BIOLOGY FORM FOUR TOPIC 1: GROWTH

Concept of Growth
The concept of Growth
Explain the concept of growth
Growth is an increase in size/mass or growth. It is the progressive development of living thing, especially the process by which the body reaches its point of complete physical development.
The growth process is not a steady one; sometimes growth occurs rapidly, at other times slowly. Individual patterns of growth vary widely because of differences in heredity and environment.
When the rate of cell increase is higher than the rate of cell loss, growth is referred to as positive growth. When the rate of cell increase is lower than the rate at which cells are lost from the body, the organism decreases in size and weight. This is also referred to as negative growth. Several factors are known to affect growth example nutrients, temperature, light and hormones.
Internal and External Factors Affecting Growth in Plants and Animals
Investigate internal and external factors affecting growth in plants and animals
Growth in plants and animals is influenced by a number of factors, which can be grouped into two categories: internal and external.
Internal factors affecting growth in humans
These are the factors which are associated with genetic make up of an organism plus all the other processes which take place in the organism’s body. These factors include the following:
  1. Heredity:A person’s physical development is strongly affected by their genes inherited from their parents. Parent’s genes predetermine the limits of an individual’s height and other characteristics including the variability in eye colour, hair colour, body composition, and skin tone.With physical attributes such as height, parents’ genes dictate the range of height their offspring can obtain. The variability in height is a result of many external factors in the environment including nutrition and events during the child’s growth.
  2. Hormones:Human growth is affected by biochemical products such as hormones. Hormones are regarded as growth-promoting substances. Probably all the endocrine glands influence growth. Most of the hormones are secreted by the endocrine glands and play a significant role in regulating the pattern of growth and development as per instructions of the genes. Examples of these hormones and their actions are as follows:
  1. Somatotrophin:The most important hormone controlling growth from birth up to adolescence is growth hormone or somatotrophin. This is a polypeptide secreted by the pituitary. It helps in growth of bones and thereby increases the height of persons. It also causes an overall growth rate of most of tissues including brain.
  2. Thyroid hormone:This hormone plays a vital role throughout the whole period of growth. The activity of the thyroid decreases gradually from birth to adolescence. In low secretion of the hormone, skeletal maturity, dental maturity and growth of the brain are all affected. During adolescence a new phase of growth occurs under the control of steroid hormones secreted by the adrenals and gonads. The gonads of both sexes secrete estrogens in small quantities from the time of birth onwards. At puberty the oestrogen level rise sharply in girls and to a much more limited extent in boys; the sex differences is possibly due to an inhibitory hormone secreted by the seminiferous tubules of the testicle.
  3. Testosterone:Testosterone, produced by the testicle, is important in stimulating growth and it is responsible for the greater growth of muscles.
  4. Gonadotrophins:Gonadotrophins are responsible for the growth of the ovaries and testis, and later on, the secretion of estrogens and testosterone responsible for the growth and development of secondary sex characters.
External factors affecting growth in humans
Growth is also affected by external factors which include the following:
  1. Nutrients:Growth is closely related to nutrition. A sufficiency of food is essential for normal growth. An adequate supply of nutrients is naturally essential for the normal growth of humans and the need varies with the phase of development. For example: Zinc plays a part in protein synthesis and is a constituent of certain enzymes. A deficiency of zinc causes stunting, interference with sexual development and falling out of hair; Iodine is needed for the manufacture of the thyroid hormones; Bone will not grow properly without an adequate supply of calcium, phosphorus and other inorganic constituents such as magnesium and manganese; Iron is required for the production of haemoglobin; Vitamins play an important part in growth. Vitamin A is thought to be control the activities of osteoblasts. In vitamin C deficiency the intercellular substance of bone is inadequately formed. Vitamin D deficiency is the cause of rickets. Malnutrition during childhood delays growth, and malnutrition in the years proceeding adolescence delays the onset of the adolescence. Malnutrition may also result to diseases which decrease the appetite or interfere with digestion and assimilation. A majority of malnourished children fail to achieve their full genetic potential of body growth and are thus stunted or wasted or both.
  2. Diseases:Diseases are alteration of the normal body functions, disorders or morbid conditions of the mind. Diseases slow down growth in humans and other animals. A child that suffers from diseases very often is likely to have his growth stunted or retarded. Such a child may end up having a small body or deformed body parts.
  3. Cultural factors:The physical growth of human beings is definitely affected by cultural factors. Culture differs from ethnic group to ethnic group. The body growth differences correlate with varied cultural groups. The physical growth of the body follows some adaptations in different geographical areas of distribution of the groups.
  4. Socioeconomic factors:Socioeconomic influence on human growth is also a well known factor. Children from different socioeconomic levels differ in average body size at all ages. It is clear that growth of the children and adults in those families with good financial status is always good compared to the case in poor families. However, growth differences are more closely related to the home conditions than to the strictly economic status of the families.Size of family exerts an indirect influence on the rate of growth. In a large family with limited income the children do not get proper nutrition. As a result the growth is affected. The number of children in the family exerts an effect on the children’s rate of growth. Children in large families are usually smaller and lighter than children in small families. Possibly this is because in large families children tend to get less individual care and attention.
