1. Photoperiodism (Light period): The effects of photoperiods or daily duration of light periods (and dark periods) on the growth and development of plants, especially flowering is called photoperiodism. The role of photoperiodism in the control of flowering was demonstrated for the first time by W.W Garner and H.A.Allard (1920). They observed that Maryland Mammoth variety of tobacco could be made to flower in summer by reducing the light hours with artificial darkning. It could be made to remain vegetative in winter by providing extra light. Later, it was found that most plants would flower only if they were exposed to light for less or more than a certain period, the critical photoperiod, each day. Subsequently, it was observed that in light dark cycle, dark period is crucial in initiating flowring and not the light period as thought earlier. On the basis of length of photoperiod requirements of plants, the plants have been classified into following categories.
   1. Short day plants (SDP): These plants initiate flowering when the day length (Photoperiod) become shorter than a certain critical period. The critical day length differs with different species. The short day plants remain vegetative, if the day length exceeds the critical periods. Most of winter flowering plants belong to this category e.g. cocklebur (Xanthium), Chrysanthemum, sugarcane, tobacco (Mutant Maryland Mammoth), soyabean, strawberry etc.,
   2. Long day plants (LDP): These plants begin flowering when the day length exceeds a critical length. This length too differs from species to species. The long day plants fail to flower, if the day length is shorter than the critical period. Some common examples of long day plants are spinach (Spinacea oleracea), henbane (Hyoscymus niger), radish, sugar-beet, wheat, lattuce, poppy, larkspur, maize etc.
   3. Day neutral plants: These plants can flower in all possible photoperiods. The day neutral plants can blossom thorughout the year. Some common examples of this category of plants are cucumber, cotton, sunflower, tomato, some varieties of pea, etc.

24 hours

 

 

18 minutes

 

 

 

 

 

Hours

 

6 hr

Short day plant

 

Day length less than 12 hrs for flowering

 

24

 

 

18                                    6

 

 

12

 

24

 

 

8                                     6

Long day plant

 

Day length less than 12 hrs for flowering

 

 

 

 

 

Neutral plant Day length immaterial

for flowering

12

Fig : The day-length requirements for flowering in three catagories of plants

4. Intermediate plants : These plants flower only under day lengths within a certain range usually between 12-16 hours of light but fail to flower under either longer or shorter photoperiods. Examples of intermediate plants are Mikania scandens, Eupatorium hyssopi folium and Phaseolous polystacous.

 

 

 

1. Ampiphotoperiodic plants: Such plants ramain vegetative on intermediate day length and flower only on shorter or longer day lengths. Example of such plant is Media elegans.
2. Short long day plants: These plants require short photoperiods for initiation of flowering and long photoperiods for blossoming. Examples of these plants are some varieties of Triticum vulgare, Secale cereale.
3. Long short day plants: These plants require long photoperiods for initiation of flowering and short photoperiods for blossoming some common examples of these plants are Bryophyllum, Cestrum.

Critical period: Critical photoperiod is that continuous duration of light, which must not be exceeded in short day plants and should always be exceeded in long day plant in order to bring them to flower. There is no relation with the total day length. Thus, the real distinction between a SDP and LDP is whether flowering is induced by photoperiods shorter or longer than the critical period. The critical day length for Xanthium (a short day plant) is

15. 6 hours and that for Hyoscymus niger (a long day plant) is about 11 hours, yet the former is SDP as it flowers in photoperiods shorter than its critical value, whereas the latter is LDP requiring photoperiods longer than its critical value. Both Xanthium and Hyoscymus niger flower with 14 hours of light per day. Thus, day length in which a plant flowers is no indication of its response class in the absence of further information.

2. Skotoperiodism (Dark period): When photoperiodism was discovered, the duration of the light period was thought to be critical for flowering.

Subsequently, it was found that when the long night period was interrupted by a brief exposure to light, the short day plants, failed to

flower. Thus, for flowering, these plants require a long night or critical dark period rather than a short day length. Similarly, long day plants respond to nights shorter than the critical dark period. Curiously, they do not need an uninterrupted dark period, Therefore, a short day plant is also called long night plant and a long day plant as a short night plant.

