GROWTH AND DEVELOPMENT OF PLANTS

Learning Objectives
After studying or reading this, you should be able to:
1. Explain the concept of growth and development.

2. Measure growth in seedlings.

3. Explain primary and secondary growth in plants.

4. Distinguish between primary and secondary growth in plants.




Living organisms are able to transform the food they take in into living materials and as a result, they increase in size and weight. This irreversible increase in size and weight of an organism is known as growth. Growth in organisms is by production of new protoplasm and by cell division. Development on the other hand refers to increse in complexity.

The beginnings of patterns of growth and development that are characteristic of plants emerge in the early stages of embryo development, which takes place within the growing seed. Within such a seed, the mature angiosperm embryo consists of either one or two cotyledons. In general, monocot embryos have only one cotyledon, and dicot embryos have two. The food in the endosperm of the cotyledons is transferred to the young embryo during germination of the seed. The food is then used during the early development of the embryo during the germination and early establishment of the young plant. Starches, fats, and oils are converted into sugars and used to nourish and sustain the young plant before its photosynthesis begins.

In the embryo. the epical meristems differentiate early, continuing to divide throughout the life of the plant. The shoot apical meristem in a plant embryo is located at the tip of the epicotyl, the portion of the axis that extends above the cotyledon. The epicotyl, together with its young leaves, is called a plumule. The axis of the embryo below the cotyledons is called the hypocotyls; a distinct embryonic root at the lower end of the hypocotyls is known as radicle and this eventually develops into the primary root.

The emergence of the embryonic roots and shoot from the seed during germination vary widely from species to species. In many, the root develops emerges hrst and anchor the plant firmly in the soil before the shoot appears. Growth in plants is said to be indehnite because they do not have a iixed number of organs such as roots, stems, flowers, fruits and leaves.




PRIMARY AND SECONDARY GROWTH IN PLANTS
    ln plants, maximum growth occurs mainly at the tips of the roots and shoots. Plants grow by means of their apical meristerm (apical growth), zones of active cell division at the ends of the roots and the shoots. The epical meristem gives rise to three types of primary meristems. They are differentiated tissues in which active cell division continuous to take place. These are the protoderm, which gives rise to the epidermis; the procambium, which gives rise to the vascular tissues; and the ground meristem, which becomes the ground tissue. This is described as primary growth.

In addition to the primary growth in length occurring at the apex of the root or the shoot. a secondary growth occurs that adds xylem, or wood, to the inside of the root and phloem towards the outside. Xylem that derives from the shoot-and root-growing points is called primary xylem. In addition, new xylem, called secondary xylem, may be added by division of the cambium cells which is located between the xylem and phloem. This division gives rise to new xylem cells towards the centre in roots and towards the outside in most stems. Phloem produced in this manner becomes involved in the formation of bark, which covers old roots as Well as old stems. Some plants, however, have little or no secondary xylem. The division of the secondary xylem gives rise to secondary growth in plants.

In dlcot plants, the vascular tissue in the stems ls arranged in a ring and true secondary growth takes place, causing stems and roots to increase in diameter. in monocot plants, however, vascular tissue in scattered bundles in the stem; and no true secondary growth occurs.




PRIMARY AND SECONDARY GROWTH COMPARED
      Primary growth results from division of apical meristem which gives rise to three types of primary meristems: protoderm, procambium and ground meristem. The protoderm gives rise to the epidermis; the procambium gives rise to the vascular tissues; and the ground meristem becomes the ground tissue. The primary growth occurs at the apex of the root or the shoot. A secondary growth results from division of the cambium cells located between the xylem and the phloem. Secondary growth adds more xylem, or wood, to the inside of the root and phloem towards the outside. Xylem derives from the shoot~and root-growing points are called primary xylem. Xylem derives from division of the cambium cells which are located between the xylem and phloem is known as secondary xylem. Secondary growth in both stems and roots takes place after the formation of lateral meristems known as vascular cambia. These cylinders of dividing cells form xylem internally and phloem externally. As a result of their activity, the girth (diameter) of a plant increases.


AUXINS
      Once a seed has germinated, the plant's further development depends on the activities 0f the meristematic tissues, which themselves interact with the environment. The shoot and the root apical meristems give rise to all other cells of the plant. The growth and differentiation of the various plant tissue and organ systems are therefore controlled by various internal and external factors. Typical example of the internal factor is plant hormones.

