The Sensitive Plant
The Sense of Touch
Climbing plants with
entwining tendrils, like bryony (Bryonia) have touch sensors on their
tendrils. The model above, and below, shows a group of epidermal cells
from the tendril of Bryonia
dioica
(red bryony). Each epidermal cell has one to three hemispherical domes
protruding from its outer surface. These domes contain cytoplasmic
projections of the cell and the cell membrane is especially thin,
giving the dome flexibility to deform when pressed. The sensors
respond most strongly, however, to being pushed from the side, as the
tendrils respond to rubbing against rough surfaces (as would happen
when the wind blows once they contact a surface). The sensors on one
side of the tendril (the ventral or lower surface) inhibit tendril
coiling, those on the other side trigger coiling. The tendrils will
therefore coil in one direction once they contact a suitable support,
wrapping around the object to support the plant.
Above: Bryonia dioica, Red-berried Bryony also called simply Red Bryony and also White Bryony (presumably because of its greenish-white flowers) but this should not be confused with Bryonia alba, also called White Bryony which has berries that ripen straight from green to black and do not turn red. Indeed, the two have been confused taxonomically in the past. As the name suggests, Bryonia dioica is dioecious: it has separate male and female plants, whilst Bryonia alba is often (if not always) monecious: with seperate male and female flowers on the same plant. Bryonia belongs to the Cucurbitaceae family (squashes, pumpkins, melons, gourds and their relatives). The fruit of Bryonia dioica is only about as large as a pea and contains 3-6 seeds. Although the plant has traditional medicinal uses, it is also very poisonous, including the berries, only the young spring shoots have ever been safely eaten, after boiling.
The tendrils respond to rough stroking textures as they brush against a support, but do not respond to raindrops. The leaves are five-lobed, with the middle lobe the longest. The leaves are rough on both surfaces, being covered with short white prickles. Note that once the tendrils grapple a support, they pull the shoot towards it by coiling and contracting. Since both ends are fixed, they can not spiral in a way that would generate twisting tension and so one part of the tendril coils clockwise, another part anticlockwise, with the two coiled segments separated by a short uncoiled hinge. A tendril will coil within minutes of rubbing against a rough surface, but if it loses its grip early on then it will often slowly uncoil and try again. Relying on nearby supports, the stem of Bryonia is not very strong and is quite brittle.
Above: the greenish-white flower of Bryonia dioica, with black angular and ovoid cremocarps of Alexanders (Symrnium olusatrum). The tendency for some Bryonia types to be monoecious and others to be dioecious, made this plant a model for studying the biology of sex determination. Bryonia has X and Y sex chromosomes, Y being the male chromosome, and this was apparently the first experimental system for which genetic control of sex was elucidated. Note, however, this is not the only system of sex determination used in flowering plants.
The diagram below is a 3D representation of the model of the tactile blebs on Bryonia tendrils as elucidated by Engelberth et al. Inside the bleb or dome is a pocket of protoplasm, continuous with the protoplast of the cell through a cytoplasmic canal - a knob-like projection of the cell, lined by the continuous cell-surface membrane. Inside the cytoplasm of this pocket is a ring of membrane-bound vesicles or sacs that store calcium ions (calcium store) and beneath this is a cytoskeletal ring. Surrounding the neck of the bleb, where it passes into the cell wall, is a ring of cellulose microfibrils and around the circumference of the bleb is a ring of callose.
Mechanism
The following account gives a likely model of the mode of action of these sensors, as suported by experimental evidence.
Forces acting on the blep deform the thin cell wall of the protrusion.
In particular, the sensor is much more sensitive to side-on (lateral)
forces (large orange arrow) than to those acting directly from above
(smaller yellow arrow). These lateral forces will deform the callose
ring (which will spring-back into shape when the force is removed) which
in turn pushes against the cell-surface membrane lining the blep cavity,
stretching and deforming it. Note that the callose ring may act as an amplifier
- focusing the force onto a small region of the membrane. When the
membrane is deformed, electrical signals due to the flow of
positively charged calcium ions initiates electrical waves within the
cell, beginning in the blep. Most of this calcium comes from a ring of
membranous organelles that store calcium inside the cytoplasm of the
blep. There is a ring of cytoskeletal proteins beneath this
calcium-storage region.
There are several possibilities: stretching the membrane may open ion
channels (e.g. calcium channels) inside the cell membrane, allowing some
calcium to enter the cell (from the cell wall spaces) making the inside
of the cell less negative (remember cell membranes act as electrical
capacitors and store charge, with the inside of the cell negative
relative to the outside in the untriggered state). This calcium signal
may then trigger the release of more calcium from the internal stores,
triggering a wave of calcium (these steps also act to amplify the
signal). Alternatively, the membrane may pass its signal to the calcium
stores by another mechanism. One such possibility involves the
cytoskeletal ring - the membrane may push on this when it is deformed,
triggering some change in this ring, active or passive, which pulls open
calcium channels in the calcium store. Alternatively, the cytoskeletal
ring may be resistant to deformation and elastic and so help to restore
the shape of the blep to its resting state.
Once the electrical signal occurs throughout the cell, it must be passed
onto others, especially if the signal exceeds a certain threshold. This
could occur via the flow of calcium through the plasmodesmata
that connect the sensory cell to neighboring cells. Alternatively, or in
addition, there may be a flow of electric current through the mesh-like
cell wall. A signal could also be passed from cell-to-cell by activating
the cytoskeleton network in the cell. There are connections between the
cytoskeletal ring of each blep and the mesh of cytoskeletal filaments
beneath the cell-surface membrane and some signal may be passed along
these (possibly a physical signal, according to tensegrity theory).
