The following points highlight the three types of movements in plants. The types are: 1. Movements of Locomotion 2. Movements of Curvature and 3. Hygroscopic Movements.
The first two types of movements are called as vital movements because they are exhibited only by the living cells or organisms.
Movement Type # 1. Movements of Locomotion:
Those movements in which whole of the plant body or the cell or cytoplasm moves from one place to another are called as movements of locomotion. These movements may occur either spontaneously or in response to a certain external stimulus and are called as autonomic and paratonic (or induced) movements respectively. Paratonic movements of locomotion are also known as tactic movements.
(a) Autonomic Movements of Locomotion:
(1) Ciliary Movements:
Such type of movements take place due to the presence of cilia or flagella e.g., Chlamydomonas, Volvox, flagellated bacteria, flagellated or ciliated reproductive cells etc.
(2) Amoeboid Movements:
Such movements are exhibited by Myxomycetes where the naked Plasmodium moves by producing pseudopodia like an Amoeba.
In living cells of many plants the cytoplasm including various cell-organelles moves around the vacuoles. This movement of the cytoplasm is called protoplasmic streaming or cyclosis. It is of two types—rotation and circulation. In rotation, which is exhibited by plants like Chara, Hydrilla Vallisneria, Elodea etc., the cytoplasm moves either clockwise or anti-clockwise around a larger central vacuole. While in circulation, which is exhibited by the cell of staminal hairs of plants like Tradeschantia, the cytoplasm moves in both clockwise and anticlockwise directions around many smaller vacuoles.
(b) Paratonic or Induced Movements of Locomotion or the Tactic Movements or Taxes:
(1) Phototactic Movements or Phototaxis:
These movements occur in response to an external stimulus, the light and are exhibited by zoospores and gametes of certain algae e.g., Chlamydomonas, Volvox, Ulothrix, Cladophora etc. They show a positive phototactic movement under diffused light and a negative phototactic movement under intense light.
(2) Chemotactic Movements or chemo-taxis:
These movements occur in response to an external chemical stimulus. Such movements are exhibited most commonly by the antherozoids in bryophytes and pteridophytes where the archegonia secrete some chemical substances having a peculiar odour towards which the antherozoids are attracted chemotactically.
(3) Thermotatic Movements or thermotaxis:
Such movements result due to an external heat stimulus. For instance, if a large vessel containing some Chlamydomonas in cold water is warmed on one side, the Chlamydomonas cells will move and collect towards the warmer side (positive thermotaxis). However, a negative thermotaxis will occur if the temperature becomes too high.
Movement Type # 2. Movements of Curvature:
In higher plants which are fixed, the movements are restricted only to the bending or, curvature of some of their parts. Such movements are called as curvature movements and may be either autonomic i.e., spontaneous or paratonic i.e., induced. The curvature movements may be of two types—variation movements and growth movements. In variation movements the curvature or the bending of the plant part is temporary while in growth movements it is of permanent nature.
(a) Autonomic Movements of Curvature:
(1) Autonomic movements of variation:
Telegraph plant (Desmodium gyrans) is an excellent example of such movements. In this plant the compound leaf consists of a larger terminal and two smaller lateral leaflets (Fig. 21.2). During day time, the two lateral leaflets exhibit peculiar and interesting movements.
Sometime they move upward at an angle of 90° and come to lie parallel to the rachis. Again, they may move downward at 180° so that they are parallel to the rachis. They may again move upward at 90° to come in their original position. All these movements occur with jerks after intervals, each movement being completed in about 2 minutes.
(2) Autonomic Movements of Growth:
(i) Hyponastic and epinastic movements:
These movements occur in bifacial organs like young leaves, flower sepals, petals etc., and result due to the differential growth on the two sides of such organs. For instance, if there is more growth on the lower side of sepals and petals the flower will close. Such movements are called as hyponastic movements.
On the other hand, if there is more growth on their upper side the flower will open. Such movements are called as epinastic movements. Examples of these nastic movements may be found in ferns where the leaves (fronds) become circinately coiled in young condition (hyponasty) and erect in older condition (epinasty) or in the opening and closing of flowers in many plants such as Crocus.
(ii) Nutational movements:
Sometimes the growth of the stem apices occurs in a zig-zag manner. It is because the two sides of the stem apex alternatively grow more. Such growth movements are called as nutational movements and are common in those stem apices which are not strictly rounded but flattened.
(iii) Circumnutational movements:
In strictly rounded apices the growth occurs in a rotational way. It is because the region of maximum growth gradually passes round the growing apex. Such movements are called as circumnutational movements.
(b) Paratonic Movements of Curvature:
(1) Paratonic movements of growth or tropical movements or tropisms:
When growth movements occur in response to an external stimulus which is unidirectional, they are called as tropical movements and the phenomenon of such a movement is called as tropism. Depending upon the nature of the unidirectional external stimulus the tropical movements are of many types:-
(i) Geotropic movements or geotropism (gravitropism):
The tropical movements which take place in response to the gravity stimulus are called as geotropic movements and this phenomenon as geotropism. The primary roots grow down into the soil and are positively geotropic.
