Plant bodies - building plants from cells and modules
Shoots may also grow horizontally (plagiotropic growth) perhaps producing a creeping stem, such as the
creeping shoots of ivy that snake along the woodland floor until they find a tree or other support to climb up.
horizontally underground stems, or
rhizomes, may act as storage organs and give off vertical branches at
intervals which grow above ground. Alternatively shoots may grow vertically (
orthotropic growth). Things are
not so simple, a branch may grow horizontally along one portion of its length and vertically along another. Such
a branch commences growth in one direction and at some point secondarily changes direction. Light and
gravity are important stimuli sensed by the plant to help it determine its growth form or habit. Some shoots may
possess a joint (pulvinus) about which they bend. A tree that falls over but remains rooted may change
direction of growth, causing new growth at the tip to bend upwards vertically (or one of its branches, now
vertical, may become the dominant shoot and form a secondary stem). Shoots may also grow vertically initially,
but curve over under their own weight. In
Prunus (e.g. plum trees) the shoot apex grows horizontally but then
later curves over so that the basal or proximal portion (oldest part of the shoot) forms a vertical (or slightly
curved) stem whilst the tip of the stem grows lies more-or-less horizontal. In
Prunus the branches also grow
more-or-less horizontally. (This growth habit is described by the model of Troll). Plane trees (
Platanus) also
grow in this way and each largely horizontal branch may arch down and form roots where it contacts the soil,
allowing a new stem to grow upwards at this point ('self-layering' a form of asexual reproduction).

Not all plants belong to the flowering plants or angiosperms, conifers produce cones with naked seeds instead
of flowers and fruit that completely enclose the seeds. Ferns, club mosses, horsetails and bryophytes (such as
mosses and liverworts) are examples of spore-producing plants that also produce no flowers. However,
angiosperms are the most evolved of the plant groups and conifers are very similar in many respects, so we
shall consider the angiosperm first. Angiosperms include herbaceous plants, such as daffodils and dandelions,
grasses, bamboos and palms, and woody plants like hawthorn shrubs/small trees and large trees like the oak.
The gymnosperms include conifers and other woody trees that have no fruit, such as yew trees, Scot's pine
and sequoia.

Plants are modular organisms

The angiosperm plant body is divided into the shoot system, which constitute the parts that are usually above
ground, and the
root system which is usually below ground. Both root and shoot systems are modular - that is
they are made from repeating modules fitted together. A module is a repeating unit, for the shoot system, a
module consists of a branching unit - typically a branch, leaf and axillary bud in the join between leaf and
branch. The plant grows by successively adding more modules, and modules to modules. Thus, a bud gives
rise to a new shoot, such as a twig, with its own leaves, while the older modules grow thicker. These modules
are not put together haphazardly, but in specific patterns - the so-called branching pattern. Each module has
all it needs to become a whole new organism - cut a shoot from a tree and plant it and it may grow roots and
become anew tree. Willow trees are particularly good at this, and can regenerate from a single twig. Indeed,
willows use this strategy deliberately to reproduce - they prefer to grow near to water and they shed twigs into
the water, get carried downstream and if they root in the bank then they may grow into new trees. The crack
willow (
Salix fragilis) is so-called because its fragile wood tends to split under its own weight, but this helps the
tree disperse itself as twigs and branches fall into the water.A leaf, however, will not normally grow into a plant
(except in special artificial culture conditions) since it is not a whole module, but only part of one.

Branching patterns

Each flowering plants conforms to one of about 24 branching patterns found among the angiosperms. Growth
may be determinate, with no branches except in the flower head which tops the single straight stem.
Determinate growth is so-called because it is genetically predetermined that the plant or shoot will grow so
long and then stop, ending in a flower. The inflorescence of a foxglove is one example. Growth may be
indeterminate, continuing more or less continuously as existing modules continue to elongate or new modules
are added (though eventually reaching a variable limit). In
monopodial growth, the stem is constructed of a
single straight shoot, bearing side-branches, with the single axis (monopodium) developing from a single apical
bud that continues to grow, continuously or periodically. The coconut palm (
Cocos nucifera) consists of
indeterminate monopodial growth. The talipot palm (
Corypha utan) has a determinate monopodium, with the
single axis ending in an inflorescence (flower bearing shoots) at which point the apical bud ceases to grow any
further. Most conifers, e.g. fir trees, are also monopodial, with branches radiating in whorls from a central axis
derived from the same terminal bud which continues to grow, producing the classic conical Christmas-tree
shape. This shape is good for shedding snow.
Prunus trees (such as plum trees) have monopodial trunks.