Slower and longer-lasting signals may also pass between the cells as
chemical messengers or hormones - an octadecanoid molecule is known to
act as such a signal.
After several seconds of suitable stimulation, the tendril begins to
coil. Initially, the coiling is probably driven by osmotic changes -
water moves from one side of the tendril to the other, leaving cells on
one side plasmolysed and flaccid, whilst cells on the other side become
stretched and turgid. This creates a force with the side that loses
water being pushed inwards, on the inside of the curve. If the stimulus
persists, then the tendril undergoes a second phase of irreversible
coiling. This phase is slower, taking 24-36 hours and probably involves
differential growth with cells on the outside of the curve elongating.
In the passion flower, Passiflora caerulea, the tendrils are
sensitive even to cotton thread, but the strength of the response
increases with the force of the stimulus and the roughness of the
surface. The tendrils do not respond to smooth polished surfaces. The
tendrils can coil around an object of any size that the tendril is able
to reach around and tendrils may also coil around one another. Prior to
receiving a stimulus, the tendrils make wide sweeping circular
movements, searching for a support. Coiling starts with about 30 seconds
of stimulation and the tendril is capable of seeking out crevices into
which it goes, and then coils to form a tight plug. Between the support
and the plant, the rest of the plant which has not managed to wrap
around the support, becomes coiled, drawing the shoot in nearer to its
support, and giving the tendril the elasticity of a spring, so that it
can resist buffeting. Each tendril can coil with weight up to about 20 g
attached to it, though the usual operational load rarely exceeds 1 g.
After coiling, the tendril matures, thickening and strengthening. It
takes about 350 to 750 g of load to break a single mature tendril, and
with many tendrils grappling a support, these vines are well able to
resist wind forces.
Adhesive Discs and Suckers
Not all climbing plants rely on tendrils which grapple supportive
structures, some, such as the Boston Ivy use tendrils armed with
adhesive discs to gain purchase on the trees they climb up. The strength
of these adhesions is immense and this has attracted the attention of
engineers wishing to mimic their properties. In the Boston Ivy, Parthenocissus
tricuspidata, a member of the grape family Vitaceae, each tendril
consists of a main axis and 5 to 9 alternate branchlets. Each branchlet
has a small swelling at its tip. This swelling fills with mucilage as
the tendril develops. Repeated contact with a nearby support is sensed
by the tendril and this acts as a stimulus for the final development of
the swelling which differentiates into an adhesive disc.
The surface cells of the disc develops into elongated epidermal cells,
resembling clusters of fingerlike protrusions (each epithelial cell is
one fingerlike process) which grow and mould to the shape of the surface
and then secrete adhesive mucilage. This mucilage contains carbohydrates
and some 21compounds rich in N, S and O: elements which tend to carry a
negative charge and so are good at forming electrostatic bonds with the
surface of the support (opposite charges attract, van der Waals forces).
In addition, the center of the disc raises up, forming a cup-shaped
structure and generating suction (acting as a true sucker) to help
cement the seal. In the mature disc the epidermis degenerates and the
disc shrinks.
Tendrils of the Passion Flower Passiflora discophora end in an
adhesive pad and also usually also bear two side-branches, near the
tendril apex, which branch again with each branch ending in an adhesive
pad, making 5 adhesive pads in all. Each pad begins growth as a callus
which penetrates gaps and spaces in a rough surface and produces
extracellular glue. The tendril axis contains a tough lignified core and
after about 2 to 6 weeks the tendrils die and desiccate with the fixed
woody tendril remaining fixed in place.
Tendrils of climbing plants may also coil, once an attachment has been
made, pulling the climber closer to its support. Virginia Creeper (Parthenocissus
quinquefolia, grape family, Vitaceae) has branched tendrils, with
each branch ending in an adhesive disc that secrete an adhesive which
penetrates tiny gaps, then the tendrils coil.
The English Ivy, Hedera helix, also uses an adhesive, but this
is secreted by root hairs as the adhesive appendages are adventitious
roots (roots growing from the climbing stem). Only these aerial roots
secrete an adhesive consisting of a nanocomposite material which
contains uniform nanoparticles which are organic and 60-80 nm (60 to 80
billionths of a meter) in diameter. It is thought that the matrix of the
adhesive not only forms electrostatic bonds with the surface, but also
with the nanoparticles, giving the adhesive itself tensile strength - it
is not always enough to simply have the adhesive stick to the surface,
but it must also be able to bond with itself so that the adhesive does
not fracture.
Circumnutation
Circumnutation is the circular motion often shown by growing shoots, roots and leaves. It is due to differential growth: in order to elongate, plant cells must transiently weaken their cell walls, so this occurs on only one side of the shoot at any one time. However, this is likely more than a simple growth phenomenon and probably helps growing plant parts avoid obstacles and locate gaps to grow through. The period of a rotation of a growing tip varies between tens of minutes to a few hours. Twining vines that climb by coiling their whole stem around a support, do so by wide circumnutations, like a cowboy circling his rope before he throws it, and when they encounter a support they continue to rotate around it in a passive response that requires no special sensory reaction. The sensitive tendrils of climbers like Bryonia also circumnutate, which helps them locate a support by random movement.
