In contrast to most animal organisms are multicellular and single-celled plants immobile. Despite this speaks botanical literature since antiquity of plant movements. The movements towards the sun was regarded as a proof for the existence of a plant soul. THEOPHRASTUS describes the opening and closing of flowers, the lifting and lowering of leaves at certain times of the day and under the influence of the shift from day to night in detail. An especially impressive example is the sensitive plant’s breaking of the pinnate leaves. Most likely is THEOPHRASTUS’ observation based on the Egyptian Mimosa asperata.
It was not before the 17th century that an interest in the reason for these movements arose. The Englishman J. RAY characterized plants in his Historia plantarum in 1686 as insensitive organisms, and explained movements by strictly physical mechanisms like the uptake or loss of water.
R. HOOKE thought that the lowering of Mimosa’s pinnate leaves after touching takes place due to a flow of water that is caused by the pressure of the stimulus. He postulated ball-and –socket joints at the base of the pinnate leaves’ stem moved by water uptake and water loss. We will learn later on that this explanation is not far from reality.
The textbooks on botany written in the first half of the 19th century spend but little time on plant movements. A. de CANDOLLE ( 1834/38) distinguished between
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the direction of the plant and its parts: vertical direction of shoots and roots, striving of the stems and branches towards light, and |
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the original plant movements that he grouped into regular movements (sleeping movements of the leaves, opening and closing of some flowers, movements of the sexual organs, etc.) and the random or irregular movements (example: Mimosa). |
The flower heads open up during sunshine and close during rain and night (pollen protection). A. Kerner v. MARILAUN: "Pflanzenleben", 1913,
He wrote – although he used the term "sleep" -:
"The similarity with the sleep of animals is only seeming since the position the leaves take is a certain one, and the rigidity of their leaf stalks cannot be compared to the limpness and flexibility our limbs show during sleep."
He counted Desmodium gyrans (Hedysarum gyrans) often cited later in botanical literature among the random movements. The leaves of this plant consist of three leaflets (pinna) of which the two lateral ones move in a continuously and jerkily fashion. One lifts while the other lowers. The arch described by each of them is roughly 50°. The movements have no visible cause.
During the second half of the 19th century were movements and their causes analyzed systematically. Among the most important contributions are that of C. DARWIN, J. v. SACHS, and W. PFEFFER. In his "Vorlesungen über Pflanzenphysiologie" (Lectures on Plant Physiology) published in 1887 (2nd edition) distinguished SACHS between the
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amoeboid movements, |
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the movements of the protoplast (circulation of the plasma, the chloroplasts and other components), |
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the sleeping movements of leaves and petioles, |
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the sensitivity of the mimosa and other similar cases (turgescence and volume changes during stimulation), |
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the winding of tendrils and climbers, |
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geotropism and heliotropism.. |
Movements, no matter of what kind, do always consume energy. J. v. SACHS wrote:
" The chemical processes and the molecular movements that make up the life of both plants and animals take place only as long as the free oxygen of the atmosphere can enter them. If the supply of this gas is interrupted, then the inner movements that cause growth stop, the current of the protoplasm – the most obvious expression of life – ceases, the periodic movements of leaves and flower parts end, and the organs usually stimulated by vibration loose their sensitivity."
Today exist different classifications of plant movements. It is distinguished between autonomous (endogenous) and induced movements. The first type has no detectable extern cause while the latter are termed movements caused by stimulation. These latter movements are again grouped into nastic responses and tactic movements. The reactions of each group can be either positive or negative. Movements are, too, classified according to the cause of stimulation.
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Tactic movements are oriented towards a certain direction. They show a clear connection between the direction of the movement and the direction of the controlling extern factor (signal). Classic examples are phototropism and geotropism. |
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Nastic movements are independent of the direction of the controlling signal. An example is the seismonasty of the mimosa. The folding up of the leaflets and the leaves occurs not in the direction of the haptic stimulus. |
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Tactic movements are based on free directional movements that occur either in the same or in the opposite direction of the source of stimulation. |
In order to understand plant movements is a clear separation between the cause of stimulation, the transmitting of information and the movement itself necessary. We have already dealt with intracellular and directional movements (and thus also with tactic movements) elsewhere. The molecular mechanisms of movements are far from being completely understood, but it seems that explanation is not far away anymore. Several models developed upon the analysis of animal cells proved to be good working hypotheses. All of them include contractile elements (microtubuli and/or microfilaments).
