| Plant bodies - building plants from cells and modules |
Above: the generalised angiosperm (flowering plant) body. This diagram shows the baulk-plan of a flowering
plant. Not all plants belong top 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').
Why do trees 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.
plant. Not all plants belong top 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').
Why do trees 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.
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).
(One of the best botany books ever written).
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| Plant cells Multicellularity Modularity |
Canopy
Leaves and photosynthesis
Flowers
Epiphytes and Climbers
Tree trunks, wood and branches
Wood and bark
Stems
Stem growth
Leaves and photosynthesis
Flowers
Epiphytes and Climbers
Tree trunks, wood and branches
Wood and bark
Stems
Stem growth