Neuroid
and Nervous System
Venus's
fly-trap, Dionaea
muscipula,
is an obvious example of a rapid movement in plants. The trap
consists of two lobes or valves edged with spines and is a
modified leaf. On the inside of each lobe are three sensory
trigger hairs, arranged in a triangle. Should an insect wonder
into the trap and touch one of these trigger hairs once - nothing
apparently happens, but if it touches the same hair repeatedly, or
several different hairs in short succession, then the trap springs
shut in about 0.3 seconds. The hairs interlock, trapping all but
the largest insects inside. The trap then secretes mucus to
entangle the insect and seals tightly shut. Enzymes are released,
the insect is digested, and then the trap reopens, ready to
operate again. Touching one of the sensory hairs generates
electrical currents within the trap. These currents are summed
over space and time (which requires the plant tissues to have an
electrical memory), and repeated stimulation of the hairs causes
greater currents to flow, until, if a threshold is reached a rapid
electrical pulse (action potential) travels across the trap,
causing it to close suddenly. This summing of the stimuli reduces
the likelihood of a false alarm - it would do no good for the trap
to close every time a rain drop lands on it, but a living, moving
insect is another matter!
Electrical (electrochemical) signals, very similar to the
electrical signals in animal nervous systems, are actually quite
common in plants and may indeed occur in all plants. As in animals
these signals or pulses, called action potentials, are used by
cells that are some distance apart in the body to communicate
rapidly. These signals are created when localized parts of the
cell membrane release stored electrical charge, resulting in
current flow. The cell membrane acts as a capacitor in a resting
cell, storing electric charge. A suitable stimulus may cause ion
channels to open in the membrane, freeing the stored charge to
flow across the membrane. This flowing current may then trigger
more ion channels in adjacent areas of the membrane to open,
causing a cascade or domino-effect as the signal pulse travels
across the cell membrane. This signal must then be relayed to
adjacent cells and the plasmodesmata may serve
this function - ions carrying the electric current can move from
cell to cell through the plasmodesmata channels that join
neighbouring cells together.
This kind of electrical junction is known as an electrical synapse
in animal nervous systems and allows electrical pulses to travel
in both directions across it. Similar gap junctions also carry
electrical signals between non-nervous cells in animals, such as
epidermal cells and cardiac cells. Animal nervous systems,
however, also have chemical synapses, which allow signals to
travel in one direction only across the synapse, which is vital
for complex processing. Animal nervous systems also have elongated
specialized cells called neurones, acting like electrical wires,
to carry signals from one place to another at great speed. Thus,
we have two distinct system-types here - a system with elongated
cells specialized in signal transduction, or nervous system, and a
more general system in which electrical signals are passed from
one cell to its neighbors. The latter is called a neuroid system,
and typically only transmits slowly and over short distances.
Some scientists refer to the plant system as a neuroid system,
others as a nervous system. By and large the plant system
functions as a neuroid system, as signals are passed from one cell
to its immediate neighbours and the signals in the fly trap, for
example, are local. Sponges are animals
that lack a nervous system, but these too have a neuroid system.
However, in some plants action potentials can travel far and do so
by traveling in the phloem vessels. As
more research is carried out in this area it is becoming apparent
that more plants than previously supposed make use of a nervous
system, from algae to apple trees, and it seems likely that all
plants make use of a nervous system. Phloem vessels, which
transport sugary sap around the plant, have living protoplasts
(unlike xylem) and so can carry these signals. In vascular
bundles, which contain phloem vessels, the bundle sheath may
provide electrical insulation - confining the signals to the
vessels, much as animal nerves are insulated. In this case, the
plant system is a little more than a neuroid system, and is much
more like a nervous system proper, even though the phloem performs
other functions and is not dedicated to signal transduction in the
way that nerve cells in animals are. Action potentials in plants
also travel at only one-hundredth to a thousandth of the speed
they do in animals (2 cm/s compared to 1-20 m/s), so the plant
nervous system is much slower.
Many plants produce action potentials. More-or-less as dramatic as
the fly trap is the sensitive plant, Mimosa. Mimosa will fold
its leaflets rapidly when touched (to deter predators, possibly
causing them to fall off the leaf if they are insects) and if the
signal is strong enough it will spread further, causing one or
more leaves to droop. However, many less obvious cases are often
overlooked. Even the humble tomato plant will send action
potentials around its body when it is touched! Many plants (if not
all) will also generate action potentials when damaged, for
example by a grazer or by fire or cold. These signals can trigger
other parts of the plant to prepare its defenses (e.g. by
producing toxins or taking measures to prevent water loss by
closing stomata, etc.).
Plants exhibit two primary types of long-range electrochemical
signal. An electrochemical
signal
is an electrical signal generated by the flow of ions (charged
chemical species). The action
potential (AP) of
plants and animals is one such signal. Action potentials are
all-or-none in character, that is the size of the AP is (usually)
of no importance in encoding information, but the number of action
potentials and the rate and rhythmicity of action potential firing
are all important. In this sense an AP is a binary signal: it is
either ON or OFF. Action potentials travel mainly in the
phloem in plants. In plants action potentials can encode a variety
of signals, such as responses to wounding, touch, change in
illumination, cold and cell expansion. For example, applying a
flame to a leaf of Aloe
vera
will generate action potentials. Thus we can say that plants
do indeed have a nervous system.
