Plant Architecture

Plant Architecture - Buds, Twigs, Phyllotaxy and Vernation

Plant Buds

The bud of the Horse Chestnut (Aesculus hippocastanum) is a closed or scaly bud in which bud scales protect the developing organs/appendages. The outer scales are dark brown, leathery and sticky with resin. Each pair of scales is at right-angles to the next pair inside of it, with each outer pair partially covering the pair inside it. Typically, moving from the outside to the inside of the bud we encounter 1) three pairs of increasingly large scales, then 2 or 3 pairs of maximum size scales; 2) 1 or 2 pairs of green scales or intermediate leaves coated with resin; 3) a pair of very hairy young green leaves and 4) the developing shoot apex, which consists of a branched group of flowers making up the inflorescence bud.

Examining buds and twigs not only allows woody plants to be identified in winter, but also gives us tremendous insight into how these plants grow and develop their final form: it gives us insight into teh architecture of woody plants. The buds of the Horse Chestnut are particularly large.The sticky resin may deter insects from eating the bud but its main function is thought to be to prevent the delicate structures of the bud from dehydrating. Note the ring-like scars from previous years' bud scales and the horseshoe-shaped scars from previous leaves, complete with sealed vascular bundles (visible as tiny dots within the scars). Alder (Alnus) and poplars (Populus) also have resinous buds, but many plants do not produce bud resin.The young immature leaves of Horse Chestnut are very hairy, which may also reduce the drying effects of wind by trapping a layer of still air close to the leaf, important in plants that bud break very early in Spring.

The outer scales of Horse Chestnut buds are corky and coated with resin whilst the inner scales are covered in interwoven hairs: these adaptations reduce water loss.

Above: the upright flower spike (flower candle) of Horse Chestnut (Aesculus hippocastanum) with accompanying leaves having emerged and expanded from a bud in Spring.

The shoot (stem or branch) of a plant grows by means of an apical meristem which elongates the shoot near the tip by producing new cells and gives off leaf primordia that develop into leaves, a leaf being an extension of the shoot tissue. In general each leaf will be accompanied by an axillary bud (lateral bud) just above the leaf's point of insertion on the shoot axis. Each axillary bud encloses its own axillary meristem. Axillary buds may grow immediately, along with the apical meristem / terminal shoot (a process called syllepsis, lit. 'occuring together'). Otherwise the axillary bud will remain dormant for some time before elongating (prolepsis - elongating after dormancy) into a side-branch to the parent stem. The axillary buds may remain dormant as long as the terminal shoot remains active, with the terminal growing point releasing auxin as a signaling molecule to inhibit the axillary buds (apical dominance). Destruction of the apical meristem, such as by wind damage or a grazing herbivore, releases apical dominance, allowing the axillary buds to shoot.Both axillary and terminal shoots may become dormant during winter or other unfavourable conditions.

The nature of bud scales

In the Horse Chestnut the bud scales are modified leaf bases, that is they are modified leaves of which only the base develops. This is similarly the case in the buds of Maples, Lilac, Lime, Ash and Rose. These modified leaves usually lack buds in their axils. (The axil refers to the angle between a leaf and the main axis - stem or branch - bearing it).

In the buds of Pine, up to 100 or more scales are arranged spirally (strictly in a helix: botanically 'spiral' usually refers to a helix) with a bud in the axil of each. Each of these secondary buds consists of a few minute brown scales (perulae) sheathing two young leaves held face-to-face. Each bud scale in a pine bud is a whole modified leaf.

In the Beech and Hornbeam the scales occur in pairs and the inner bud scales bear leaf rudiments between each pair. This indicates that the bud scales in these trees are modified stipules. Stipules are a pair of small lobes that develop at the base of a typical leaf stalk.

Above:a terminal bud of Hornbeam (Carpinus betulus).

In Poplars the scales are stipules and three are visible externally: the basal scale is a single anterior stipule and within this is a posterior scale that warps around, overlapping its edges anteriorly and inside this is a third visible scale at the tip of the bud. Willow buds (below) are similar except in the Willow the basal scale is double, representing two fused leaves (or stipules?). The scales are also modified stipules in Oaks, Beech, Alder and Magnolia.

In Ivy (Helix)in contrast, the bud is of the naked or open type with very few leaves that are similar to, but smaller than the ordinary foliage leaves. Such open buds represent dormant shoot apices in which growth has been suspended but no special structures have been developed, the immature leaves serving to protect the apical meristem and leaf primordia.

A table of some characteristics of the buds of a selection of species of woody plants is shown below. Can you fill the gaps with your own observations?

Notes: 'several' means between 5 and 12 inclusive.