Internal factors affecting growth in plants
The internal factors that influence plant growth include following:
Hereditary factors
Heredity factors are internal factors that affect the growth of plants. They affect the physical appearance and the size of a plant
Hereditary units called genes are found in chromosomes inside the nucleus of all plant cells. These units control the various characteristics of plants such as flower colour number of floral parts, growth pattern and so on. Genes are passed from parents to off spring. For example, tall plants produce tall offspring and short plants produce short offspring.
Growth hormones
Certain hormones such as growth hormones are known to affect growth. Hormones are chemical substances that influence physiological processes. Drastic changes in their concentrations in the body will, therefore, affect growth.
There are several known growth hormones. Some of them, like auxins, cytokinins are growth-promoting while others like abscissic acid and ethylene are growth inhibitors. Most of the growth regulators are synthesized by plants while a few are synthetic in nature.The table below summarizes the role of certain plant hormones on growth of plants and seeds.
HormoneRole in plant growth
Indoleacetic acid(IAA)—the main auxin. Other three auxins seem to have rather marginal importance for plants in natural environments.
  • Promotes cell division
  • Promotes cell enlargement
  • Promotes response of shots and roots to stimuli such as light, water and gravity
  • Promote growth of adventitious roots
  • Induces parthenocarpy (formation of fruits without fertilization)
  • Causes formation of the abscission layer at the base of the leaf stalk, leading to falling of leaves (abscission).
  • Inhibits development of lateral buds, thus promoting apical dominance
  • Causes formation of callus tissue. Callus tissue forms at the site of an injury to bring about healing in the plant.
  • Controls division in the vascular cambium and xylem differentiation.
  • Used as the rooting hormones in stem cuttings.
  • 2-4 D is used as an herbicide to kill broadleaf, dicotyledonous weeds.
  • Promotes flowering in pineapples.
Gibberellins
  • Promote cell division and elongation of internodes in dwarf plants.
  • Induce parthenocarpy by initiating formation of indoleacetic acid (IAA)
  • Promote lateral bud development
  • Inhibit development of adventitious roots
  • Inhibit formation of the abscission layer on the leaf petiole
  • Promote germination of seeds
  • It helps in inducing seed germination by breaking seed dormancy and initiating the synthesis of hydrolases enzymes for digesting reserve food.
Cytokinins
  • Stimulate cell division
  • Stimulate formation of callus tissue
  • Promote flowering
  • Break seed dormancy
  • Promote formation of adventitious roots
  • Promote development of lateral buds by inhibiting apical dominance.
  • Low concentration of clytokinin induces cell elongation and causes ageing of leaves
  • Help in the production of new leaves, chloroplasts, and adventitious shoots.
  • Help in delaying senescence by promoting nutrient mobilisation.
Ethylene (ethane)
  • Promotes ripening of fruits
  • Causes formation of callus tissue, leading to falling of fruit and leaves
  • Stimulates thickening of the stem while inhibiting stem elongation
  • Helps in breaking seed and bud dormancy.
  • Promotes root-growth and formation of root hairs.
Abscisic acid (ABA)
  • induces seed dormancy by inhibiting seed germination, growth of stems, and sprouting of buds
  • Causes fruits and leaves to fall (abscission)
  • Promotes flowering
  • Stimulate apical dominance by suppressing development of lateral buds
  • Stimulates stomatal closure during water stress
Indolebutyric acidSynthetic plant hormone that promotes elongation of stems and roots


Apical dominance
An apical bud is found at the top of the plant. Apical buds are responsible for increase in plant’s height (apical growth). Lateral buds are found on the sides of the plant. Lateral buds are responsible for the formation of branches. Apical dominance is the inhibition of the growth of lateral buds by the presence of the growing apical bud. Apical dominance causes plant shoots to have a conical shape.
The apical bud produces auxins that diffuse to the lower parts of the plant. These auxins retard the development of lateral buds. The lateral branches of such a plant are short. A plant that has strong apical dominance gains more height in comparison to its width. Thus the plant assumes a conical outline.
Cutting the apex of the shoot causes the lateral buds to sprout. The dominance is overcome since the source of auxins at the apex is removed. The lateral buds sprout, branches develop, and the plant assumes an umbrella shape. Tea bushes are pruned so that they can develop many side branches. Rose plant, cypress and bougainvillea plants are pruned so that they can make a good hedge.