The trees with which most people are familiar, such as oak trees and maple trees, exhibit
sympodial growth.
Sympodial plants are the  truly modular plants, with the stem consisting of a series of modules stacked one on
top of the other, with each terminal (apical) bud ending in a flower at some stage in growth and the main stem
or branch continuing its growth with the extension of the previous modules axillary bud, forming a new module in
which again the apical bud ends as a flower whilst the axillary bud continues growth. Examples of this
indeterminate sympodial growth include the oak tree and the maple (broad-leaved trees). The sympodial plant
may  still appear to have a single straight axis (formed from a series of modules), however, but close analysis
reveals the history of its growth.

Sometimes, the trunk may be monopodial and the branches sympodial, as in
Sterculia species ('tropical
chestnuts').

Below: bifurcation ratio and branch order:

Bifurcation Ratio

Below the European Oak, Quercus robur. The trunk and branches are both sympodial,helping to give the trees an undulating serpentine appearance. (Click image for wallpaper size!).

Oak
              Tree

There are additional ways to describe the branching patterns of plants. One of these is the branching or bifurcation ratio, BR. BR is the number of distal branches divided by the  number of proximal branches. for example, in shade-adapted Fraxinus americana (American Ash) the stem and each branch terminate in two branches, on average, giving a BR of 2:1. In contrast, sun-adapted Fraxinus americana have a BR of 9:1, that is the main stem will put our 9 lateral branches, on average, giving a more 'fir-tree' like appearance. Work on shrubs has shown that only three orders of branching represent the branching pattern of the entire shrub, since the pattern repeats. In other words, one need only consider the smallest (primary) twigs, their parent twigs and the branches the parent twigs are attached to, higher order (larger) branches will reflect the same pattern 9in fractal fashion: zooming in causes the pattern to repeat).

According to one study, sun or shade made little difference to the BR of Quercus rubra (Red Oak) but the first order terminal shoots were elongated (by about 50%) and the average branch angle (the angle between daughter branches at the apex of the parent branch) increased from 46 degrees in the sun to 77 degrees in teh shade. Thus, shade-adapted plants had more spreading branches which were elongated.

The order of the branches is indicated in the figure above. This system of ordering branches is also used in the study of rivers and is the Horton-Strahler system. In this system, the smallest terminal branches are order 1 and two order 1 branches converge to form an order 2 branch. If branches of different order meet then the order is not changed, so an order 1 branch meeting an order 2 branch does not change its order (in other systems it might change the order of that branch segment to 3, such a system rating the order by physical size). Essentially, we have computed branching ratio (BR) as the number of branches of a given order, divided by the number of branches of the next highest order. Alternatively, we can use the following formula for the whole plant:

    Rb=N-NmaxN-N1{R}_{b}=\frac{N-{N}_{max}}{N-{N}_{1}}

Where: Rb is branching ratio, N is the total number of branches of all orders, Nmax is the number of branches of the highest branching order and N1 is the number of branches of first order. For example, for the tree shown above right: there are 15 branches in total (including the main stem) and 1 order 4 branch, so Nmax = 1, giving us (15 - 1 = 14) 14 on the top; there are 8 first order branches, giving us (15 - 8 = 7) 7 on the bottom and thus the branching ratio = 14:7 = 2:1 (or 2) as required.

Above: the generalised angiosperm (flowering plant) body. This diagram shows the baulk-plan of a
flowering plant. Based on (redrawn and simplified) a diagram in Esau, Anatomy of the Seed Plant 2nd
edition, Wiley. (One of the best botany books ever written).
Plant cells             Multicellularity               Modularity
Trunk_section_tilted
Plant cell
Why do trees (and other plants) branch?

A tree or shrub absorbs both light and carbon dioxide from the atmosphere. Plants these need things in
order to grow. The light energy is used to convert the carbon dioxide into fuels and materials to build the plant
body, in a process known as
photosynthesis. (Water and minerals are also required for photosynthesis and
these are generally obtained from the soil supplied by the roots). Carbon dioxide and light are absorbed by the
leaves. The stem and branches serve to position the leaves high in the air, so that the plant can access the
light and carbon dioxide without its neighbours stealing these resources first - taller plants will overshadow
shorter plants. The question now becomes: why not have a solid green sphere instead of a branching canopy?
Each leafy branch absorbs carbon dioxide from the surrounding air, leaving a zone of carbon dioxide depleted
air around it. It then relies upon
diffusion (the random motion of molecules like carbon dioxide) or advection
(bulk air movements or wind) to bring in new supplies of carbon dioxide from the surrounding air further from
the branch. If the branches are packed too close together (or if the canopy is a solid mass) then neighbouring
parts of the canopy will compete and some regions will not obtain sufficient carbon dioxide. In fact
computer
simulations
(using the diffusion equation) have shown that alternating regions of low and high carbon dioxide
concentration would develop around the canopy. It makes no sense for a tree to position foliage in the areas of
low concentration, since such foliage will consume more fuels and materials than it produces. The solution is to
break the canopy up into branches and position the branches an optimum distance apart so that they do not
deplete one another's supplies of carbon dioxide. Computer simulations predict that some 20 or so different
branching patterns achieve this optimum and most of these are seen in nature, but no tree species confirms to
any pattern of branching that is sub-optimum.