However, plants also pass a type of long-range prolonged graded
potential, called a variation
potential (VP).
A graded potential is an electrical response in which the height
of the signal generally encodes stimulus strength and the duration
of the signal may also encode stimulus duration. Thus, a VP is not
all-or-none like an AP and is 'analogue' rather than 'digital'.
Graded potentials are used in neuroid systems in many organisms
and are usually local in nature, since the height of the signal
diminishes with distance from the signal source. (If an AP loses
signal height this does not matter as long as the AP is
discernible above background noise, so an AP is generally used for
longer-range signalling). Plants are unusual in using the VP for
long-range signaling too. This is almost unknown in the animal
kingdom, though some jellyfish neurons do use long-range graded
potentials in addition to action potentials.
Variation potentials encode a variety of signals, often connected
to wounding, local burning, organ damage or removal and also in
response to hydraulic signals. Changes in xylem pressure can
travel as pressure waves along the xylem at the speed of sound
(about 1500 m/s). These might be produced by an increase in turgor
pressure following rainfall, an embolism (blockage) in a xylem
vessel, bending of the plant, and also wounding. These hydraulic
waves
travel through the xylem, generating variation potentials in
parenchyma cells which are then carried to the phloem. Such a
signal can travel through a region of dead tissue (which is able
to conduct water) as an hydraulic wave before being regenerated as
a VP in living tissue on the other side. The speed of variation
potentials varies and they may be slower than action potentials
(in darkness) or faster (in light). Such signals may, for example,
signal stomata closure after wounding to conserve water.
Differences in electric potential can be measured between cells in
different parts of woody plants, such as between the stem and the
leaf. Electric potential is the potential energy per unit charge
and as gravitational potential energy drives the acceleration of
falling objects, so electric potential energy drives the flow of
electric charge or electric current. These differences follow a
daily rhythm, changing with the light-dark cycle. It is possible
that electrical signals in plants have some function in regulating
and coordinating daytime and nighttime activities in plants and
may be part of their biological clocks.
Gravitropism
Among the many signals plants can detect is
gravity. Shoots and roots can sense the direction of the gravitational
field, with roots growing down towards it (positive gravitropism) and
shoots growing upwards away from it (negative gravitropism). The
photograph below shows gravitropism in conifers growing on a slope -
the roots have grown downwards, growing down around obstacles like
boulders along the way.
How do roots sense gravity? The sensors appear to be cells located in
the root cap of growing root-tips. These cells contain heavy starch
grains which move under gravity, for example if a root is dislodged to
become horizontal, these sensors will detect this change in orientation.
A chemical signal, or plant hormone (phytohormone) called auxin
plays a key role here. Auxin is produced in the growing tips of shoots,
in the tip meristems or shoot apical meristems (SAMs) (a meristem is a
region of cell division in a plant, where new cells are made). This
auxin is carried down the shoot and into the roots, to the root tip,
through the phloem.
A growing root consists of several zones. Behind the root cap, which
shields the root and produces mucus to assist its movement through the
soil, is the root apical meristem (RAM) where mitosis gives rise to new
cells. Behind this is the zone is elongation, where the new cells expand
and elongate, and then behind this is the zone of differentiation, where
the cells develop or differentiate into the different cell types that
make up the mature root. The phloem develops in this zone of
differentiation, however auxin can move outside the phloem, across the
tissues from cell-to-cell in specifically controlled directions, by a
process called polar auxin transport. This allows the auxin to
reach the RAM or root-tip meristem. From here, polar transport carries
the auxin back along the rows of new cells, up the root. High
concentrations of auxin inhibit root elongation. In a vertical root, all
sides have an equally low auxin concentration and all elongate equally.
In a vertical root the auxin descending from the shoots will be
uniformly distributed on all side of the root and the root will grow
straight down. This is illustrated below:
Above: auxin movement in a vertical root (blue arrows) leads to a uniform distribution of auxin and the root grows equally on all sides and remains straight - growing straight down.
However, in a root which is horizontal, the gravity sensors in the root cap send signals to the root to alter the transport of auxin back along the root from the RAM by polar transport - it transports more auxin back along the lower side of the root than along the upper side. This will cause high concentrations of auxin to accumulate in the lower half of the root, and causing the underside to elongate less than the upper side, since the high auxin concentration here inhibits its elongation. The result is that the root curves downwards as the upper surface elongates more. This is illustrated below:
Nature
of the Gravity Sensor
Growing root tips have gravity-sensitive cells (gravireceptors)
in the columella of the root cap. The root cap is a protective cap of
tissue which protects the growing region of the root tip and secretes
mucilage to help the root tip move through the soil. The columella (lit.
'little column') is the central column of cells in the root tip.
Gravity-sensing cells also occur in the endodermis (innermost layer or
layers of cortical parenchyma cells surrounding the vascular tissues) of
green shoots and the pulvini of leaves responsible for correct
positioning of the growing leaf.
These gravirecptor cells have one feature in common: the presence of amyloplasts
(modified chloroplasts which lack chlorophyll but store starch, i.e.
starch grains). The starch makes the amyloplasts weighty and if a cell
is turned upside- down the starch grains will sediment at the bottom of
the cell in about 5 minutes. Complete sedimentation is thought not to be
necessary for triggering a response which is thought to occur after only
a few seconds post-reorientation. This is the starch-statolith
hypothesis. A statolith is a weighty 'stone-like' structure in an
organism which is involved in sensing gravity.