The secondary roots growing at rights angles to the force of gravity are called as Diageo tropic. While those growing at some intermediate angle (between 0° and 90° to the vertical) are said to be plagiogeotropic (plagiogravitropic). On the other hand, the primary stems are negatively geotropic.
Geotropism in primary roots and stems can easily be demonstrated by sowing certain maize seeds in the soil so that their radicles lie in different directions. After a few days it will be noticed that irrespective of their position, the radicles in all the seeds always go down while the coleoptile always grow in upward direction (Fig 21.3).
That the geotropic curvature results due to unilateral gravity stimulus can be demonstrated by using a clinostat (Fig 21.4). If a young potted plant is fixed on a clinostat in horizontal position and rotated, neither the root will bend down nor the stem will curve upward. It is because in such case, the effect of gravity will be uniform all round the stem and root.
If however, the plant lies in horizontal position and is not rotated, the stem and the root will receive gravity stimulus only on their lower sides or the effect of gravity will be unilateral. This will results in a positive geotropic curvature in root and negative geotropic curvature in stem.
In case of roots, the gravity stimulus is perceived only by the root cap which covers the root tip. However, the geotropic curvature takes place a little behind the root tip, in the region of cell elongation. The effect of the unilateral stimulus of gravity causes unequal distribution of growth hormone auxin in the root tip i.e., more auxin concentrates on the lower side than on the upper side. This in turn results in more growth on the upper side and less growth on lower side, and ultimately a positive geotropic curvature is observed (Fig. 21.5).
But, in case of stem the higher concentration of auxin on the lower side promotes more growth on that side so that a negative geotropic curvature is observed. (The root cap is a thimble-like mass of cells which covers the root tip. It consists of a central cylinder called columella in which cells are arranged in regular tiers. Columella is surrounded by one or more layers of peripheral cells. (literally, thimble means a cap-like cover with a pitted surface worn in sewing to protect the end of finger that pushes the needle).
Microsurgical removal of root cap abolishes the gravitropic response of the root without however, interfering with its elongation or growth. Replacing the root cap or regeneration of root cap after an interval of time, restores the gravity response of the root. Within the root cap, the cells of columella especially the innermost ones, are sensitive to the gravity stimulus and not the peripheral cells of the root cap.
In the beginning of the 20th century, Nemec (1901) and Haberlandt (1902) put forward starch-statolith hypothesis independently to explain mechanism of gravitropic response by roots. According to this hypothesis, some specialized plastids called amyloplasts containing a few starch grains inside them are present in columella cells of the root cap. These amyloplasts and the columella cells, which contain these can sense gravity stimulus and have been called as statoliths and statocytes respectively.
When root tip along with its root cap is placed horizontally, the statoliths sediments under the influence of gravity stimulus on the basal sides of the statocytes and provide the basic perception mechanism for gravitropic response. The starch-statolith hypothesis has been supported by many scientists but also rejected by others and it has never received universal acceptance).
(ii) Phototropic movements or phototropism:
The tropical movements which occur in response to an external unilateral light stimulus are called as phototropic movements. These movements are commonly found in young stem tips which curve towards the unilateral light stimulus and thus, are called as positively phototropic.
This can be observed very easily by placing a potted plant in a room near an open window. After a few hours, the stem will be seen bending towards the window, the latter being the unilateral source of light (Fig. 21.6). The roots in some plants also exhibit phototropic movements but they are negatively phototropic.
When the stem tip receives uniform light all around, the concentration of the growth hormone auxin also remains uniform in the tip. But when the tip receives unilateral light, the conc. of auxin becomes more in the shaded side than in the lighted side. Consequently, the higher conc. of auxin in the shaded side causes that side to grow more resulting ultimately in a positive phototropic curvature (Fig. 21.7).
If however, a small young potted plant receiving unilateral light is fixed on a clinostat in a vertical position and rotated, there will be no phototropic curvature in the stem. It is because in this case the stem tip will be receiving unilateral light all around its tip and there will be no unequal distribution of the auxin.
Unilateral blue light is also known to be effective and essential in causing phototropic curvature.
(iii) Thigmotropic or haptotropic movements:
These movements take place in response to a touch or contact stimulus and are very common in plants which climb by tendrils (Fig. 21.8).
In such plants e.g., Passiflora, the tip of the tendril in the beginning moves freely in the air. 33ut as soon as it comes in contact with a solid object which may provide it support (i.e., it gets the contact stimulus), it twines round the object so that the plant may climb upward. The twining of the tendril around the support is due to less growth on that side of the tendril which is in contact with the support than the more growth on the free opposite side.
(iv) Hydrotropic movement or hydrotropism:
The tropical movements occurring in response to water stimulus are called as hydrotropic movements. These are commonly found in young roots and can be demonstrated by the following simple experiment: Some seeds soaked in water the previous night are kept on a wire gauze covered with saw dust. The water gauze is then kept slanting in humid condition. After a few days, the radicles will be seen bending towards the moist saw dust (Fig. 21.9).
