Above: a growing twig of horse chestnut (Aesculus hippocastanum) with a terminal bud
beginning to open. The terminal bud or apical bud would have been dormant over winter
and is protected by the reddish scale-leaves (cataphylls) which secrete a protective sticky
substance. These cataphylls protect the bud from extremes of winter weather, insects and
disease. They are the last leaves to develop the previous growing season and do not fall
when other leaves are shed.
Along the twig can be seen triangular scars with a horeshoe-shaped array of dots. These
are leaf scars formed when earlier leaves were deliberately shed. As twigs grow at their tips
(primary growth) before thickening (secondary growth) the most recent leaf produced the
leaf scar nearest the end of the twig: this would have been one of last summer's leaves
which was shed in autumn during leaf fall. In leaf fall, the leaves are deliberately cut-off by a
layer of corky tissue (containing suberin) which grows across the base of the leaf stalk.
Finally the ends of the broken vessels are sealed, forming the horseshoe row of dots. This
is the process of leaf abscission.
In addition to leaf scars an array of annular scars can be seen - circular ridges
encompassing the girth of the twig at intervals. These are the scars formed by previous
cataphylls and mark a previous over-wintering terminal bud. The distance between one set
of annular scars and the next thus indicates one year's growth. A nice little project is to
measure these distances to plot the amount of growth in a twig over recent years and to
compare exposed and more shaded twigs, or plants in more shaded parts of a wood to
those in more sunlit parts.
Also visible on this twig are what appear to be a few scale insects, presumably the
horse-chestnut scale insect, Pulvinaria regalis. These are the wingless females, which once
mature remain immotile in scale insects, fixed on one spot where they feed on the tree's sap
with their piercing mouthparts. The winged males do not feed and only live a day or two.
Plants generally grow in length from the terminus of their shoots. At the terminus is the
apical meristem, a region of cell division where new cells are produced. This will be
protected by developing leaves which surround it, or by the terminal bud. This meristem
suppresses growth in side-shoots by apical dominance: it releases hormones which travel
down the shoot to reduce the growth of side-branches and buds. Flowering plants produce
a bud in the axil of every leaf, the axillary bud but the apical dominance of the terminal bud
keeps these axillary buds dormant, unless the terminal bud is damaged or removed, in
which case the main axis can no longer elongate and in this case one of the axillary buds
becomes active and produces a new branch.
Above and below: growing twigs of ash (Fraxinus excelsior) with the characteristic ash-black
non-sticky buds. The dormant axillary buds are clearly visible, one above each leaf scar.
Apical Bud Types
Terminal buds may open to produce leaves or they may open to produce a flower, as in elms
and cherries. If they produce a flower then growth of the terminus will end and one or more
axillary buds will produce a branch when growth resumes. In the horse chestnut, a pair of
axillary buds resume growth and the twig forks. Some trees, e.g. plum trees, can also
produce mixed buds which give rise to flowers on some side-shoots, leaves on others, so
that growth can continue.
In some trees, only the stipules of the outer bud leaves form the cataphylls, the rest of the
leaf expanding when the bud opens, e.g. Tilia (linden, lime tree). Some trees do not form
cataphylls, but simply form naked buds where the last leaves cease growth and serve to
protect the meristem and resume growth when dormancy ends. This occurs in some
eucalypts. Junipers do not form distinct buds at all, but produce new growth from
meristematic tissue concealed beneath the bark of the twig.
Ashes, horse chestnuts, oaks, beech, hornbeams, walnuts, some maples and conifers
preform next year's shoot and package it in a miniaturised form inside the bud. When the
bud opens growth is extremely rapid as the shoot simply expands over a few days or weeks.
This is called determinate growth. In some trees, only part of the shoot is preformed,
whilst the rest is produced de nova, and these usually give rise to basal leaves which differ
in shape and/or size from those produced de nova further along the growing shoot. In silver
birch (Betula pendula) the preformed leaves end-up distinctly wider. This kind of unrestricted
or continuous growth, which is not pre-determined, is called indeterminate growth and
occurs in elms, Tilia (lime), cherries, birches, junipers, coastal redwood, western red cedar
and ginkgo. In both cases, trees may show a second burst of growth from the terminal buds
later in summer, typically early August and thus called lammas growth. In some trees there
may be several cycles of growth and bud formation in a single year (rhythmic growth) as in
some fast-growing pines.
Apical dominance ensures that not too many buds open. If all buds opened then the leaves
would be overcrowded and shade one-another, reducing the efficiency of photosynthesis.
However, trees produce more buds than they need in case frost, disease, insect pests or
grazing animals, or structural damage, destroy many of the buds. Buds that end-up surplus
to requirements may be aborted or remain dormant. Whole twigs and branches which
become unproductive due to shading can be deliberately shed in some trees. In this
process, called cladoptosis, cork cells grow across the base of the branch, sealing it off
until it detaches or abscisses. Branches up to 1 metre in length may be shed this way. Elm
trees are notorious for suddenly dropping much larger branches, up to 0.5 m in diameter.
This occurs by a different mechanism: in hot conditions water stress and thermal expansion
may crack the wood and it simply loses strength. Willow trees have a brittle zone at the base
of small branches which can thus easily break off in the wind. Such twigs may flow
downstream, since willows typically grow near water, and are able to root downstream and
develop into a new tree.
What makes a bud dormant? The plant hormone (phytohormone) abscisic acid (ABA) (a 15C
isoprenoid synthesised from three 5C isoprene units) is the most widely used growth inhibitor
in plants. This and perhaps other growth inhibitors accumulate, for example, in the dormant
buds of ash trees and the levels of these chemicals in the buds declines when dormancy is
broken. ABA is primarily synthesised inside plastids (chloroplasts and related organelles).
This hormone is transported mainly in the phloem, but also in the xylem and parenchyma of
the vascular bundle sheath (in the latter transport appear to be non-polar, that is it occurs
equally in either direction, in contrast to the polar transport of auxins).
ABA has several different functions in plants. It seems to be responsible for closing stomata
during water stress or when photosynthesis stops in the evening. It also induces bud
dormancy and seed dormancy. It also appears to trigger abscission in the leaves, flowers
and fruits of a small number of plant species.
In the case of bud dormancy, the ABA is manufactured in the chloroplasts of leaves and
transported to the buds when the leaves sense decreasing day length (via the phytochrome
sensor). Short days and low temperatures sensed by the plant trigger terminal bud
The dormancy or slowed growth of lateral or axillary buds that normally occurs to a greater
or lesser extent behind the shoot tip is due to the suppression of these buds and meristems
by the apical meristem and referred to as apical dominance. This is the result of the
production of the hormone auxin by the apical meristem. The auxin travels down the shoot in
the parenchyma cells (by polar transport). This causes axillary buds to remain dormant or to
produce shoots which grow more slowly as horizontal branches. Removing the apical shoot,
such as by pruning, releases this inhibition and more axillary buds may break their dormancy
and existing branches increase their growth, with one or more becoming new dominant axes
which may switch to vertical growth. This often increases the plant's bushiness.
Beneath the cover of higher vegetation, the ratio of red to far-red light is lower than it is in
direct sunlight. Plants can sense this using phytochrome. Plants that normally grow in the
open will respond by inducing axillary bud dormancy, investing their growth instead in the
main axis which then has a better chance of reaching open light. However, plants which
normally grow in the shade of taller plants will not respond in this way - a herb has no
chance of out-competing trees for height!
Thomas, P. 2000. Trees: their natural history. Cambridge University Press.
Above: a twig with buds and flowers of a cherry tree, Prunus.