Thus, trees branch so as to break up their canopy in such a way that maximises their absorption of carbon
dioxide and light. The final pattern of branching is both genetic (and so dependent upon tree species) but also
a result of how the tree responds to its environment as it grows. Growing shoots will seek out light (and
presumably carbon dioxide) and leaves will position themselves to catch the light. In some plants these
movements occur as the shoot grows, but in many plants the leaves maintain some ability to move about when
mature and in some species they may undergo daily movements as they follow the sun.

Leaves are also hinged - look at the end of a leaf stalk where it joins the branch and you will see a swollen
region (the
pulvinus) which permits movement of the leaf. The pulvinus and leafstalk allow the leaves to rustle
in the breeze. This rustling movement serves to mix the air around the leaf, replenishing the carbon dioxide
around the leaf, and also helps keep the leaf cool and helps it to resist high winds (a stiffer structure may more
likely break). Leaves may also break up their contour, forming lobed leaves with finger-like lobes, such as in
the maple - this also optimises absorption of carbon dioxide and leaf cooling. Indeed in some plants, the 'sun
leaves' at the top of the plant may have a very different shape to the 'shade leaves' near to the base of the
plant, and this probably reflects the greater need of the sun leaves to keep cool. Trees such as the aspen and
poplar have particularly mobile leaves and are famous for the way their leaves rustle in even the slightest
breeze.
Above: simplified branching patterns

The simplest branching pattern is
dichotomous branching (left) in which the growing tip forks into two,
more-or-less equal branches, each of which continues growth with repeated forking a certain number of times
(in this example there are three orders of branch plus the main stem). In true dichotomous branching, the
growing tip meristem itself splits into two meristems. This kind of growth is seen in many seaweeds, liverworts
and only a few flowering plants, such as certain cacti and palm trees. Dichotomous branching is rare in
flowering plants, and when it does occur it may involve the growth of two lateral shoots on either side of the
central shoot whose growth is suppressed, this is called false-dichotomy.

Sympodial branching (centre left) is similar to dichotomous branching except that one branch in each fork
dominates, with the dominate branch alternating (left-right-left in the example shown). The alternate shoots
may grow upright initially, before leveling off, giving rise to a single sympodial trunk. This growth pattern is
common in broad-leaved trees and examples include the oak tree (
Quercus).

Monopodial growth, right, occurs when one central axis and its terminal meristem dominate growth. This is
seen in many flowering herbaceous plants, in which the central axis eventually terminates in an inflorescence
(cluster of flowers) in determinate growth, as in many orchids. Some orchids, however, are sympodial.

The flowering plant body


When one first examines the patterns and morphology of plants in detail, the diversity of shapes and forms
can be bewildering! Things make more sense once one understands a bit about plant development. Plants
grow from
meristems, localised regions where cell division occurs. The cells produce enlarge and mature,
differentiating as they do so into the full variety of plant cell types needed by the growing organ. Apical
meristems
occur in the tips of growing shoots and roots and cause these organs to elongate. Lateral
meristems
may be involved where the plant continues to grow thicker, such as the cylinder of cambial cells
between the inner bark and wood of a tree or shrub.

Many of the specialised structures that plants produce are modifications of the basic structures. Flowers are
compressed (shortened shoots) on which the whorls of leaves develop instead into sepals, petals, carpels
and stamens. Flower clusters may form where an entire portion of the shoot system, complete with all its
branches, forms a cluster of flowers, which may, for example, be arranged on a central spike or on a single
platform resembling a single flower, as in Asteraceae (Compositae). For example, the 'flower' of a daisy is in
fact a compressed shoot system made up of a number of flowers and reduced leaves (ligules) with the flowers
on the margin being modified to resemble petals.

