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A Place in the Sun

An Article published in The Clematis, the journal of the British Clematis Society, Winter 2003

Introduction

There is a climbing rose, Rosa ‘Danse Du Feu’, and a clematis, Clematis viticella ‘Etoile Violette’, that grow on the southern corner of my pergola. The rose took a couple of years to climb to the top whilst the clematis does it every year in just a couple of months, and whereas the rose needed tying in the clematis can generally manage the climb without such assistance. In a more natural situation the pergola would be a tree and we would see a third strategy for finding a place in the sun - a large self-supporting plant with its leaves ready placed in full sun, after an initial period of many years growth. The clematis’s strategy means that it does not need to invest energy in building up structural tissue nor in providing winter protection to its sensitive growing tips. It does have the disadvantage that every year it must store enough reserves in its roots to be able to find a support and grow high enough, rapidly enough to get its leaves into the sun for long enough to harvest enough energy both to flower and do the same thing all over again next year.

All herbaceous climbers adopt this strategy. An essential key to their success is the ability to cling to the support. Climbing plants use various techniques for this and, as gardeners, we are familiar with all of them - tendrils (sweat peas), clasps (Virginia creeper), adventitious roots (ivy), twining stems (wisteria) and hooks (rose). Finally, there is the method chosen by the clematis tribe. We have all seen that the leaflets curl around and bind tightly to supports and also several shoots binding together to form a self supporting erect growth of several feet, but have you ever wondered how this works? How do shoots know to grow upwards? How do they find a support? How do they know when they have found a support?

Responses to Environmental Signals

For centuries, botanists and plant physiologists have studied the mechanisms by which plants detect what is going on around them and then use this sensory information to go about their business - getting roots to go down, shoots to go up, leaves to follow the sun, stems to curl around supports, leaves to catch insects, and so on and so forth. Darwin, always a meticulous observer, spent many years carefully recording the growth movements of a vast variety of plants and published his conclusions in one of his seminal works The Power of Movement in Plants in 1880. In it he describes many of the responses that may be used by climbing plants and carefully experimented to try to find the environmental signals that cause them.

Further work followed and today scientists place these responses into two basic categories. Movements which occur in a specific direction, either toward or away from the stimulus, are called tropisms, whilst movements that have no directional aspect, like the sleep movement of leaves, are called nastic responses. There is also a miscellany of other important responses that do not fit easily into these groupings, for example movement in response to innate rhythms. By no means all of these are involved in the climbing mechanism of clematis, but let us have a brief look at the likely candidates.

Upward Growth

Clearly, in climbing plants there must be a basic tendency for shoots to grow upwards - there are exceptions, like ivy, but even in this worst case shoot growth is horizontal and not downwards!

Gravity is the fundamental environmental signal that plants use to orientate themselves to ensure that roots go down and shoots go up. Geotropism, as this response is called, was demonstrated back in 1806 when a scientist called Knight attached bean seeds to a spinning wheel and showed that instead of growing up the shoots grew towards the axis of the wheel because in the spinning wheel the centrifugal force was stronger than gravity. In the intervening years scientists have carefully studied the response but they still do not know how the sensitive regions of the plant, which are the shoot and root tips, detect gravity, although it seems likely to be the tendency of a solid body within the sensitive cells, possible stain grains, to settle under gravitational force.

Geotropism is modified by various other environmental signals, perhaps the most important being light. Again, Darwin ran some elegant experiments demonstrating that many plants or plant parts bend towards a source of light, a response that is now called phototropism. He was ingenious enough to be able to show that in shoots the very tip detects the light signal whilst the growth response occurs some distance from the tip. Scientists have since shown that the hormone auxin is the likely agent by which the signal is transmitted from the point of detection to the site of response. All gardeners will be familiar with this phototropic response, especially the tendency of leaves to orientate themselves towards to the light. Indeed, some plant organs follow the sun on a daily basis. Lupin leaves and sunflower heads are great examples. One should not expect the growing shoots of climbing plants to be strongly phototropic, however, because they might actually be better served by moving towards the shade where there will be more supports.

Finding a Support

Geotropism modified by phototropism gets the shoots growing upwards, but how do they find a support? One method is circumnutation which is an oscillating movement of a growing plant organ. Darwin found that "the growing parts of all plants circumnutate" but in climbing plants the movement is "greatly increased in amplitude". Circumnutation is caused by an internal rhythm and not some environmental signal, a fact that has been demonstrated by many experimenters from Darwin, who observed it in plants growing in darkness, to astronauts, who have observed it in sunflower seedlings growing in the zero gravity of space.

As they extend circumnutating organs describe a helix which, when viewed head on, appears to be an ellipse. In a 4- to 5-day-old sunflower seedling, for example, the ellipse traced is 6-8 millimeters long and takes about 110 minutes. In the shoots of rapidly growing climbers the amplitude is much greater - using time-lapse photography I have observed ellipses of 50cm in shoots of kiwifruit growing in controlled environment chambers. Such extreme movement presumably helps these climbing shoots in their search for a support.

Circumnutation appears to be most pronounced in plants like kiwi-fruit that climb by twining their stems around the support. I could not see any obvious circumnutation movement in the shoots of my Clematis ‘Etoile Violette’, and it has be shown to be disadvantageous for horizontally growing shoots in honeysuckle, because those that do circumnutate cannot root as frequently and so produce fewer vertical climbing shoots.