In the night interruption experiments, when the short day plants were exposed to a flash of light before achieving a critical dark period, flowering was prevented. It is called light break reaction. If this was followed by exposure to far-red light (740 nm), the effect was reversed. Red, far red exposures given in succession showed that plant response is determined by the last exposure. Thus, photoperiodic response (flowering) is a phytochrome mediated process. The phytochorme shows reversible change is red (660nm) and far-red (730nm) wavelength.

On absorbing red light Pr is converted into Pfr. The Pfr becomes Pr either rapidly by absorbing far-red light or slowly in darkness. Thus, darkness or far-red light promotes Pr

Day                                   Night

 

 

 

 

 

White light

 

 

 

 

Red light (R)

 

 

 

 

 

Far red light (FR)

 

 

 

 

R       FR

 

 

 

 

 

R      FR       R

 

 

Flowering

 

 

 

 

No-flowering

 

 

 

 

No-flowering

 

 

 

 

 

 

Flowering

 

 

 

 

 

 

Flowering

 

 

 

 

No-flowering

Fig : Effect of night (Dark) interruption on flowering in a short-day plant

 

 

formation and stimulates flowering in short day plants, on the contrary, sunlight or red light promotes Pfr formation and stimulate flowering in long day plants.

(3)Mechanism of photoperiodism

1. photoperiodic perception: Experiments have demonstrated that photoperiodic stimulus is perceived by the fully developed leaves. Very young or first few leaves are commonly insensitive. In Xanthium (a short day plant) single leaf or even one eight part of a leaf was sufficient for this purpose. Further, a single leaf exposed to short days was able to induce flowering, when it was grafted on to a plant kept under non- inductive conditions.
2. Photoperiodic induction: Conditions under which the effect of suitable cycle of light and dark periods can persist in a plant and leads to flowering is called photoperiodic induction. It generally occurs when the plant has achieved certain minimum vegetative growth.e.g. 8 leaves in Xanthium strumarium. Minimum vegetative growth provides the plant with ripeness to flower. Some plants are however, exception to it and can be photo induced even in their cotyledonary stage.e.g. Chenopodium rubrum. The minimum number of appropriate photoperiods (inductive cycle) required for induction varies from species to species e.g. one for Xanthium.
3. Photoreceptor: The chemical which perceives the photoperiodic stimulus in leaves is phytochrome. The wavelengths of light are absorbed by the leaves. This becomes evident by the fact that defoliated (leaves removed) plant does not flower. Presence of even a single leaf is sufficient to receive required amount of photoperiod. Partially mature leaves are more senstitive to light while very young or mature leaves are much less sensitive to photoperiodic induction.

Garner and Allard’s early worked led to the discovery, isolation and much of the characterization of the pigment responsible for absorbing light involved in photoperiodic phenomenon of plants. Borthwick, Hendricks and their colleagues later termed this pigment phytochrome. Pigment was isolated by Butter et al. (1959). This pigment controls several light dependent developmental processes in plants besides flowering, phytochrome exist in two interconvertible forms. The red (660nm), absorbing form Pr and the far red (740 nm), absorbing form Pfr . Pr is converted to Pfr on absorbing far red light. Pfr is converted to Pr rapidly absorbing far red light or slowly in darkness. The slow conversion to red absorbing form is under thermal control. During the day when white light available, Pfr accumulates in the plant. This form of phytochrome is inhibitory to flowering in short day plants and stimulatory to flowering in long day plants. In evening, Pfr undergoes thermal and spontaneous decay to change into Pr. This pigment is stimulatory to flowering in short day plants and inhibitory to flowering in long day plants.

 

 

 

 

 

 

 

 

 

 

 

Fig : The phytocrome concept

 

Therefore, in SOP interruption of dark periods with a flash of red light converts Pr into Pfr and flowering is inhibited.

 

 

1. Structure/Chemistry  of  phytochrome:  The clarification of he chemical structure of phytochorme was due to isolation efforts and purification of phytochrome from several plant sources by Borthwick, Hendricks and their colleagues. Phytochrome was initially isolated from cotyledons of etiolated turnip seedlings. Siegelman and Firer were responsible for a highly purified extract that led to further purifications and analysis of the phytochorme structure.

