In this article we will discuss about the growth and movement in plants.
Growth in Plants:
Growth is one of the fundamental features of the living organisms. Non-living objects like rocks and crystals may also grow by accretion or deposition of new materials, but growth in living things is something distinctly different. It is really a complex phenomenon regulated by protoplasm.
During anabolic activities complex food matters are synthesised and ultimately assimilated. As a result, an increase in size occurs and new tissues are formed. In Aatabolic activities complex matters are broken down for the liberation of energy, which is essential for life processes.
The two processes go on simultaneously, but anabolism gains over katabolism and the ultimate result is growth. Growth may be defined as a permanent increase in mass or volume, accompanied by irreversible change of form or structure.
Usually growth is attended by increase in weight. But in germinating seeds or sprouting potato tuber there is a loss of weight during growth, obviously due to utilisation of stored food matters.
In the lower plants, like unicellular organisms, growth is simple enlargement of the cell; but in the vast majority of plants, besides enlargement, new cells are continually formed which enlarge and mature in due course.
The process of growth consists of three phases:
(a) Phase of formation, involving formation of new cells. In plants it is localised in the meristematic regions. The apical meristems of stem-tip and root-tip are primarily responsible for the growth in length of the axis; and lateral meristems, cambium and cork-cambium, are instrumental to the secondary increase in thickness in the stems of dicotyledons and gymnosperms and roots of dicotyledons.
(b) Phase of elongation lies next to the phase of formation. Here the cells enlarge, protoplasm gets more and more vacuolated ultimately lining the cell wall. This is the region of active growth.
(c) Phase of maturation is the final phase where the cells assume permanent shape and form.
The rate of growth is imperceptibly slow in plants and so can be hardly detected and measured without the help of suitable instrument. Certain plant parts like tendrils of Cucurbita, shoot of bamboos, etc., however, exhibit rapid growth.
Auxonometer is a common instrument used for measuring growth by which the actual growth in length may be magnified many times and conveniently recorded. Lever aoxonometer or are-indicator has a pointer fixed to a wheel and a graduated arc as shown in figure 181.
A cord attached to the growing tip of a potted plant is passed over the wheel and allowed to remain suspended at the other end by a weight. With the growth of the tip the indicator moves and the increase is recorded on the graduated are, of course, on a magnified scale. From the record thus obtained the actual increase may be calculated.
Factors Affecting Growth:
The rate of growth is influenced by the simultaneous operation of a number of external and internal factors.
Some factors affecting growth are discussed below:
1. Supply of Nutritive Materials:
Growth, as we know, is the ‘result of the predominance of the constructive phases of metabolism over the destructive ones. For growth, plant organs always need sufficient supply of food which is digested and assimilated to form substances like cellulose to construct the skeleton of the plants. Like animals, plants also require a ‘balanced ration’, i.e. food matters in proper proportion for their activities and growth.
2. Moisture or Water:
Turgidity of the cells is a pre-requisite for growth, for the maintenance of which an ample supply of water is necessary. Water is also essential for carrying food matters to the growing regions. Protoplasm itself remains saturated in a high percentage of water. Generally plants of habitats with abundant water supply have luxuriant growth, as found in the tropical forests.
The nature of the soil, and particularly the nutrient matters present in the soil, profoundly influence the growth of plants, as we have seen in water culture experiments.
A supply of oxygen is necessary for respiration, by which process energy for the vital activities is released.
It has a marked effect on the rate of growth. A suitable temperature, which varies with climate and plants, is most conducive to growth. There are three cardinal points, viz. minimum, below which growth will not take place; optimum, which is most favourable and gives the highest rate of growth; and maximum, above which growth will not be possible. The optimum temperature, on the average, is near about 30°C. Within proper limits the rate of growth is roughly doubled with the rise of temperature by 10°G.
Though at the initial stage light, instead of accelerating the process, retards growth, yet it plays a very important role in the growth of the plants. It is the ultimate source of energy that is stored up during photosynthesis in the carbohydrates without which growth would not be possible. Besides, it has influence on the formation of chlorophyll and on the opening of the stomata.
Some lower plants may carry on throughout their life in darkness, but higher plants get etiolated in the absence of light. The organs become elongated and soft, are deprived of chlorophyll and thus –grow or become yellowish in colour. These plants ultimately perish for want of food. Only plants with sufficient stored up food matters can continue to live in darkness for a long time.