External factors that affect growth in plants
The external factors that affect plant growth include light, nutrients, temperature, relative humidity, water, carbon dioxide and oxygen, soil condition, biotic factors, and pollutants. Each of these factors is explained in detail below:
  1. Light:The effect of light on growth can be studied under three headings: light intensity, light quality and duration of light. Growth is generally favoured by darkness, but light is necessary because of its role in the manufacture of food. Young plants growing in the absence of light develop elongated thin stems with narrow leaves and poorly developed shoot system. Such plants are said to be etiolated.In weak intensity of light the internodes are short and the leaves are expanded. In strong intensity of light, the plant assumes a normal height. Very low light intensity reduces the rate of overall growth of the plant, by lowering the rate of photosynthesis.Growth in full spectrum of visible light is found to be better than the growth in any one of the different colours of light. Red colour seems to be the most favourable for growth.The duration of light has a pronounced effect on the growth of vegetative as well as reproductive structures. The influence of duration of light is most marked in inducing or suppressing flowering. This phenomenon is termed as photoperiodism.
  2. Nutrients:Availability, quality and quantity of food substances will automatically affect growth. For growth to occur in living things, food must be broken down to release energy. In areas where nutrients and water are adequate, competition is reduced and population increases. In case of shortage of nutrients and water, competition sets in and most individuals die.There are different mineral nutrients required for optimum plant growth. These nutrients are classified as either macronutrients or micronutrients. Macronutrients are those nutrients required by plants in high doses while micronutrients are the nutrients required in small quantities. Examples of macronutrients include nitrogen, potassium, magnesium, calcium, phosphorous and sulphur. Micronutrients include iron, zinc, molybdenum, manganese, boron, copper, cobalt and chlorine.
  3. Temperature:Atmospheric and soil temperatures are very crucial for plant growth as it affects many plant processes such as photosynthesis, metabolism, respiration, transpiration, breaking of seed dormancy, seed germination, protein synthesis, translocation, and flowering. At high temperatures the translocation of manufactured food is faster so that plants tend to mature earlier.Growth can take place between 0°C and 50°C. But the optimum temperature for the growth is between 20° and 30°C. Low temperature, however, is necessary for many plants to flower. Different physiological processes such as photosynthesis and respiration are controlled by enzymes. The enzymes are affected by temperature and pH. Enzyme activity and the rate of most chemical reactions generally increase with rise in temperature. Up to a certain point, there is doubling of enzymatic reaction with every 10°C temperature increase. But at excessively high temperatures, denaturation of enzymes and other proteins occur.It follows, therefore, that drastic changes in temperature and pH will affect growth.
  4. Relative humidity:Relative humidity (RH) is the amount of water vapour in the air, expressed as the proportion (in percent) of the maximum amount of water vapour it can hold at certain temperature. For example, an air having a relative humidity of 60% at 27°C temperature means that every kilogram of the air contains 60% of the maximum amount of water that it can hold at that temperature.The relative humidity affects the opening and closing of the stomata which regulates loss of water from the plant through transpiration as well as photosynthesis. Transpiration is slower in humid conditions. This is because diffusion of water vapour out of the leaf slows down if the leaf is already surrounded by moist air.
  5. Water:As mentioned earlier, water is a primary component of photosynthesis. It maintains the turgor pressure or firmness of tissue and transports nutrients throughout the plant. In maintaining turgor pressure, water is the major constituent of the protoplasm of a cell. By means of turgor pressure and other changes in the cell, water regulates the opening and closing of the stomata, thus regulating transpiration. Water also provides the pressure to move a root through the soil. Among water’s most critical roles is that of a solvent for minerals moving into the plant and for carbohydrates moving to their site of use or storage. Gradual evaporation of water from the surface of the leaf near the stomata helps stabilize plant temperature.
  6. Carbon dioxide and oxygen:The oxygen and carbon dioxide in the air are of particular importance to the physiology of plants. Oxygen is essential in respiration for the production of energy that is utilized in various growth and development processes. Carbon dioxide is a raw material in photosynthesis. However, a high concentration of carbon dioxide reduces growth because of its effect on the closing of stomata, and maintenance of dormancy. If the concentration of carbon dioxide in the plant leaf is higher than the surrounding air, the stomata will open to let in more of the gas from the surrounding air so as to balance the equilibrium of the gas between the two media (air and leaf air spaces). The opposite is the case if the concentration of the gas is higher in the air than in the leaf.
  7. Soil condition:The characteristics of soil play a big part in the plant's ability to extract water and nutrients. If plants are to grow to their potential, the soil must provide a satisfactory environment for plant growth.Plant growth is influenced by the soil properties such as texture or structure, salinity, acidity, waterlogging, or compaction.
  8. Biotic factors:Diseases, plant pests, weeds and harmful substances released by roots (allelopathy) affect plant growth drastically. Weeds compete with plants for moisture, nutrients, and light. Root knot nematodes reduce absorption, so more fertilizer is necessary. All of these have negative impacts on plant growth and development.
  9. Pollutants:Pollutants can hamper plant growth. Many pollutants composed of poisonous gasses (such as carbon monoxide, sulphur dioxide, hydrogen fluoride, hydrogen sulphide) are capable of restraining growth, even bringing plants to death. Pollutants from household or industrial wastes are also able to restrain plant growth.

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