Experiments have confirmed that the starch grains are required for
gravity sensing. A working hypothesis is that the amyloplasts are
attached to part of the cell's cytoskeleton. Movement of the amyloplasts
can then send a signal throughout the cell via the cytoskeleton. The
movement of the amyloplasts triggers calcium ion release from internal
stores (via the PIP2 - IP3 signalling system which serves many diverse
functions in eukaryotes). This rise in the cytosolic concentration of
calcium within the cells occurs in two phases in Arabidopsis seedlings:
a rapid spike in calcium concentration, followed by a slower more phasic
response, in which calcium concentration is proportional to stimulus
strength and gradually fades over 25 minutes or so. (See: Plieth, C and
AJ Trewavas, 2002. Reorientation of Seedlings in the Earth's
Gravitational Field Induces Cytosolic Calcium Transients. Plant
Physiology 129: 786-796). Interestingly, the seedlings also responded to
puffs of air by mobilising intracellular calcium ions, but in this case
only the rapid spike was seen. The gravitropic response is accompanied
by polar transport of calcium ions in the apoplast (plant cell wall
network) which is essential for polar auxin transport.
The diagram below illustrates the current model of polar auxin transport. Two neighbouring cells are illustrated. Essentially protein pumps, possibly a protein called PIN, which span the cell-surface membrane, export the auxin from one end of each cell and another type of protein pump (called AUX1) imports it across the cell-surface membrane at the other end or pole of the cell. This ensures that the net flow of auxin is from the top to the bottom in the diagram. Additionally, when outside of the cell in the cell-wall matrix, which tends to be acidic, auxin possibly exists largely as a neutral molecule IAAH (indole acetic acid), but when inside the cell the higher pH causes the auxin to lose a proton (H+ ion) forming an anion, indole acetate or IAA-, which being a large and charged molecule cannot easily cross the cell-surface membrane. Thus the auxin entering the cell at one end is changed into a form that cannot passively diffuse back across the membrane and out the way it came. This would help ensure that the flux of auxin continues in one direction.
Plant
Muscle and Reaction Wood
It is not just roots that respond to gravity.
Shoots tend to grow away from the center of a gravitational field,
upwards and hopefully toward the light. The branches and trunks of
trees also respond to gravity. A branch has to maintain its position
against its own weight and also reorient itself should the tree become
dislodged and slanted from the vertical. For example, if a young
sapling is dislodged so that it stands at an angle, the branches will
sense the change in gravity pulling on them and alter their position
as they continue to grow, as indeed will the stem which will be pulled
back into a vertical position as new wood forms. This is achieved by
the formation of compression wood in conifers and tension
wood in angiosperms or broad-leaved hardwood trees (though there
is some evidence that angiosperms may also form a type of compression
wood).
A branch which is more-or-less horizontal will tend to grow vertically
elongated, giving it an oval cross-section. This strengthens it
against its own weight acting to bend it downwards. A conifer branch
will also produce specialized wood that forms where the weight of the
branch would tend to compress the wood on the underside. This is the
compression wood (CW) which is distinctly reddish in a cut log and has
shorter tracheids.
This wood is designed to offer greater resistance to the compressive
forces and tends to push against the weight of the branch to keep it in
position, or to correct its position as needed. Angiosperms form tension
wood (TW) toward the upper surface of the branch, which pulls against
the weight of the branch to achieve the same result.
More mature trees may have too much weight to right themselves if dislodged into a large angle. However, in a thick trunk reaction wood is still important in maintaining shape. It forms wherever a kink or burr in the trunk displaces the stem from its center-line of gravity and helps to keeps the trunk from curving too far as it grows. If a tree falls, but remains rooted, then one of its side-branches that now points upwards will become the new leader shoot, or perhaps the tip of the trunk will curve upwards as it grows toward the light, perhaps assisted by reaction wood. In this way trees with apparently horizontal trunk sections may form. It is the combination of a plant's responses to gravity and light that keep it growing as intended.
In many cases, at least, the contraction generated by reaction wood in angiosperms is generated by G-fibers, specialized sclerenchyma fibers with gelatinous cell-walls. These fibers are not only found in wood, but in secondary xylem more generally and may occur in primary and secondary phloem. The mechanism is not fully understood, but the gelatinous material swells when hydrated, but shrinks when it dries out. This shrinkage would generate tension or a contractile force. G-fibers then function as the muscles of dicotyledonous angiosperms, though many plants can also achieve movement without them. Nevertheless, G-fibers also occur in tendrils, like those of Bryonia dioica, generating the contractile force that causes them to coil, and also in the stems of many twining plants. They also occur in the curious sinusoidal stems of the Monkey-ladder Vine, Entada gigas, a member of the Pea family, Fabaceae.
Phototropism
in Shoots
A
tropism is a growth away or toward a stimulus. Photropism is
growth toward or away from a source of light. Roots tend to grow
away from the light - they exhibit negative photropism, whereas
shoots are positively phototactic and grow towards the light.
If a growing shoot is evenly illuminated on all sides, say from a
light source directly overhead, then it will grow straight up
toward the light. If, however, the light is brighter on one side
than the other then the shoot tip (or the region of the shoot just
behind the tip) will bend toward the light.