The duration of light has a positive influence on growth and particularly on flowering and fruiting. The response of plants to the daily period of illumination is called photoperiodism.
It has attracted the attention of many investigators in recent years who have shown experimentally that the rate of growth and time of flowering could be profoundly affected by the length of day-light. It may be of much economic importance, in view of the fact that by artificially changing the length of day-light the vegetative growth and flowering may be considerably controlled.
7. Growth Hormones:
These are organic growth-stimulating substances secreted by the plants. Much have been heard about hormones secreted by ductless glands in the animal bodies. Even plants secrete hormones which regulate growth, regeneration and the healing of wounds. Like vitamins very minute quantities of hormones are needed for proper action. Plant hormones go by the name auxins.
Grand Period of Growth:
The rate of growth is not uniform in plants. Plant cells or organs or even the whole plant show variations in the rate of growth. It starts slowly, the rate is gradually accelerated till it reaches the maximum, then falls slowly and ultimately comes to a standstill. The whole period from the initiation to the stoppage is referred to as the grand period of growth. It may be easily determined by measuring any growing plant organ at regular intervals.
The rate also varies with factors influencing growth. It is more rapid at night than in the day, obviously due to inhibiting influence of light. This is called diurnal variation of growth. There are seasonal variations as well. For many plants winter is very unfavourable for growth, whereas spring is the favourable period.
Vernalisation is the process by which seeds are subjected to treatments before sowing, to hasten flowering in the plants they would give rise to. The famous Russian botanist Lysenko suggested this pre-treatment method. The principle of vernalisation, to put very briefly, is as follows: Seeds are soaked as usual.
When germination has commenced moisture is controlled to arrest further growth. Now the seeds are subjected to treatments like exposure to light and darkness, high or low temperature. Seeds sown after these treatments would bring about earliness in flowering.
As vernalisation has immense possibilities in agriculture, it has created enough of sensation in recent years. Soviet Russia is the only country where it has been extensible practiced in fields with good results. Vernalisation is receiving the attention of investigators all over the world. In our country some scientists are engaged in research on vernalisation of economic plants.
According to some, vernalisation is an old practice under a new name. A process similar to vernalisation was recorded in an American reference book on agriculture sometime in the middle of the last century. Indian farmers also subject the seeds to pre- treatment (by cutting off light) before sowing, with good results without really knowing anything about the principle of vernalisation.
Movement in Plants:
Movement is another characteristic feature of the living things. Since most of the plants remain rooted to the soil they are unable to move from place to place like the animal. The capacity of movements of plants is often consequently doubted. Many lower forms of plants like unicellular organisms have the distinct power of locomotion. The higher plants show movements by changing position of the organs in various ways.
Opening and closing of the flowers, behaviour of the sensitive tendrils while coiling round a support, drooping of the leaflets of the sensitive plants when touched, movements of aerial stems and underground roots in response to light and force of gravity are some glaring instances of plant movements. There is no denying of the fact, however, that the rate of movement in higher plants is usually very slow.
Movements may be of two classes:
(A) Movement of Locomotion, where the entire organism may move from place to place.
(B) Movement of Curvature, where the organs of the stationary plants change positions and thus curve in different ways.
Both the kinds of movements may be:
(i) Autonomic or spontaneous, i.e. without any external influence, or
(ii) Induced, i.e. due to some kind of external influence or stimulus. The capacity of responding to stimulus is irritability which, we know, is an inherent property of protoplasm. Irritable movements help the plant organs to adjust themselves favourably to the environmental conditions, so that the vital activities may be most advantageously carried on.
A. Movements of Locomotion:
Amoeboid and ciliary movements of naked masses of protoplasm and streaming (rotatory and circulatory) movements of protoplasm in walled cells are instances of autonomic movement of locomotion. This type of movement of many algae may be seen under the microscope.
Movement of the entire organism may be influenced by the external stimuli like light, chemical substances, etc. These are called taxisms or tactic movements. Phototaxis is the induced movement of locomotion, where the stimulus is light.
Many algae show photo-taxis by moving towards weak and away from strong light. Chemotactic movement in response to stimulus of chemical substances is exhibited by the male gametes of fern and moss.