Bud scales may be dry, brown and papery or leathery as in the Beech and Prunus avium (Wild Cherry or Gean) or they may be greenish as in Lilac and Sycamore / Sycamore Maple (Acer pseudoplatanus). They may be covered in hairs, as in Walnut (Juglans), Downy Birch (Betula pubescens), or velvety as in Ash (Fraxinus) or covered in scaly hairs as in Sea Buckthorns (Hippophae) and Silverberry (Eloeagnus) or covered in resin, as in Alder (Alnus glutinosa), Poplars and Horse Chestnut.

Corky tissue in bud scales, thickened cuticles, hairs and resin all serve to reduce water loss. For example:

bud scales of Oak have thickened cuticles, outer scales are corky and the innermost scales covered in interwoven hairs;
those of Lime and Honeysuckle have layers of corky tissue;
scale leaves of Maples, Sycamore and Beech are hairy;
in Silver Fir, Pines and Birch, the outer scales are corky and coated with resin.

The Arrangement of Buds and Leaves on a Shoot - Fibonacci Series and the Golden Ratio!

Phyllotaxy is the arrangement of leaves on a shoot. In many plants leaves are arranged in a spiral (strictly a helix) along the shoot. Many phyllotaxies can be represented by a fraction, for example ½ means that leaves are arranged in two rows along the shoot (the shoot is distichous). More specifically, the numerator (top number) tells us how many 360-degree turns around the shoot axis we have to travel from one leaf to reach the next leaf (or more exactly its point of insertion on the shoot axis) on exactly the same side of the shoot, whilst the denominator (bottom number) tells us how many leaf-insertions we pass on the way, excluding the leaf-insertion we start at. Thus, if there are two rows of leaves on a shoot then traveling 180 degrees around the axis we reach the next leaf on the opposite side of the shoot and traveling another 180 degrees we reach the next leaf on the same side of the shoot, thus we have traveled 360 degrees and met two leaf-insertions: ½ phyllotaxy. Note that this distichous arrangement means that the leaves are exactly alternate: alternating from one side to the other of the shoot as we travel along it. This arrangement occurs in grasses, Lime, Elm and Beech (some sources give Beech as ⅓). For fractions smaller than ½ the phyllotaxy is spiral.

Other spiral phyllotaxies occur.Sedges (Cyperaceae), Alder and Birch display predominantly ⅓ phyllotaxy. This means that there are 3 rows of leaves along the shoot with each leaf separated from the next one by 120 degrees. Thus we have to travel 360 degrees to get to the next leaf in the same row, but pass by two other leaf-insertions along the way, making a total of 3 leaf-insertions excluding the one we started at. This arrangement is also called tristichous.

In Oak, Willow, Pear, Apple and Currant we have ⅖ (2/5, pentastichous) spiral phyllotaxy. This means there are five rows of leaves, separated by 144 degrees (2/5 x 360 = 144): to reach the next leaf in the same row we have to make two full 360 degree turns around the shoot, touching five leaf-insertions on the way. This is best seen in the diagram below:

These phyllotaxic ratios are simplifications, in Oaks and Chestnuts, for example, the angle between successive leaves changes up the stem.

Other spiral phyllotaxies occur, such as 3/8 (ochtostichous) phyllotaxy found in Holly as well as in Radish, Cabbage, Flax, Monkshood and Plantain. Similarly 5/13, 8/21, 13/34, ... also occur in nature.

What is the pattern here?

A Fibonacci series is a series of numbers in which each term is the sum of the two preceding terms, e.g.

0, 1, 1, 2, 3, 5, 8, 13, ...

In spiral phyllotaxy both the numerator and the denominator follow a Fibonnaci series:

1/2, 1/3, 2/5, 3/8, 5/13, 8/21, 13/34, ...

This series of fractions tends to (but never reaches) a function of the irrational limit phi, also called the golden ratio:

What do I mean here exactly? The series of fractions gets closer and closer to (but never reaches) the value:

1/ (φ + 1) = 0.381966011 ...

(1/ (φ + 1)) x 360 degrees = 137.5077 ... degrees

The ratio of one term divided by the previous term in the Fibonacci series tends to phi.

What is the significance of the golden ratio?

The golden ratio allows a shoot to fit a tight mosaic of leaves together without any leaf overlapping another, that is no leaf would be directly above another on a vertical shoot, so there would be a minimum of self-shading! This clearly optimizes light capture. Real plants only approximate this ratio by adopting terms lower in the series. However, plants do not simply rely on genetics to optimizes light capture: a leaf may tilt the plane of its blade to face the sun (as in Maples), petioles (leaf-stalks) may elongate to bring a lower leaf up into the light and petioles can twist. Twisting of the petiole is pronounced in Yew, Box, Hazel and in climbers like Ivy and plants with prostrate stems, e.g. Salix repens (Creeping Willow). Therefore, the phyllotaxy refers not to the positions of leaves exactly, but to the positions of leaf-insertions.