The blue arrows indicate the movements of auxin, synthesised in the
shoot tip. The Cholodny-Went theory (1937) across the width of the
shoot). Thus, as this auxin moves back along the shoot the darker side
receives more auxin. Similar to roots, shoots grow from a meristem in
the shoot-tip, which produces new cells by cell division (mitosis).
Behind the tip is a region of cell elongation, where the new cells
elongate, before they mature further down the shoot. The auxin
concentrations found in shoots stimulate cell elongation (in contrast to
roots in which a high auxin concentration inhibits cell elongation) and
so cells in the darker side of the shoot elongate more, causing the
shoot to bend toward the light. This theory has been hard to test
directly, however, it is known that transport of auxin is necessary for
phototropism in shoots. Phototropism occurs specifically in response to
blue light (which is a component of white light and natural yellow
sunlight). The theory is that a pigmented light receptor (called
cryptochrome) respond to the blue light signal and then trigger a
cell-signaling cascade which results in a change in the level of
phosphorylation of the auxin export pumps required for polar transport
of auxin.
Nyctinastic
Movements
These are literally 'sleep movements' and are
movements related to rhythmic changes in day and night conditions. Thermonasties
are responses to diurnal temperature changes, whereas photonasties
are responses to diurnal light changes.
Many flowers will only open at certain times of day and then close
again. This ensures that they expose their parts when pollinating
insects are at their busiest, but close to protect their parts (and to
conserve aromatics and nectar) when pollinating insects are inactive.
Flowers that rely on moths for pollination will open at night-time. In
the crocus and tulip, the sepals are like petals and form part of the
flower, in such a case the term 'tepals' is used to refer to both the
petals and sepals. In warm temperatures and in high light intensities,
the upper surface at the base of the tepal expands faster than the
lower surface and the flower opens. A reversal in these growth rates
close the flowers at night. These movements are actual growth
movements - the tepals are growing a little bit each day. The optimum
growth temperature of the upper tepal tissues is 10-17 degrees C, that
of the lower tepal tissues, 3-7 degrees. Surprisingly, this creates a
very sensitive system and a change in temperature of just 0.2 to 1 C
can be enough to trigger flower opening/closing.
Leaf
Movements
Leaves are positively phototropic and will
position themselves so as to intercept the maximum amount of light. If
one leaf become shaded by other leaves, then it will grow out from the
shade toward the light. It is the leaf-stalk or petiole that responds
and grows in this way. This enables leaves to avoid overlapping one
another and leaves tend to form instead a leaf mosaic - a canopy
of 'interlocking' leaves.
The leaves of many plants also move so as to track the sun a sit moves
across the sky, keeping themselves optimally illuminated. This movement,
unlike phototropism in shoots and petioles, is not a growth movement and
is truly reversible. It is caused by water moving in and out of cells in
the pulvinus (the swelling at the base of the petiole which forms a
joint). If the cells on one side of the pulvinus swell with water, then
these cells will exert a pushing force, whilst at the same time cells on
the opposite side lose water and become soft and flaccid and so stop
pushing. In this way the pulvinus can push the leaf one way or another.
This movement of water is apparently driven by the movement of potassium
ions. If potassium exits a cell, then this will lower the water
potential outside the cell, causing water to leave the cell, following
the potassium by osmosis (as water will move from a region of higher
water potential to a region of lower water potential by osmosis - see transport in plants).
This solar tracking undertaken by leaves is also called heliotropism.
Many leaves undergo nictinastic movements - opening out during the day
and closing at night. Extensor cells in the cortex on one side of the
pulvinus open the leaves by swelling with water and becoming turgid and
stiff to create a pushing force, whilst flexor cells in the cortex on
the opposite side of the cortex lose water and become soft and flaccid.
Leaf closing is brought about when the flexor cells become swollen and
turgid and the extensor cells falccid. These movements are not growth
movements (unlike the opening and closing of Venus' fly-trap) and are
perfectly reversible. In some plants, e.g. Mimosa, >Abizzia,
the leaves fold upwards when closing at night and the extensor cells are
in the uppermost part of the pulvinus. In plants like Phaseolus
and Samanea, the leaves fold downwards when closing and the
extensor cells are in the lower part of the pulvinus. The structure of a
typical pulvinus (as seen in transverse section) is shown below:
The leaves may respond to light and dark, opening in the light and
closing in the dark, or there may be a circadian rhythm (which is set by
the light-dark cycle) in which the leaves continue opening and closing
at the correct times of day even when kept in 24 hours light or total
darkness.
Mechanism
of Pulvinus Action
The flexor actively closes leaves, whilst the
extensor acts to open the leaves.
When the leaves are open (in the light) the potassium ion concentration
in the protoplasm of cells in the extensor is high. When the leaves
begin to close much of this potassium is transferred into the walls of
the extensor cells, where it enters the apoplast pathway.
Chloride follows, driven by the electrochemical gradient (the negatively
charged chloride ions are pulled along by the positively charged
potassium ions). Water will also follow by osmosis, so that the extensor
cells become flaccid and pliable, reversibly buckling under the weight
of the leaf. Recall that there are two main pathways of water flow
across plant tissues: the symplast, consisting of the protoplasts of
cells connected by plasmodesmata, and the apoplast, consisting of the
connected walls of plant cells which resemble porous fibrous meshes.