Phytochrome is a chromoprotein with a chromophore (Pigment coloured protein) prosthetic group (e.g. chromoprotein). The chromophore group is a linear tetrapyrrole that differs in the conformation and absorption spectrum of its Pr state clearly from its Pfr state. A similar group with comparable conformational changes occurs in the bilirubins of red algae, though they bear an ethyl group instead of the vinyl group at their D-ring. Further there is probably one chromatophore for each phytochrome molecule. The chromatophore is linked to the protein at ring

3.  

Apparently the photo-conversions of the Pr and Pfr forms involve electronic changes in ring I, with either addition or loss of a proton. Conformational (structural) changes in the protein probably contribute to dark conversion and possibly decay.

 

Fig : Structure of chromophore of phytochrome and its relation with protein element

The protein is a dimer of two identical subunits with molecular weights ranging from 120000 to 127000 in different plant species. It is an allosteric protein.

2. Importance of phytochrome : Phytochrome is located in plasma membrane. Phytochorme far red Pfr form is considered to be biologically active form and is responsible to initiate a number of physiological process such as.
   • Elongation of stem and leaves.
   • Plastids morphology and differentiation of stomata.
   • Seed germination.
   • Photoperiodism and transpiration.
   • The florigen complex (Flowering hormone): When the proper amount of light is perceived by leaves, they produce a chemical (flowering hormone), which undergoes stabilisation in dark. Later on, this chemical passes to shoot apex and causes its differentiation into flowering shoot.

The various experiments discussed in the foregoing section provide strong evidence for the production of flowering hormone in plants under suitable photoperiods. Chailakhyan (1936) a Russian investigator on photoperiodism, proposed that it be called ‘florigen’. According to him (1958) the “Florigen complex” the true flowering hormone includes two groups of substances formed in leaves :

 

 

 

1. Gibberellins : Which are necessary for formation and growth of stem.
2. Anthesins : Substances which are necessary for flower formation.

Acting together Gibberellins and Anthesin produce the effect as ascribed to florigen.

Induction of flowering

 

 

CO₁

Light

A

Florigen

A

 

 

 

 

 

Fig : Photoperiodic induction and formation of florigen which is translocated to growing point for formation of flowers

The flowering stimulus moves readily not only through the plant but also from plant across the graft union between a flowering plant and a non-flowering or vegetative plant. Lang performed grafting experiments and demonstrated that every type of grafting is possible e.g., intervarietal, interspecific and intergeneric.

Short day (12 hrs)                                                                Long day (16 hrs)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1. (B)                                       (C)

 

Fig : Demonstration through grafting showing that flower inducing stimulus is a chemical

 

5. Photomorphogenesis: When plants are grown in continuous darkness they become etiolated i.e. such plants are longer, weaker, having yellowish half opened leaves, while light grown plants do not show such conditions. When etiolated plants are kept in light they gradually develop green colour and become normal. The effect of light in reversing etiolation involves two kinds of action; one the biochemical level for the synthesis of the chorophyll and secondly at the level of morphogenesis light acts to promote expansion of the leaves and inhibits elongation of the internodes. This phenomenon is called photomorphogenesis and is independent of the direction of light.

The action spectrum of photomorphogenesis reveals that plants are most sensitive to red light, but blue light is ineffective.

 

 

 

4. Vernalization: Many plants, especially biennials do not flower before they experience a low temperature. They grow vegetative during the warm season, receive low temperature during winter, grow further and then bear flowers and fruits. Russian agronomist Lysenko coined the term vernalization (1929-30). According to him vernalization may be defined as the method of inducing early flowering in plants by pretreatment of their seeds at low temperatures. Chourad (1960) has defined it as the acquisition or acceleration of the ability to flower by chilling treatment. The low temperature requirement for flowering was first noticed by Klipport (1857) while working with winter varieties of cereals such as wheat, barley, oat and rye. He observed that, these varieties when sown in spring failed to flower the same year but grow vegetatively. Such winter varieties, when

 

 

Chilled for two months at 5°C, planted at ordinary temperature

Vernalised

 

 

Slightly germinated seed

sown in the autumn, they flowered in spring of the same year.