Examples of plants with alternate and distichous phyllotaxy: Lime, Vine, Ivy, Planes, Elms, Beech, Hazel, Chestnut, Cranberry and Cherry Laurel.

Examples of plants with alternate and spiral phyllotaxy: Holly, Broom, Prunus, Pear, Rose, Hawthorn, Blackberry, Ribes (Currants and Gooseberries), Rhododendron, Alder, Birch, walnut, Oaks, poplars, most Willows and most Conifers.

Opposite Phyllotaxies

In spiral phyllotaxies, there is one leaf per node. Although strictly alternate leaves only occur in distichous shoots, other spiral arrangements are often referred to as 'alternate'. In opposite phyllotaxy there are two leafs per node with the leaves on opposite sides, that is separated by 180 degrees around the stem. This is found in Lilac, Elder, Honeysuckle, Maples, Horse Chestnuts, Ash, Cypress (Cupressus) and Cedars (Thuja). Usually the arrangement of the opposite leaves is decussate, meaning that each opposite pair is at right-angles to the pair immediately below and above it. Thus there are four rows of leaves along the shoot. This alternating arrangement of opposite pairs along the stem helps optimize light capture without leaves overshadowing one-another.

It should be noted that distinctions are not always absolute: opposite and alternate leaves may occur on the same shoot axis, as in Salix purpurea (Purple Osier or Basket Willow), Atriplex and occasionally Ash.

Examples of plants with opposite and decussate phyllotaxy: Clematis, Horse Chestnut, Field Maple, Norway Maple, Sycamore, Mistletoe, Dogwood, Elder, lilac, ash, Privet, Box, cedars, Cypress and wayfaring Tree (Viburnum lantana).

Whorled Phyllotaxies

Leaves occur in whorls (verticels) when 3 or more occur at each node. This arrangement is called whorled or verticellate. For example, in Juniper there are usually 3 leaves per whorl; in Paris quadrifolia (Herb Paris) and Heather there are usually 4 leaves per whorl. Numerous leaves per whorl occur in Horsetails and Mare's Tail.

Pseudowhorls occur when there is so little extension of internodes that a number of nodes cluster together, creating the appearance of whorls, but with only one or two leaves per node. This occurs on the crowded dwarf shoots (see below) of Larch and Cedars for example.

Arrangement of buds on the shoot

Buds generally follow the arrangement of the leaves, since in general one bud forms in each leaf axil. thus buds may be opposite and generally decussate, as in Clematis, Maples, Mistletoe, Lilac, Ash, Horse Chestnut, Spindle Tree, Sycamore, Dogwood, Elder, Wayfaring Tree, Privet and Honeysuckle.

False whorls or false verticels of buds occur in Pines, Spruces and Silver Fir, especially towards the branch tips.

A spiral arrangement of buds occurs in: Alder, Walnut, Willows, Red Currant and Gooseberry, Oaks, Poplars, Roses, Blackberry, Apple, pear, Hawthorn, Cherry and Bird Cherry, Almond and Blackthorn.

Things are not always so clear-cut and for example the bud arrangement is described as sub-opposite (i.e. not quite opposite) in Salix purpurea, Silver Fir (Abies pectinata) and Norway Spruce (Picea excelsa). In Oak most buds occur at the tips of twigs and in some Cherries they are clustered on short dwarf shoots. In Planes (Platanus) the axillary bud is hidden by the base of the petiole which almost completely encircles it (except for a small aperture). No axillary buds occur in the lowermost couple of leaves of Birch, Hornbeam, Beech and Lime and are completely lacking in the crowded dwarf shoots of Pine, Larch, Cedar, Beech and Alder. Conifers typically have buds in only a few leaf axils on the long shoots. Conversely, more than axillary bud may occur in a leaf axil, as in Ash, Rubus, Willow, Sycamore and some species of Prunus.

Above: a generic woody twig in its winter dormant state. (Based on: Ward, H.M. 1904. Trees Vol. 1, Cambridge University Press). The ring-like scars on the main axis represent the attachment sites of previous years' terminal bud scales with the main axis resuming growth each Spring. Each branch grew from an axillary bud, the ring-like scars of the bud scales of the axillary bud being visible at each branch base. Beneath each branch base is the leaf-scar of the leaf the axillary bud formed in the axil of. This pattern of axillary branching is repeated for the secondary branches (branches of branches). This gives us a clearer picture of how woody plants grow.

Epicormic Buds

Above: a longitudinal section of a tree trunk showing the vascular supply to the dormant
epicormic buds. (Based on Busgen and Munch, 1929, in Thomas,2000; trees: their natural
history, Cambridge University Press).