Once in the apoplast pathway, the potassium and chloride ions can move
through the collenchyma cells in the leaf. Collenchyma cells are
supporting cells which have thickened cellulose cell walls.
Closing takes about 20 minutes and during closing the portassium and
chloride ions flow to the flexor where they enter both the cell walls
and protoplasts of the flexor cells. Water follows by osmosis,
so that these cells become swollen and turgid and exert positive
pressure to move the leaf. In darkness, when closed, potassium ions
remain at high concentration in the collenchyma apoplast which acts as a
potassium ion reservoir. concentration. Closing takes about 20 minutes.
Some potassium and chloride are also probably shunted through the symplast pathway.
It is thought that specialised transfer cells pump potassium and
chloride in and out of the symplast of the phloem. The reverse process
happens when the leaves open - potassium and chloride leave the flexor
cells, which become flaccid, and enter the extensor cells which become
inflated, such that the action of the forces is reversed.
Mimosa pudica, the Sensitive Plant, is a good demonstration of
pulvini power. Repeated touching of the leaves will cause the leaflets
of each compound leaf to close, which again involves action of the
pulvini.
Thigmomorphogenesis
Thigmomorphogenesis is a change in plant
growth in response to touch. The most obvious example is the way
climbing plants spiral around a supporting object, such as the stem of
another plant. Many plants respond to repeated rubbing or bending by
growing shorter with thicker stems. This is simulating movements caused
by wind. Plants growing in windy places need to more resistant to the
wind and so grow thicker stems. They are also generally shorter, either
because this reduces loading on the stem, or because they have limited
nutrients to invest and have invested more in growing wider, or because
wind-exposed plants are likely to be isolated and so need not compete so
much for light.
Trees growing in isolation are exposed more to high winds and grow
shorter but thicker. Trees growing grouped together compete for the
light and are sheltered by one-another and so invest in producing a
narrower but taller trunk. Plants that are repeatedly shaken but not
directly handled also show a similar response (also called
seismomorphogenesis). This raises the possibility that this response may
also occur in response to repeated browsing by herbivores. Browsing,
pruning or damaging a tree also tends to cause a proliferation of
shoots, especially in hardwoods.
Pollarding a tree, in which the canopy is cut back about two
metres above the ground, results in the activation of dormant buds or
production of new buds on the stem, called epicormic buds, which
results in more shuts growing back. Clearly if a tree has lost a major
limb, then replacing this with several limbs is a good insurance policy
- if the tree is exposed to winds or herbivores, then producing more
branches is a good insurance policy. Pollarding was done to encourage
the growth of numerous new shoots, which had many uses, for example as
fence posts, whilst keeping the new shoots out of the range of pigs,
sheep and deer that may graze the woodland. Where such grazers are not
kept Coppicing is often carried out - in which the tree is cut
done near the base, leaving a tree stump to grow new shoots. Most
conifers do not survive coppicing, but hardwoods have many epicormic
buds to produce new shoots. A regenerating tree stump will draw upon
food reserves in the roots, the parenchyma of the remaining wood, and
also on nutrients supplied by other trees. Tree roots often naturally
graft together beneath the ground, connecting their xylem together,
which allows some nutrients to pass from nearby trees to the stump, or
indeed any tree that is struggling. Sometimes such root grafts even
occur between trees of different species.
These responses pose several problems for woodland management. First of
all, pollarded trees are often pollarded at intervals, failure to do so
can result in too many branches maturing, making the crown top-heavy,
despite thickening of the trunk. This may cause the trunk to fail and
split as the tree starts to fall apart. This is not as disastrous for
the tree as it sounds, however, since pollarded trees extend their
life-span and such failing pollards are often immensely old in any case.
Also, the fragments may remain rooted and grow as separate trees.
Felling trees that were close together, leaving some remaining trees
rather isolated can increase windfall or wind-damage since a tree that
previously grew taller and thinner, relying on its companions for
support may now suddenly be more exposed to high winds.
Mechanical stimulation may produce shorter and stockier plants, but it
may also reduce fruit yield. Spraying tomato plants once a day can
severely reduce the yield, since the plant has invested more resources
in strengthening its body and so has less to invest in fruit production.
Many plants only require shaking for a few seconds each day in order to
induce a response. These responses also pose problems for plant
scientists, since when comparing treatments on plant growth they must
ensure that each group of plants are handled in a similar way.
Phytochrome
Phytochrome is a blue pigment present in plants (in
low concentrations, so it does not make plants look blue!) that absorbs
red light and, in so doing, is converted to a green form that absorbs
far-red light. When the far-red sensitive form absorbs far-red light it
is converted back to the red sensitive form. Thus, phytochrome
alternates between a form that is sensitive to red light at about 660 nm
(peak absorption occurs at 666 nm) and a far-red sensitive form
sensitive to far-red light at about 730 nm.
Pr is the form that absorbs red light (at 666 nm wavelength) and Pfr
is the form that absorbs far-red light (at 730 nm wavelength). Far-red
light is light between 700-800 nm (infra-red light is light above 760 nm
wavelength). Pfr also reverts back to Pr in the dark.
Phytochrome
is used by plants to sense light
Plants clearly utilise light-energy for
photosynthesis, but they also sense light using phytochrome. Light
induces or promotes the following reactions in germinating plant
seedlings:
1. Chlorophyll synthesis (seedlings grown in the dark are pale and NOT
green).
2. Leaf expansion (if leaves unfold under the ground it causes
problems!).