1. Site of vernalization: The stimulus of vernalization is perceived

No chilling planted at ordinary temperature

Not vernalised

only by the meristematic cells such as shoot tip, embryo tips, root apex, developing leaves etc.

Fig : Experiment to show effect of vernalization on winter Rye.

2. Requirement of vernalization : Vernalization treatment requires three conditions (a) Low temperature – Low temperature required for vernalization is usually 0-4^oC is most of the cases. The chilling treatment should not be immediately followed by high temperature (i.e., about 40^oC), otherwise the effect of vernalization is lost. This phenomenon is called de-vernalization. (b) Duration of low temperature treatment

– It varies from species to species from a few houses to a few days, (c) Actively dividing cells- Vernalization stimulus is perceived only by actively dividing cells. Therefore, vernalization treatment can be given to the germinating seeds or whole plant with meristematic tissues and other conditions are (d) Water- Proper hydration is must for perceiving the stimulus of vernalization (e) Oxygen – Aerobic respiration is also a requirements for vernalization.

3. Process of vernalization: Usually vernalization treatment is given to the germinating seeds. The seeds are moistened sufficiently to allow their germination.They are then exposed to a temperature of 0-4^oC for a few weeks and sown to the fields. Lysenko develops the process of vernalization it is completed in two stages.

1. Thermostage: Germinating seeds are treated with 0-5^oC in presence of oxygen and slight moisture. The seed dormancy is broken.
2. Photostage: The stage is very essential to initiate the reproductive phase. After vernalization plants must be subjected to a correct photoperiod in order that they may produce flower.

4. Mechanism of vernalization : The stimulus received by the actively dividing cells of shoot or embryo tip is translocated to all parts of the plant and prepare it to flower. The stimulus has been named as vernalin (reported by Mechlers). It can be passed from one plant to another through grafting in case of Henbane but not in others. However, vernalin has not been isolated and identified. In some plants cold treatment can be replaced by gibberellins. It was reported by Lang. It has also been observed that the endogenous level of gibberellins enhances in vernalized plants. Therefore, it is suggested that the stimulus of vernalization that induces flowering could be particular gibberellin or a mixture of gibberellins. However, the correct mechanism is still not known and needs through investigation.

(v)Importance of vernalization

1. Vernalization is believed to overcome some inhibitor and induce synthesis of growth hormones like gibberellins.
2. It reduces the vegetative period of plant.
3. It prepares the plant for flowering.
4. It increases yield, resistance to cold and diseases.
5. Vernalization can remove kernel wrinkles in wheat.
6. Vernalization is beneficial in reducing the period between germination and flowering. Thus more than one crop can be obtained during a year.

 

 

 

 

 Senescence and Death .                                                                                                                                     

Plant and their parts develop continuously from germination until death. The production of flowers, fruits and seeds in annuals and biennials leads to senescence. The latter part of the developmental process, which leads from maturity to the ultimate complete loss of organization and function is termed senescence. Several workers equate ageing and senescence as same process. Ageing is a sum total of changes in the total plant or its constituents while senescence represents degenerative and irreversible changes in a plant. The study of plant senescence is called phytogerontology.

1. Types of senescence : Plant senescence is of four types- whole plant senescence, shoot senescence, sequential senescence and simultaneous senescence. The last three are also called organ senescence.

1. Whole plant senescence : It is found in monocarpic plants which flower and fruit only once in their life cycle. The plants may be annual (e.g. rice, wheat, gram, mustard etc.), biennials (e.g. cabbage, henbane) or perennials (e.g. certain bamboos). The plant dies soon after ripening of seeds.
2.  

Shoot senescence: This type of senescence is found in certain perennial plants which possess underground perennating structures like rhizomes, bulbs, corm etc. The above ground part of the shoot dies each year after flowering and fruiting, but the underground part (stem and root) survives and puts out new shoots again next year. e.g. banana, gladiolus, ginger etc.