Plant hormones are specialized chemical substances produced by plants and are the main internal factors controlling plant growth and development. Hormones are produced in one part of a plant and transported to other parts where they are effective in very small amounts. Depending on the target tissue, a given hormone may have different effects. Thus. auxin, one of the most important plant hormones, is produced by growing stem tips and transported to other areas where it may either promote growth or inhibit it.


ROLE OF AUXINS lN STEMS AND ROOT ELONGATION
     Auxins are plant hormones that control cell elongation. In stems, for example, auxin promotes cell elongation and the differentiation of vascular tissue, whereas in roots it inhibits growth in the main system but promotes the formation of adventitious roots. It also retards the abscission (dropping off) of flowers, fruits, and leaves.


GIBBERELLINS
   These are other important plant-growth hormones; more than 50 kinds are known. They control the elongation of stems, and they cause the germination of some grass seeds by initiating the production of enzymes that break down starch into sugars to nourish the plant embryo. Cytokinins promote the growth of lateral buds, acting in opposition to auxin; they also promote bud formation. in addition, plants produce the gas ethylene through the partial decomposition of certain hydrocarbons, and ethylene in turn regulates fruit maturation and abscission.





ROPIC, NASTIC AND TACTIC MOVEMENTS (RESPONSES) 0F PLANTS
      Various external factors, often acting together with hormones, are important in plant growth and development. One important class of responses to external stimuli is that of the tropisms. Tropisms are directional movement responses that occur in response to a directional stimulus. Plants are not able to relocate if they happen to start growing where conditions are suboptimal. However, plants can alter their growth so they can grow into more favorable conditions. To do so require the ability to detect where the conditions are better and then alter their growth so that they can "move" in the appropriate direction.

One of the most commonly observed tropic responses in plants is phototropism, in which plant stems grow towards light. As anyone who has grown plants near a window knows, the plants tend to lean towards the window where the light is usually stronger than inside the room. Another commonly observed tropic response is geotropism, where a plant will grow so that it stays oriented relative to the source of gravity (the earth). Thus, if a plant is knocked down the shoot will grow faster on the lower side until the shoot is more-or-less standing up again. Stems are negatively geotropic, growing upwards, whereas roots are positively geotropic, growing downwards.


Tropic responses result from differential growth. Phototropism is a blue-light-dependent response controlled by the action of specific blue light photoreceptors called phototropins. Geotropism is dependent on the presence of starch-filled plastids (amyloplasts) in specialized cells. When the orientation of the cells changes, the mass of the starch-filled plastids causes them to sink to the lower end of the cell. The tumbling of the amyloplasts triggers, through unknown mechanisms, differential growth that causes curvature to develop.

Phototropism is particularly important in the initiation of flowering. Some plants are shortday, flowering only when periods of light are less than a certain length. Other variables such as the age of the plant (internal) and temperature (external) are also involved with the complex beginnings of flowering.

Nastic movements are rapid, reversible responses to non-directional stimuli such as temperature, humidity, and light irradiance. These are diffused stimulus and therefore do not come from any particular direction. The opening and closing of flowers provides a good example for such nastic responses. For example, crocus flowers open when it is warm and close when it is cold (thermonasty) and the flowers of certain daisies open in the light and close in the dark (photonasty). In all these cases, a response involves differential growth in one part of the plant, resulting in a localized bending movement. In crocuses, for example, a rise in temperature causes accelerated growth on the inner side of the petals, so that the petals bend outwards and the flower opens.

Nastic movement is caused by turgor pressure change due to movement of water in cells as opposed to tropic movement which is actual growth and therefore irreversible. The rate or frequency of these responses increases as intensity of the stimuli increases. An example of such a response is the opening and closing of flowers (photonastic response). Nastic movements are generally slow and can be observed by time-lapse photography. Such movements as those of developing buds, which swell, open up, and eventually fall off, are examples of internally directed, or autonomic, nastic movements. Nastic responses are usually associated with plants.

Roots often grow towards moisture (hydrotropism) or towards certain chemical substances in the soil (chemotropism). A good example of chemotropism is the growth of the pollen tube towards the ovary in flowering plants. Some plants give a growth response to touch (chemotropism). The best example of this is provided by the tendrils of climbing plants which bend round an object with which they come into contact. in this case growth is slowed down on the side of the tendril experiencing the stimulus of touch. A rapid response of touch, comparable with animals in its speed of action, is seen in the spectacular closing of the leaves of Mimosa pudica in response to touch, and reactions of carnivorous plants such as sundew and Venus flytrap to contact with prey.