It becomes apparent that plants are actually made-up of repeated branching units or
modules, which may be
more-or-less modified. The typical angiosperm body consists of the shoot and root systems. The shoot
system consists of one or more main axes bearing side-branches at regular intervals on modified sections of
stem called
nodes. The segments of stem between nodes being called the internodes. The side-branches
develop from buds in the axils of leaves, the
axillary buds. The shoot grows as new cells form behind the
apical meristem in the shoot tip or apex. The leaves may alternate from left and right, sometimes forming a
spiral or helix around the stem or branch axis; or they may occur in pairs opposite one-another at the same
level on either side of the axis. The branching may adopt a three-dimensional pattern with branches and
leaves surrounding the main axis in circles or spirals called whorls, or the plant may show some degree of
planation or tendency to branch in a two-dimensional plane, assuming a flattened form. Typically one shoot
apical meristem, perhaps the main central stem, will assume
dominance and other side branches will only
grow to a lesser degree or even remain as dormant buds.
The angiosperm body
Branching Patterns in Horizontal and Vertical Shoots

As an example, Petrokas (2011) has given a detailed description of branching patterns in Wych Elms (Ulmus glabra). As a small tree that sometimes grows as a shrub, Wych Elm displays a variety of branching patterns in its vertical (orthotropic) shoots. Vertical shoots may sometimes branch near the base, forming two co-dominant axes, or several axes may be present at soil level (multi-dominant axes), each such axis being a dominant or leader axis. Alternatively, the main axis may fork or trifurcate (branch into three) near ground level with one branch becoming the dominant leader; or the stem may divide higher up or not divide at all, putting out lateral branches.

Plants are modular organisms, typically one module being added to each growing shoot each year. Even in tropical climates, with more-or-less constant conditions, most plants exhibit rhythmic growth. The lateral shoots put out each season by the elongating leaders can differ in their position upon the new segment of stem. They can also differ in their vigour of growth or extent/rate of elongation. It is important to note whether one is referring to the whole plant or a single growth module. In acrotony, the shoots are either positioned near the apex, or the apical shoots grow more, in basitony the basal shoots are given preference and in mesotony the middle shoots, as illustrated below:
In monopodial conifers these growth forms give rise to whorls of long branches (whorl branches) interspersed by smaller interwhorl branches. These forms are typical of a genus, but can differ with species. Atlas Cedar (Cedrus atlantica) is acrotonous, for example.

acrotony

mesotony

basitony

Additionally, further levels of branching can be described, in particular the lateral shoots branching from a parent or principle shoot (P) which is horizontal or nearly horizontal (plagiotropic) may develop asymmetrically. When the main branch (M) emerges from the underside (hypotony) and finally, if the lateral side-shoots are favoured, then this is known as amphiotony. Amphitony occurs in Fir trees and Wych Elm, for example, and results in planar branches, although the side-shoots may angle upwards (amphitony-epitony) as is often the case in Wych Elm (Petrokas, 2011) or downwards (amphitony-hypotony).

horizontal branches

horizontal branches

Hypotony occurs in the cactus Opuntia fulgida, for example, and epitony in the Black Walnut (Juglans nigra). These terms can also be applied to leaves on the plagiotropic shoots of many plants (the leaves on orthotropic shoots tend to be arranged in symmetrical whorls). These terms can also be applied to sympodial trees, in which case we speak of secondary acrotony, etc.

Castellanos et al. (2011) studied branching in Buxus vahlii (Vahl's Boxwood). The diagram below is based on their work: on the left is a young juvenile tree, less than 1m tall, and on the right a mature tree about 5m tall. Note that the branching pattern is sympodial with axilary buds giving rise to the next module in the series. The lower branch of the mature tree is a later addition, produced in a process called reiteration. This occurs either in favourable conditions or in response to damage and involves the activation of a dormant (epicormic) bud which repeats or reiterates the growth form of the entire parent plant on a smaller scale. It is important to realise that the final form of the tree depends much on environmental factors, in addition to the underlying inherent branching algorithm which is essentially genetically determined.

In senescent trees some of the axes begin to die, in the case of Buxus from the top down, and reiteration occurs on some of the branches and may also occur on the main stem as dormant buds become activated to help compensate for the loss, which of cause eventually wins and becomes total.

buxus branching
References

Barthelemy, D. and Y. Caraglio, 2007. Plant architecture: a dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny. Ann. bot. 99: 375-407.

Castellanos, C.; D.A. Kolterman and H.F.M. Vester, 2011. Architectural analysis of Buxus vahlii Baill. (Buxaceae) in two different environments in Puerto Rico. Adansonia ser. 3, 33(1): 71-80.

Courbet, F.; J.-C. Herve; E.K. Klein and F. Colin, 2010. Diameter and death of whorl and interwhorl branches in Atlas cedar (Cedrus atlantica Manetti): a model accounting for acrotony. Ann. for. sci. 69: 125-138.

Courbet, F.; S. Sabatier and Y. Guedon, 2007. Predicting the vertical location of branches along Atlas cedar stem (Cedrus atlantica Manetti) in relation to annual shoot length. Ann. for. Sci. 64: 707-718.

Petrokas, R. 2011. Height growth and its relation to the branching habits of Wych Elm (Ulmus glabra Hudson) in Lithuania. Baltic Forestry 17(1): 83-90.


Article last updated:

22/2/2014
8 Oct 2018