 

Whole plant senescence (A)

Progressive or sequential leaf senescence

2.  

Shoot senescence (C)

Simultaneous leaf senescence (D)

Fig : Types of plants and senescence (shaded region is the part undergoing senescence)

3. Sequential senescence: This is found in many perennial plants in which the tips of main shoot and branches remain in a meristematic state and continue to produce new buds and leaves. The older leaves and lateral organs like branches show senescence and die. Sequential senescence is apparent in evergreen plants e.g. Eucalyptus, Pinus, Mango.
4. Simultaneous or synchronous senescence: It is found is temperate deciduous trees such as elm and muple. These plants shed all their leaves in autumn and develop new leaves in spring. Because of this shedding of leaves, autumn season is also called fall. e.g. Dalbergia, Elm, Mulberry, Poplar.
   2. Theories of senescence: Several theories have been put forth regarding senescence. Some important ones are given below.
      1. Wear and tear: According to this theory, senescence occurs due to loss of activity and cells undergo wear and tear due to disintegration of organelles.

 

 

2. Toxicity: It is viewed that senescence takes place due to accumulation of toxic and deleterious substances in all.
3. Loss of metabolites: It is assumed that senescence leads to gradual depletion of essential metabolites in a cell.

(iv)Genetic damage

5. Differences between senescence, ageing and death

 

Characters	Senescence	Ageing	Death
1. Definition	Senescence: It refers to all collective, progressive and deteriorative process which ultimately leads to complete loss of organization and function.	Ageing: It includes all the chemical and structural changes, which occur during the life span of a plant or its organ.	Death: It is the ultimate termination of functional life of plant part.
2. Changes	It includes only degenerative and deteriorative changes in a plant or its parts.	It is sum total of metabolic changes that occur in plant or its parts.	It is a regular feature of the annual cycle of plants which is usually preceded by senescence.
3. Occurrence	Senescence occurs as a result of ageing and leads to death.	Ageing is a permanent feature of all living organisms.	Death is a permanent feature of all living organisms.

(3)Characteristics of ageing and senescence

1. There is general decline in metabolic activities decline in ATP synthesis and also decreased potency of chloroplast.
2. Decrease in RNA and DNA
3. Decrease in semipermeability of cytoplasmic membranes.
4. Decrease in the capacity to repair and replace wornout cells.
5. There may be accumulation of chromosomal aberrations and gene mutations with advancing age as a result of these changes protein synthesis becomes defective.
6. Increased production of hydrolytic enzymes such as proteases and nucleases.
7. Deteriorative change in cell organelles and membranes.
8. Decrease in the internal content of auxin and cytokinins and increases in the production of abscisic acid or ethylene.
4. Importance of senescence : Biologically senescence and death have following advantages :
   1. It maintains efficiency since the old and inefficient organs are replaced by young efficient part like leaves, buds, flowers and fruits. etc.
   2. During senescence, the cellular breakdown results in release of many nutrients including amino acids, amides, nucleotides, simple sugars and minerals. The same are withdrawn from the senescing organs into the main trunk and later utilised in the growth and developed of new parts.
   3. Shoot senescence is a mechanism to help the plants perennate during the unfavourable periods.
   4. Simultaneous or synchronous leaf fall occurs in autumn prior to winter. It reduces transpiration, which is essential for survival in survival in winter, when the soil in frozen and roots can not absorb water.
   5. Litter of fallen leaves and twigs is an important source of humus and mineral replenishment for the soil.

 Abscission .                                                                                                                                                           

The process of shedding of leaves, fruits or flowers by a plant is called abscission. The shedding of plant parts takes place by the formation of a special layer of cells called abscission layer, within the region of attachment. The

 

 

 

middle lamella between certain cells in this layer in often digested by polysaccharide hydrolyzing enzymes such as cellulase and pectinases.

Certain other degenerative changes also occur making the region soft and weak. The organ from the plant is then easily detached whenever there is heavy rainfall or wind, etc.

