Irreversible Turgor Movements – Moisture Absorption Movements and Cohesion Movements

This section comprises movements serving mainly the distribution of seeds and spores. They are consequently mainly – though not only – observed in fruits, seeds, and the capsules of spores. It has to be distinguished between the irreversible turgor movements of living cells and moisture absorption movements (changes of the cell wall’s volume). The latter can also occur in dead cells.

Irreversible turgor movements have their origin in the development of an osmotic pressure that cannot be reversed by physiological means. It may therefore cause ruptures of the tissue after crossing a threshold value resulting in explosive movements. Two impressive examples are

1. the catapult action of the fruits of all Impatiens- species, and
2. the squirting movement of the squirting cucumber Ecballium elaterium.

In the case of the first example, anatomical studies showed the existence of preformed breaks that define the direction of the movement. The seeds are produced in the upper part of the fruit at the inner side of the pericarp. The outer cell layers in the lower part start to expand, but their movement is stopped by a resistant tissue at the inner side. The outer cell layers are called erectile tissues. When the maximal turgor has been reached (about 9 – 14 Bar), do the longitudinal connections between the carpels rupture, the now free carpels do instantly roll up and catapult the seeds sitting on them away by using the trip mechanism. The energy set free by rupturing is thus used for the distribution of the seeds.

Similar phenomenons exist with the stamens of some species (in Pellionia daveauana, for example). The lower side grows faster than the upper side during an early stage of the filaments’ development. The resulting tensile stress cannot be lowered by an increased growth of the upper side during a second developmental stage, since the anthers are glued together at their bases. As soon as the connection is removed, the filaments do explosively shoot to the outside.

In the case of the second example, the fruits of the squirting cucumber are surrounded by a multi-layered, elastic pericarp, that resists the strong inner pressure of the fruit. The seeds are within the fruit, between the large parenchyma cells. The osmotic pressure within these cells increases during ripening from 8.5 to 14 Bar. A less resistant separating tissue is located near the fruits base. As soon as the pressure becomes higher than a critical value, the fruit is catapulted away, the parenchyma cells rupture, and the seeds are discharged through the opening at a ballistically favourable angle of 40 – 60 degree and high velocity due to the pressure set free. They do easily overcome distances of more than 10 meters. The requirements for such explosive movements can conclusively be outlined as follows:

1. A deformable tissue specialized for movements that is under a stress higher than usual is needed.
2. A resisting tissue has to balance the elastic stress at first.
3. The possibility to finally remove the resistance (rupture of the tissue at the preformed breaks) has to exist.
4. The parts of the tissue, organs, or units that are to be distributed during the explosion have to be arranged in an optimal fashion.

Moisture Absorption Movements

The cell wall of plants consists of several stacked cellulose layers (texturing). The orientation of the cellulose fibrils changes from layer to layer, and is only outlined here by the terms transverse texture, longitudinal texture, and screw-like texture. The wall contains further structural elements besides cellulose (fibrillar macromolecules) the proportion of which differs from layer to layer. They are – just like all macromolecules – able to store water thereby increasing their volume (hydration, absorption). The molecular classes involved can be grouped according to their capacity to bind or store water as follows:

pectin > hemicellulose > cellulose > lignin

If layers of differing abilities to absorb water are tightly coupled, then they will elongate or shorten at different degrees. The resulting tensions cause a bending of the respective cells and thus also of the tissue.

It has to be noted that solely the cell wall’s state of hydration, i.e. that of the single layers of the cell wall, is of importance. The atmosphere’s amount of humidity is enough to deform the cells (or tissues) that are under stress and to cause their rupturing. The cell’s plasma is, if still existing, not involved.

The opening and closing of some seed capsules, the movements of the specialized distribution gadgets of several seeds, the peristome movements of mosses, and the torsion of the carpels of many Leguminosae are an expression of the changed state of hydration (= hygroscopic movements). Everything comprised in wood-processing under the sentence "The wood warps." is caused by locally unequal changes of the state of hydration. These changes may cause stress and ruptures of the wood.

The asymmetric structure of the cell wall and the surface tension of the water cause, too, another kind of movements, the so-called cohesive movements. The classic – and thus nearly ubiquitously cited – example is the opening mechanism of the fern’s sporangium. The cells of the mono-layered wall thin. Only the cells of the anulus that surrounds the sporangium like a meridian or a nearly closed hoop form an exception. A preformed area (stomium) at the front of the sporangium is left blank.

The inner walls and the radial walls between neighbouring cells of the anulus are thickened, while all outer walls are thin and elastic. The cells loose water during the ripening of the sporangium, and the thin outer wall is drawn to the inner part due to cohesion of the remaining water and its concurrent adhesion to the walls. This process brings about a tension of the wall that spreads to the radial walls, too, and causes them to come closer together. A tissue tension leading to a rupture of the sporangium’s preformed areas results since the same occurs simultaneously in all anulus-cells. The cohesive force of the water is overcome by the subsequent entering of air. The cells and thus the whole anulus return to their original position. An exchange of the air by water can start the whole process again.

The hydration process of a Lythracea’s seed hairs is another example. The electron microscopic and the scanning electron microscopic analysis showed that this hydration is a process of turning outside during which a moist hair gains length compared to a dry one.