3. Stem elongation inhibition (dark-grown seedlings have long stems).
4. Root development.
The process by which light affects development in plants is called photomorphogenesis.
Large seeds have abundant food reserves and once they germinate they do
not need to photosynthesise for several days. Dark-grown seedlings are
said to be etiolated (French: etioler: to grow pale or weak)
because of their long, thin pale stems. It is a strategy to enable the
seedling to place all its resources in growing a long shoot to break
through the soil. Once in the light it will then turn green, thicken and
unfold its leaves.
Light
is also used as a signal to control flowering.
Often both light and temperature are involved, enabling the plant to
determine the right time of day to open its flowers, or the right time
of year to shed its leaves. In many cases these responses (and the role
of phytochrome) are incompletely understood.
The role of
phytochrome
In sunlight there are slightly fewer far-red than red
photons (the Sun is a yellow star!) although the levels of these two
wavelengths are roughly the same. To amplify this effect, Pr absorbs red
light more efficiently than Pfr absorbs far-red light, so that sunlight
acts as a red-light stimulus. For example in lettuce seeds: red light
promotes germination; far-red light inhibits germination.
Q. Which form of phytochrome (Pr or Pfr) promotes germination, and which
form inhibits it?
A. Pr senses red-light more efficiently than Pfr absorbs far-red light
in direct sunlight. This Pr converts into Pfr, so in direct sunlight
there is more Pfr present and so it is Pfr that stimulate germination.
In darkness, Pfr converts back to Pr and so Pr inhibits germination!
Seeds that require light for germination are said to be photodormant
seeds. (Imbibition: is the first uptake of water by a seed and is
required for germination to begin).
As a further example, in French beans, red-light (Pfr) promotes
unbending of the hypocotyl hook and triggers leaf expansion. The
hypocotyl is the stem of the seedling, between the radicle (first
seedling root) and the cotyledons (first pair of"leaves"). The hypocotyl
is arched over at its tip, called the hypocotyl hook, protecting the
cotyledons as it pushes through the soil. Once through the soil it
straightens and the cotyledons unfold).
Photoperiod
The photoperiod is the day length.
Short-day plants are plants that flower when the daylength
(photoperiod) is short, e.g. spring flowering plants like tulips and
daffodils and autumn/winter flowering plants like snowdrops. Flowering
in short-day plants is inhibited by Pfr and stimulated by Pr, since Pr
accumulates in darkness and Pfr accumulates in sunlight.
Long-day plants flower in summer (when photoperiod is long) and
are stimulated to flower by Pfr, which is the predominant phytochrome
form in sunlight.
Leaf
canopy shading
More far-red light than red light penetrates tree
canopies. Thus, shaded plants have more Pr. In order to compensate for
this, germination in shade-tolerant plants is less inhibited by far-red
light, than it is in shade-intolerant plants.
Other
light-sensitive pigments in plants
Chlorophyll: responsible for absorbing the
light energy used in photosynthesis. Chlorophyll is green since it
absorbs blue light (400-500 nm) and red light (600-700 nm). Chlorophyll,
however, does not appear to play any major role as a light sensor.
Cryptochrome: absorbs blue and violet light. This pigment
triggers phototropism: the growth of some parts of a plant
towards light (and other parts away from light) as described above.
Cryptochromes occur in animals and plants and are important components
in the biological clock that regulates circadian rhythms.
they are also implicated in responses to the detection of magnetic
fields, for example in birds. In some plants and animals it has been
shown that responses to magnetic fields depend upon the response of
cryptochrome to blue light. (More information will be provided on this
topic as it becomes available).
Other
responses of plants
Many parts of the plant are sensitive to a variety
of stimuli. For example, the nodes of many grass stems and/or the bases
of leaf sheaths are sensitive to gravity, exhibiting gravitropism
(moving the plant as it grows to grow away from gravity). They contain
statoliths and responses to a change in gravity start to occur after
about 30 seconds if the plant is re-oriented. These movements occur at
the same regions that sense the gravity, called pseudo-pulvini
or false pulvini (singular: pseudo-pulvinus) to distinguish them from
the pulvini of leaves that bring about reversible movements rather than
growth responses. Thus the false pulvinus is both a gravity sensor and
responds to gravity by re-orineting parts of the plant. Stamens, flower
stalks, fruit, leaves and other parts of the plant may also respond to
gravity. This area is still poorly researched.
One of the emerging themes of this article is that plants respond to a
very wide variety of stimuli, albeit in slow motion compared to many
reactions of animals. These responses are often poorly researched. The
role of the plant nervous system, with its action potentials, remains
poorly studied. Additionally, like the cells of other multicellular
organisms, plant cells can communicate with each other by chemicals as
well as electrical signals. Chemical messengers, such as phytohormones,
cause signals within the responding plant cell, an intracellular
cell-signalling mechanism. In contrast to studies on animals and the
cellular slime mould Dictyostelium, and indeed bacteria,
cell-signalling is very poorly understood in plants. Clearly much more
research is urgently required in these fields of botany / plant science.
Download
a pdf table of plant hormones.
References and
Bibliography
See
also: Plant Trichomes
Article
updated:
22/2/2014
- damaged parts of the file replaced along with some additional
material;
13 Jan 2016 - information added on adhesives in climbing plants.
21 May 2019 - Page maintenance (file was damaged again!)
11/Feb/2024