 

 

 

 

 

Steam

Fruit

 

Leaf

 

 

 

No abscission

 

 

 

 

 

 

 

 

 

Leaf

 

 

 

 

 

 

Fruit

 

Vessels Cortex Epidermis

Cells of abscession layer

Abscission

 

 

Fig : Leaf and fruit abscission due to the formation of abscission layer

The abscission occurs due to a change in the hormonal balance. It has been observed that the abscission layer formation occurs rapidly when the auxin gradient becomes less i.e., less auxin on distal side than the proximal side of the leaf, flower or fruit. The plant hormones like ethylene and abscissic acid promote the abscission. A high concentration of auxin prevents the formation of a abscission layer.

Dormancy and germination of seeds

(See detail in Embryology module -II)

Important Tips

• SDP’s contain anthesins and synthesize gibberellic acid for flowing. Whereas LDP’s contain GA and synthesize anthesins for flowering.
• Leaves show maximum expansion in violet light.
• Impaction is the treatment given to seeds when they are shaken vigorously.
• The term negative growth is sometime used for senescence.
• Knott (1934) found that the locus of photoperiodic induction is the leaves.
• Wellensick (1964) found that the locus for perception of cold treatment is the meristmatic cells (at all places) especially the shoot apex .
• Reduced availability of auxin stimulates leaf fall while presence of auxin slows down leaf fall. Cytokinin prevent senescence through stimulating anabolic activity. They are called artiageing hormones Florigen hormone synthesized in the leaves.

 Plant movements .                                                                                                                                                

Movement is a change in position or place of an organ or organism. The movements in plants is not as much apparent as in the case of animals. But plants also show movements though they are fixed. They show movements of their parts. Such movements are not apparent except when observed after a time interval. They can however, be seen with the help of time lapse cameras.

 

 

Usually higher plants exhibit growth movements. Plants show movements in response to a variety of stimuli. Stimulus can be defined “as a change in external or internal environment of an organism that elicits response in the organism”. The reaction of plant to a stimulus is known as response”. The power or ability of a plant to respond to a stimulus is called sensitivity or reactivity or irritability.

The movements which occur without the effect of external stimulus are called autonomic or spontaneous movements. Thus spontaneous movements are brought by definite internal stimulus. And if the movements are produced in response to external stimulus, they are known as paratonic or induced movements.

The area which perceives a stimulus is called perceptive region, while the plants part showing the response is known as responsive region. The minimum duration or time required for a stimulus to be applied continuously on the perceptive region to produce visible response is called presentation time. The duration between the application of stimulus and production of visible response is called latent time or reaction time.

Classification of plant movements

Plants movements are broadly classified into two types:

1. Movements of locomotion
2. Movements of curvature

1. Movements of locomotion: In this case, plant moves physically from one place to another. The movements of locomation are of two type-autonomic (occurs spontaneously) or paratonic (induced by external stimuli).

1. Autonomic movement of locomotion : These movement of locomotion are due to internal stimuli they are of following types.

1. Ciliary movements: Certain motile algae (e.g. Chlamydomonas, Volvox, etc). Zoospores and gametes of lower plants move from one place to another by means of cilia or flagella.
2. Amoeboid movements: It is the movement of naked mass of protoplasm by means of producing pseudopodia like process e.g. members of Myxomycetes (slime fungi).

1

2                                                                                                                                      Protoplasm

 

7

 

3

6

5

4

1. Protoplasm        (B)                                                           (C)                                                 (D)

Fig : Locomotory movements (A) Ciliary (B) Amoeboid (C) Rotation (D) Circulation

 

3. Cyclosis: These are movements of cytoplasm with is a cell (also called protoplasmic streaming). These are of two types.
   • Rotation:       When the protoplasm moves around a single central vacuole in either clockwise or anticlockwise direction e.g. leaf cells of Hydrilla, Vallisneria.
   • Circulation: When the movement of protoplasm accurs around different vaculoes in different directions within the cell e.g. staminal hair of Tradescantia, shoot hairs of gourds.
4. Excretory movements : Apical part of oscillatoria like a pendulum. It is considered that such movements are due to exerction of substances by the plants. (movements opposite to the side of exerction).