Fern spermatozoids rush towards the egg being attracted by malic acid and salts of malic acid secreted by the archegonium. Similarly sucrose secreted by the moss archegonia attracts the sperms. Many bacteria move towards the source of oxygen.
B. Movement of Curvature:
In fixed plants autonomic movements of organs are associated with growth or variations in the turgidity of the cells. Growth movements are clear in twiners and leaves. A stem twines round the support only on account of unequal growth on two sides of the organ. The same is the case with rolled or folded leaves, which remain rolled due to rapid growth on the under surface and open out due to the rapid growth of the upper surface.
Autonomic movement due to variations of the turgidity of the cells is noticed in Desmodium gyrans or Indian telegraph plant, where the two lateral leaflets move up and down from morning till evening, but the terminal one remains fixed (Fig. 183).
Induced movements of curvature are caused by external stimuli like light, force of gravity, moisture, etc. These movements are again of two kinds, viz. tropisms or tropic movements, where the direction of stimulus determines the direction of movement; and nastiest or nastic movements, where the direction of movement is independent of the direction of stimulus.
Tropic movements are usually of the following types:
Phototropism or heliotropism is the movement initiated by the stimulation of light. Stems always grow towards light. So they are positively phototropic. The roots go away from light and are negatively phototropic. Leaves orient themselves at right angles to the source of light just for absorbing the maximum solar energy required for photosynthesis.
They are transversely phototropic or diaphitotropic. When potted plants are placed on windows, the stem is always found to go outwards, i.e. towards the source of light. Phototropic curvatures may be shown by growing a mustard seedling in culture solution in a jar (Fig. 184).
The capacity of roots and stems to respond to the force of gravity is called geotropism. Roots always go downwards showing positive geotropism and stems grow upwards and are thus negatively geotropic.
The lateral roots and branches are transversely geotropic. If a seed is germinated and the axis is placed horizontally on a tray in a dark room (for eliminating the stimulus of light), in a day or two it will be found that the root is bending downwards and the stem upwards showing positive and negative geotropism of root and stem respectively. Curvature usually takes place in the region of elongation behind the tip.
It is the induced movement caused by the stimulus, moisture. Roots always grow towards the source of water and are thus positively hydro- tropic. An interesting example of negative hydrotropism can be observed in the pneumatophores of mangroves where the roots are simultaneously negatively hydrotropic and negatively geotropic.
This is the movement of curvature in response to contact with foreign bodies. Tendrils quickly coiling round a support, nicely exhibit haptotropic movement.
This movement in response to chemical stimulus is noticed in the leaves of Drosera, where the nitrogenous matters induce the bending down of the tentacles. The movement of the pollen-tube through the style towards the embryo-sac is also an instance of chemotropism.
Nastic movements are independent of the direction of stimulus, i.e. the stimulus acts from all directions and the direction of the response bears no relation whatsoever with any particular direction of the stimulus. Movements exhibited at the opening and closing of flowers in response to the variations of light intensity and temperature or both, are called nyctinastic movements.
Flowers like lotus, lily, sunflowers open in the morning and close in the evening; similarly nocturnal flowers like jasmine, Cestrum (B. Hasnahana) open in the evening. Leaves of many plants, particularly of family Leguminosae, show sleep movements resulting in the folding up of leaflets with the fall of light and temperature.
A movement as a result of shock stimulus is called seismonasty. An unusual but interesting type of seismonastic movement is exhibited by the leaves of sensitive plant, Mimosa pudica (B. Lajjabati). – Leaflets of this plant close up on touch (Fig. 185).
Though not directive, intensity of stimulus markedly affects the degree of movement. On slight shock one or two pairs of leaflets collapse but stronger stimulation results in closure of many leaflets and the ultimate drooping of the leaf.
Rapid conduction of the stimulus to all the aerial leaves is a striking feature of this movement in sensitive plant. Movements are due to the changes in the turgor of the cells of the swollen pulvinus (leaf-base).
Here the stimulus is perceived by the leaf, whereas the action takes place at a distance from it; there is apparently a sensory organ which receives the stimulus, and the motor organ is the swollen pulvinus which brings about the movement. An analogy is apparent here with the nervous mechanism in animals.