Causal observation of clematis suggests that its search for a support is rather disorganised and random. The shoots grow up until they find a support or they collapse under their own weight and then grow up from the tip again. In this way they can repeatedly spread and search. But once they have found a support they quickly attach to it, because their leaf stalks (petioles) are sensitive to touch.

Attaching

Thigmotropism, from the Greek root thigma, meaning "touch", and trope, meaning "turn", is the term used to describe a plant's response to physical contact. Sensitivity to touch has a number of uses to plants, including capture of prey and escape from being eaten, but its mainstream function is probably helping plants to attach themselves to supports whilst climbing. The twining and clasping leaves and tendrils of climbing plants are said to be positively thigmotropic - they curl towards the physical contact.

Clematis petioles are positively thigmotropic and its fun to make your own observations of this phenomenon. I found that a young actively growing leaflet on my Clematis ‘Etoile Violette’ would curl 360º around thin air in response to me gently stroking what became the inner edge of the petiole every hour for just four hours. What is more I could make it uncurl by then stroking the outside edge.

More Thigmotropism

Plants of some species are incredibly sensitive to touch, in fact more so than humans. For example, the tentacles on sundew plants that trap insects with their sticky blobs also detect the movement of the insect and cause the leaf to curl over it. These tentacles respond to a thread of weight 0.0008mg being drawn over them. The tendrils of climbing plants can be even more sensitive - Sicyos tendrils respond to a thread weighing just 0.00025mg. In contrast, the minimum thread weight detectable by human skin is 0.002mg, 10 times less sensitive than some plants. It is not just weight that is important, however, because heavier but smooth or friction-free objects like droplets of water or mercury do not elicit a response.

The speed of reaction can be very fast indeed. The record holders are probably those plants that catch insects by springing traps, like the Venus fly trap or the bladderwort. Here the response to the stimulation by the moving insect is sub-second. Mimosa, the sensitive plant, responds to touch by folding all of its leaflets and drooping its leaves within a few seconds, even leaves that have not been touched will fold in this way within a minute. The tendrils of some plants will curl around an object within as little as 3 to 10 minutes. My Clematis petioles showed signs of curling within an hour after a single short period of stroking.

We still have a lot to learn about the way thigmotropism works. Indeed it may vary between plant species. Since the response occurs in many more cells than have actually been touched we know that there must be detection of the signal, transduction of the signal into some sort of message, transfer of the message to the site of action, and then the action itself. Research goes on and many pieces of the jigsaw have been found.

We know that the rapid movements are due to changes in the amount of water in the plant’s cells, whilst the slower movements are due to growth, and that the effect must be more pronounced on one side of the stem than on the other in order to cause the curvature. In Mimosa, where the response to touch is extremely rapid, there are specialised hinge-like organs that attach the leaflets to the stalk. The leaflets are made to fold upwards when touched by water moving out of the cells in the top of hinge, so that they shrink, and as a consequent the cells at the bottom of the hinge, which are held under pressure by water, expand. In Mimosa there is evidence for both a hormone and an action potential as the agent that transfers the message from the point of touch to the hinge. (An action potential is essentially an electrical impulse, so interestingly this method of message transfer is rather like a nervous impulse in animals.)

The initial detection must be done by the cells on the outer layer of plant tissue often by specialized cells shaped like hairs. The surface of most tendril tips have a very dense distribution of these "hair" cells. The Venus Fly Trap, on the other hand, has a few rather large hairs on its leaves which can be made to snap shut by touching the very tip of one of these hairs. Looking at the petioles of my clematis through a high power lens reveals not only lots of appressed hairs, mostly on the upper side, but also a very fine pubescence all over the petiole.

The Thale Cress, Arabidopsis thaliana, is a small insignificant annual weed that is hugely influential in modern plant physiology. It is a relatively simple plant and so is used in many laboratories as a model on which to experiment. Four genes have been found in Arabidopsis which are switched on in as little as 10 minutes by touch. Since they are sensitive to touch they are called TCH genes. When switched on they cause the manufacture of various proteins that themselves control other chemical processes within the cell. One of these proteins is the enzyme XET which breaks bonds between the cellulose threads in the plant cell wall, so that the wall is loosened and can more easily expand. The TCH genes are known to be turned on by high levels of calcium ions, and it is also known that touch causes an increase in the concentration of calcium ions.

In the tendrils white bryony, Bryonia dioica, a wild flower often found scrambling through hedgerows, the concentration of a compound called Jasmonite is increased when the tendril is touched. When Jasmonites are fed to tendrils without touching them they curl in a way that is indistinguishable from tendrils that have been touched. Jasmonites are a by-product of one of the many chemical processes that go on in plant cells and it seems from this experiment that they can act as a message transfer agent.


Putting all of these jigsaw pieces together we have an intriguing theory for the mechanism of thigmotropism, as follows. When the plant is touched the hairs bend and buckle, and the cell membranes inside them are deformed. This activates stretch-activated ion channels which start pumping calcium ions into the cell, increasing the concentration of calcium ions. TCH genes are thus turned-on and cause the manufacture of increased levels of XET and/or other proteins and these, in turn, loosen the cell wall and otherwise affect cell chemistry. Thus, the cell expands or messages are sent, whichever is appropriate. Quite complex I’d say! Whether this theory turns out to be true or not two things are certain, the scientists will continue to investigate and my clematis will continue to climb.