Cheinotropic movements occur in response to some chemical stimulus and are best exhibited by fungal hyphae and pollen tubes.
(vi) Thermotropism and Aerotropism:
These tropical movements are not very important. When they occur in response to temperature stimulus, they are called as thermotropic movements. In case the stimulus is air, they are called as aerotropic movements.
(2) Paratonic movements of variation or nastic movements:
When growth movements occur in response to an external stimulus which is not unidirectional but diffused, they are called as nastic movements. These movements occur only in bifacial structures like leaves, sepals, petals etc., and may be of many types:
(i) Nyctinastic movements (or sleep movements):
In many plants the leaves and flowers acquire a particular but different position during day and at night. Such movements are called as nyctinastic movements or sleep movements. If these movements result in response to the presence or absence of light, they are called as photonastic movements e.g., Oxalis sp. (Fig. 21.10) where the flowers and leaves open in the morning and close at night. In other plants such as Crocus and Tulip the flowers open at higher temperatures. Such movements which occur in response to temperature stimulus are called as thermonastic movements.
(ii) Seismonastic movements:
These movements are best exhibited by sensitive plant (Mimosa pudica) and occur in response to a touch or shock stimulus including shaking or wind, falling of rain drops, wounding by cutting and intense heating or burning.
In this plant the leaves are bipinnately compound with a swollen pulvinus at the bases of each leaf and similar but smaller pulvinules at the bases of each leaflet or pinna. If a terminal pinnule of a leaflet is touched or given a shock treatment, the stimulus passes downward to the pulvinule and all the pinnules of that leaflet get successively closed in pairs. Now the stimulus passes to the other pinnae or leaflets so that their pinnules also close down and finally it reaches the pulvinus resulting in drooping of whole of the leaf (Fig. 21.11 A, B). Whole of this process is completed just in few seconds.
The pulvinus contains a number of specialised large thin walled parenchymatous cells called motor cells which undergo reversible changes in turgor in response to the stimulus. When stimulus reaches the pulvinus, the osmotic pressure of motor cells is decreased. Consequently, water is released from them into intercellular spaces and they suddenly collapse resulting in drooping down of the leaflets and the leaf.
After the lapse of sometime, the leaf recovers from the shock or touch stimulus, the turgor of motor cells is restored and the leaflets and the leaf come in their normal erect position (see, Fig. 21.11 A & C). It is now well established that almost any part of Mimosa plant can perceive the stimulus and transmits it to the pulvinus as electric pulses through phloem sieve tubes at velocities up to 2 cm s-1. The appearance of the action potential is correlated with rapid uptake of protons (H+).
When action potential reaches the pulvinus, it stimulates rapid efflux of both K+ and sugars from motor cells into the apoplast (cell walls and intercellular spaces) decreasing their osmotic pressure. Consequently, water is released from the motor cells, which now become flaccid due to loss of turgor and collapse resulting in drooping down of the leaf. After sometime, reverse changes occur to restore the turgor of motor cells and the leaf comes in its original straight position again.
Some scientists especially Hermann Schildknetcht (1983, 1984), have found certain chemical substances isolated from phloem sap of Mimosa pudica and Acacia Karroo (the latter plant is not senstive to touch but exhibits nyctinasty) which activate pulvini in these plants when applied to cut end of the stem. These chemical substances have been called as turgorins by Schildknetcht and are identified as β-D-glucosides of gallic acid. Chemical structure of one of the most active turgorins (previously known as periodic leaf movement factors (PLMFs) is given in Fig. 21.12.
It is believed that turgorins may give rise to action potential in a manner similar to the neurotransmitter acetylcholine in animals but at a much lower velocity (Action potential travels along the animal nerve cells at velocities of tens of meters per second; plants do not have nerve tissues and the action potential may travel only upto 2 cm s-1). In both the cases (plants and animals), action potentials are caused by flexes of specific ions across the cell membranes.
(Turgorins are so named because they act on turgor of pulvini. These have been isolated from over a dozen higher plants which exhibit nyctinastic movements and are believed to be hormones that control nastic movements).
(iii) Thigmonastic or haptonastic movements:
The movements are found in the leaves of Drosera (Sundew) and Dionaea (Venus Fly Trap) and result in response to the touch stimulus of the insects. In Drosera, as soon as an insect sits on the leaf, the tentacles curve inward to trap the insect. Similarly in Dionaea, the two halves of the leaf curve upward along the midrib. These parts of the leaves come to their normal position after the insect has been digested.
Movement Type # 3. Hygroscopic Movements:
These movements are found only in dead parts of the plants which are hygroscopic in nature and result either due to loss or gain of water by them from the atmosphere. Hygroscopic movements can best be observed in the elaters in bryophytes, peristome teeth in moss capsules, elaters of Equisetum spores etc.