OF PANICLED CYME-FLOWER HEAD OF THE PRIVET
ILLUS. 124. EXAMPLE OF CYME-LIME TREE
flowers on a domed or even line--you construct a Corymb (Fig. 259) ; example, Hawthorn (Illus. 121).
The Umbel.--This differs from the corymb in having all the axillary stalks, that are placed on a main axis, so short as to make them appear to radiate from one common point like the spokes of an umbrella (Fig. 260). At the point of insertion, they are encircled by a necklace of collected bracts. Each bract subtends a stalk ; but, from the compressed length of the axis, they, like the stalks, seem to start from one point. These flower-bearing stalks are of nearly equal length, so that the head of the flower-cluster is flat or slightly domed. The flowers of the Crab Apple (Illus. 122) and Gean (Wild Cherry) are examples. Umbels are not restricted to the simple form of a main axis, bearing secondary stalks with flowers ; the secondary stalks themselves may be branched, and with the tertiary stalks (that carry the flowers) repeat pound Umbel the construction of the original plan (Fig. 261).
(2) Definite Inflorescence. --This arrangement, as like main axis we have said, consists of a main axis terminated by a flower (so that new flowers have to spring from points on the axes below the flower already formed). This arrangement is known under the general name of Cyme (Fig. 262). The branching of the secondary stalks is carried out in the same manner as they were divided from the main axis, and the tertiary stalks again in their turn follow same routine. When three lateral axes radiate from one point below the terminal flower so that it resembles a compound Umbel, it is called a Cymose Umbel. (Fig. 263.) Other compound forms of the Cyme are known with descriptive adjectives, as Panicled Cyme, Corymbose Cyme. The Privet (Illus. 123) is an example of the former, and the Elder of the latter. Comes are formed by the flower-clusters of the Lime, Rowan, Spindle, and Elder (5-branched).