 

 

 

2. Paratonic movement of locomotion (Tactic movement): These movements take place in whole small plants. e.g. chlamydomonas or small free ciliated organs e.g. gametes. These movements are due to external factors like light, temperature or chemicals and are of following types.

1. Phototactic movements or phototaxisms: It is the movement of free living arganism towords or away from light. e.g. movement of Chlamydomones, Ulothrix, Cladophora, Volvox etc. towards suitable light intensity. Three types of arrangement present in columular cells in chloroplast of dorsiventral leaves.
   • Parastrophe : In intense (maximum) light chloroplast cells arranged in longitudinal wall as a sequence manner.
   • Apostrophe : In minimum light chloroplast cells arranged in different manner.
   • Epistrophe: In dark chloroplast cells are arranged in transverse wall as sequence manner.

 

            

Parastrophe (In light)

Apostrophe (In darkness)

Fig : Photolactic movements on leaf of Lemna

Epistrophe (In shade)

2. Chemotactic movements or chemotaxisms : It is the movement of plant or plant parts from one place to another towards or away from chemical substance. e.g. Male gametes (antherozoids) of bryophyta move towards archegonia under the influence of sugars produced by neck canal cells and also in pteridophyta male gametes move towards archegonia due to the malic acid produced by disintegration of neck canal cells and ventral canal cells.
3. Thermotactic movements or thermotaxism: It is the movement of free living organism in response to external stimuli of temperature. e.g. Chlamydomonas move from cold water to medium warm water and from very hot water to medium temperature.

2. Movement of curvature : In these cases, plants are fixed, thus they fail to move from one place to another. Somehow, movement is noticed in the form of bend or curvature on any part of the plant. Movement of curvature can be classified into.
   1. Mechanical movement
   2. Vital movement

(A) Mechanical movements: These movements depends upon the presence or absence of water and occurs in non-living parts of plants. It is of two types.

(i) Hydrochasy:  This movement occurs due to the absorption of water.

Example: (a) Peristomial teeth of moss protrude out when the capsule is dry and curve when capsule is wet.

2. Spores of the Equisetum coil and uncoil in the presence and absence of water respectively.

 

 

 

Uncoiled elaters

Elaters

 

 

 

 

 

(A)

Coiled elaters

 

11

 

 

 

 

(B)

Fig : (A) Equisetum spore (B) Peristomial teeth of moss

 

 

 

 

 

 

 

(ii) Xerochasy : This movement occurs due to the loss of water.

Example: When water is lost from the annules of the sporangia of fern, it burst from stomium and spores are thus liberated out.

 

Fig : Movements in the annulus of fern sporangium

 

3. Vital movement: These movement are of two types :

1. Growth movements : These movements are due to unequal growth in different parts of an organ and are irreversible.

They are further divided into two types-autonomic (occurs spontaneously) and paratonic (induced by external stimuli).

(a)Autonomic growth movements

• Nutation (Nutatory movements): These movements occur in the growing stem of twiners and tendrils. The stem exhibits a kind of nodding movements in two directions. This is because the stem apex shows more growth on one side at one time and a little later there is a greater growth on the opposite side. It is called nutation. In spirally growing stems the region of greater growth passes gradually around the growing point resulting in the spiral coiling of stem and tendrils. Such a movement is called circumnutation. Coiling of a tendril after coming in

contact with a support is a thigmotropic movement.

• Nastic movements: They are non-directional movements in which the response is determined by the structure of the responsive organ

and not the direction of the stimulus. The responsive organ has

 

(A)

Fig : Nutations :

1. Nodding movement
2. Circumnutation

an asymmetrical or dorsiventral structure. Greater growth on one side causes the organ to bend to the opposite side. Greater growth on the adaxial side is called hyponasty. e.g. circinate coiling and closed sepals and petals in a floral buds. Whereas

More growth on lower surface

More growth on

upper surface

more growth on abaxial side is called epinasty. e.g. opening of fern leaf and spreading of sepals and petals during opening of the floral bud.

 

 

Hyponasty

(A)

Closing

 

Opening

 

 

Epinasty

(B)

(b)Paratonic growth movement (Tropic movements