Epicormic buds are buds that remain dormant but grow just enough to keep themselves at or near the surface of the trunk. They may remain dormant indefinitely but may activate if the plant loses part of its crown and the buds receive sufficient light (and any auxin block from leading shoots is sufficiently diminished).

Above: epicormic buds (small wart-like protuberances) on the surface of a Beech trunk (Fagus sylvatica).

Types of Shoot

Woody plants produce long shoots called leaders that dominate the growth of the plant and reach out into the light. More shaded buds, however, may remain dormant, abort or undergo slow growth in which the internodes elongate little. These are called dwarf shoots or spurs. On dwarf shoots the nodes become crowded together so that the leaves (and their attachment scars) may appear to be in whorls (pseudowhorls) or rosettes. Dwarf shoots are typically lateral secondary branches. If damage to the crown of the plant opens up spaces then dwarf shoots may activate and become leaders, growing towards the light to plug the gap.

Above: a dwarf shoot of White Poplar (Populus alba).

Dwarf shoots are also frequent in conifers, e.g. Pine, Larch and Cedar. In Pine they develop in the axil of a scale leaf on the parent axis and have few leaves and no axillary buds and often occur lower down on the parent axis. in Willows, on the other hand, all shoots are typically long, more-or-less. In some species, dwarf shoots may eventually bear flowers and fruit or develop into thorns.

Vernation - The folding of leaves in a bud

The folding of an individual leaf prior to expansion is called ptyxis, but the way leaves are folded 9if at all) to package them together into a bud is called vernation (not to be confused with venation which refers to the patterns of veins on a leaf). Flower parts may be similarly folded to package them into a flower bud, a phenomenon called aestivation.

In the Blackthorn (Prunus spinosa) the vernation is convolute, meaning that the leaf blades are rolled vertically in a cylindrical fashion with the upper surface facing inwards, with one leaf margin tucked inside the margin of adjacent leaf on one side and the other margin overlapping the adjacent leaf on the other side.

In conduplicate ptyxis/vernation the leaf blades are folded along the midrib with the two halves of the blade folded together, like the wings of a butterfly, with the upper surfaces closed together. This is found in, for example: Oak, Elm, Hazel, Cherries, Blackberry, Rose, Vine, Ash, Magnolia and Lime.

Apple, Pear, Black Poplar, White Poplar and the leaflets of Walnut all exhibit involute folding in which the edges of the leaf roll inwards towards the midrib, such that the upper surface of the leaf is concave. In revolute folding, in constrast, the edges of the leaf are rolled underneath such that the underside of the leaf is concave and this occurs in the Plane and Rhododendron, for example.

In plicate folding the leaf is folded in the manner of a traditional hand fan. This occurs, for example, in Alder, Beech, Birch, Maple, Sycamore, Black Currant, Red Currant, Hornbeam and the Wayfaring Tree.

In some plants, the leaves are not specially folded but simply miniaturized and compressed together in the bud, as in Mistletoe, Fig, Firs, Cedars, Yew, Privet, Spindle Tree, Broom, pines, Juniper, Honeysuckles, Ivy, Holly, Chestnut, Willows and Larch.

Bud Burst - Bud opening and shoot extension

In some trees seasonal growth resumes from the terminal bud as in many conifers: Pines, Firs, Larch, Cedar, Cypress and Yew, but also in Maples and Sycamores. In some trees, however, the terminal bud may abort and growth may resume from an axillary bud at the end of the branch as in: Birch, Aspen, Hazel, Elm, Willow, Lime, Hornbeam, Poplars and Prunus (though the trunk of Prunus is apparently monopodial). In Beech and Oak, the terminal bud may or may not abort.

When the bud breaks dormancy and bursts open the shoot elongates by lengthening of internodes. The internodes between the basal nodes, which bear scale leaves, barely elongate and produce no axillary buds. The scale leaves soon fall. The middle nodes elongate the most, slowly at first in Spring and reaching a maximum rate in early Summer. Trees that are shade tolerant, such as Silver Fir, Beech, Hornbeam, Lime and Spruce grow slowly. trees that are less shade tolerant such as Birch, Alder, Aspen, Ash and Scots Pine elongate quickly.

Some trees, such as the Oak, may under certain circumstances put out a second bud-burst following trauma such as loss of foliage leaves due to caterpillar attack. This causes the shoots to prematurely enter the winter twig stage and then to open a second round of buds to resume growth whilst conditions remain favorable. Some trees such as Beech regularly put out new shoots called Lammas shoots, perhaps after a dry Summer or during an exceptionally long Summer. similarly, Horse Chestnuts may undergo a second period of flowering.