In contrast to intracellular movements are the plant movements that we will discuss now mostly based upon local growth and turgor changes, i.e. changes in the osmotic pressure of the cells involved in the movements. In addition have swelling and cohesion movements that can be explained by the physico-chemical properties of cell walls to be mentioned.
In a strictly formal sense and based upon several well-chosen examples is it distinguished between irreversible growth movements and often reversible turgor movements, but both processes, especially in multicellular plant parts – as we will learn a little later – , are usually dependent on similar causes. In the same way are the movement and the developmental physiology of plants tightly coupled, and the two terms are often not more than two sides of the same problem.
Growth can be described as an irreversible increase of volume. As we have seen before exist two types of growth: growth by division and elongation. Here, we will focus exclusively on elongation. Cells capable of division contain usually no or only very small vacuoles, while cells going to elongate have full-size vacuoles and therefore the capacity of maximal water uptake.
Besides contain the walls of cells capable of elongation an elastic and also a plastic component. Plasticity is based upon the ability to deposit new wall material in a stretched cell wall, thus stabilizing the condition. A consequence is the increase of the cell wall surface and thus also of the cell’s volume. Walls of fully differentiated cells can still stretch elastically, but are unable to incorporate new wall material. They do therefore return to their original state after the intracellular osmotic pressure has stopped. It is thus the quality of the cell wall that determines primarily whether an increase of the turgor causes an irreversible cell growth or a reversible, temporal increase in size. Both phenomenons cause local deformations within the plant’s tissue that act like levers upon neighbouring tissues. This causes the change of their position that we perceive as movements.
If two opposite sides of an organ have - for some time or over an extended period of time – different growth rates, then the direction of growth changes necessarily: the organ bends, and it is spoken of a growth movement. Paradoxically is this term rather unusual in zoology, although it is especially in animal germination (in contrast to that of plants) that cells do really change places (without remarkable changes of volume). Just think of gastrulation or the cutting off of the neural tube.
As has been said at the beginning is it distinguished between tactic and nastic movements. Phototropism and geotropism, for example, occur typically in organs of radial symmetry, like the shoot, or the axis of the main root, while nastic movements are typical for dorsiventral organs. The dorsiventral structure of an organ is the expression of an asymmetric organization of the single types of tissues (example: leaf). This means that the single tissues have different capacities for extension and that an uneven growth of the two flanks (upper and lower flank) means also a programmed direction of movement that is independent of the direction of stimulation. As a consequence is a movement caused by stimulation primarily explained by the anatomy of an organ and thus by its mechanic properties.
Most turgor movements differ from typical growth movements in that they are reversible. When a cell takes up water, increases its turgor and consequently also the pressure exerted on its walls: the cell increases in size due to a certain elasticity of its walls. As has been explained elsewhere is this pressure opposed by that of the neighbouring cells. If their pressure increases simultaneously and in the same way, then considerable tensions within the tissue result that might lead to deformations of that tissue. Such a deformation can be the cause for the spatial shift of whole plant parts. Sometimes are the cells participating in such a movement surrounded by cell walls of different sizes, so that the pressure spreads into a certain direction. The guard cell movements within the epidermis is a prime example.
Turgor movements are reversible only if the osmotic pressure within the cells can decrease again after some time. Such changes can be observed in some petiole joints that cause the circadian lifting and lowering of leaves.
In other cases (tissues) builds up an osmotic pressure causing tensions that cannot be reversed by physiological processes. After a critical maximal value has been reached causes the pressure a rupture of the tissue (often at sites specially designed for such a rupture). These ruptures are the explanation for the explosive movements or catapult actions of certain fruits. Their biological sense is seed distribution.