TACTIC MOVEMENT (TAXIES)
Tactic movement is the movement of an entire cell or organism (that is locomotion) in response to, and directed by, an external stimulus. The direction of the movement is obtained by the direction of the stimulus. Examples of tactic movement include: phototaxis and chemotaxis in phototaxis, the stimulus is light: and in chemotaxis the stimulus is chemical substances. Euglena, unicellular algae, swims towards light. Chioroplasts move towards light, fruit flies fly towards light. That is they are positively phototactic. Earthworms, blowfly larvae, woodlice and cockroaches, move away from light. That is they are negatively phototactic. Sperms of liverworts, mosses and ferns swim towards substances produced by the ovum, motile bacteria move towards various food substances. That is they are positively chemotactic. Mosquitoes avoid insect repellents. That is they are negatively chemotactic.



MECHANISM OF TROPIC MOVEMENTS
      The tropism response is brought about by an unequal distribution of the hormone rather than failure of the cells to respond to it. For example, a directional light causes an unequal distribution of auxin in the shoot of a plant. This is brought about by the destruction of auxin on the illuminated side of the shoot. in the shoot, auxins enable the cell walls to be stretched more easily by the osmotic forces developed in the vacuoles, allowing the cellulose micro iibrils to slide past each other. There is also evidence that auxins can promote cell division as well as elongation. A striking demonstration of this is seen if the tip of a stem is removed and the cut surface treated with [AA The result is a timorous growth of loosely~ packed cells at the cut end of the stem.

Nastic movement is generally caused by elastic changes in the size of special motor cells within the plant tissue. These changes are generally produced by changes in osmotic pressure due to an influx or efflux of ions that cause water to move in or out of the cells.‘ In many plants, shrinkage of the motor cells causes the overall movement of the plant. The nastic movements are reversible movements that require a motor organ called pulvinus, found in many legumes.


EXPERIMENT TO DEMONSTRATE THE EFFECT OF AUXINS ON ELONGATION OF SHOOT TIPS 0F A SEEDLING.

i) The response of a young shoot of maize seedling to directional light can be demonstrated using agar blocks. ‘

ii) Place the tip of a decapitated shoot on an agar block which has been divided into two by an impermeable barrier.

iii) Illuminate the shoot from the right hand side with lighted electric bulb.

iv) Place the block on the cut end of the shoot.

v) Observe the direction of growth of the shoot for some few days and record your observations.




vi) Discuss your observations with your friends and your teacher.



RESULTS
    It will be observed that, the shoot bends over towards the right hand side (in the direction of the source of light).


EXPLANATION
     The left half of the agar block contains more hormones than the right half, and this is confirmed by direct chemical estimation of the amount of hormone on the two sides of the agar block. The light causes an unequal distribution of the hormone in the shoot, more on the dark side than the light side. So directional light causes an unequal distribution of auxin in the shoot and phototropic response is therefore brought about by an unequal zone of elongation of plants. The light does not distribution of hormones (eg. Auxin) in the prevent the cells responsible for growth from responding to the hormone on the illuminated side of the shoot.






Did you understand what you have just studied or read?

To make sure you really UNDERSTOOD, ANSWER the QUESTIONS below:

1. a. Explain the following terms:
i. Tropism
ii. Nastic movement

b. Describe briefly an experiment to demonstrate that growth hormones are produced at the apex of the coleoptiles.

2. State briefly what happens when a seedling is subjected to a unilateral source of light and explain the mechanism involved.

3. a. What is meant by growth in living
organisms?

b. How does growth in plants differ from that in animals?

c. Give an account of the factors that affect the rate of growth in plants and animals.

4. Describe an experiment to measure the rate of growth in the main root of a seedling.

5. a. What is meant by growth in organisms?

b. Distinguish between primary growth and secondary growth in plants.

6. a. What is an auxin?

b. Describe an experiment to demonstrate that the auxins are responsible for increase in the stem length of a seedling.

7. a. What is meant by phototropism?

b. Describe on experiment to demonstrate the response of a short to a unidirectional source of light.

8. Explain how auxins influence phototropic movements in plants.

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