This is a representation or plot of any growth parameter (such as height, mass) measured with sets of time interval. When any growth parameter is measured with sets of time intervals and a graph of that growth parameter with time is plotted, a sigmoid curve is obtained. The sigmoid growth curve. (Fig. 22.2, Page 759 BS)
A to B: This is the lag phase. There is gradual or slow increase in growth because there are few cells which are dividing slowly. The cells are not yet adapted to their environment.
A to C: This is the log or exponential phase. Here, there is rapid increase in growth because there are many rapidly dividing cells which are well adapted to their environment in obtaining enough resources e.g. food, oxygen. There is no or less metabolic wastes in case of unicellular organisms during this growth phase. The point at which growth is maximum and beyond which growth start to decline is called the Inflexion point.
A to D: This is the decelerating phase. The growth of an organism increases gradually because of their internal factors (e.g. genes, hormones) or external factors (e.g. food shortage) or interaction of both factors limiting the growth. For unicellular organisms, the increase in metabolic wastes can also reduce the growth rate.
A to E: This is the stationery phase or plateau phase. During this phase, growth ceases or remain constant and the organism maintain itself until it dies, the rate of tissue break down is equals to the rate of their repair. There is no net increase or decrease in dry mass of the body.
Note: In different species of organisms the sigmoid growth curve is modified in different ways, e.g. in annual plants, the dry mass decreases at an early stage of during germination (i.e. negative growth occurs) due to hydrolysis of the stored food and its breakdown into energy inform of ATP i.e. during respiration of the seed embryo and thereafter, growth increases due to accumulation of organic food substances during photosynthesis as the seedling develop leaves.
The stationery phase of growth is not constant throughout, but tail off or decreases due to seed and fruit dispersal and senesce before it dies. The human growth curve is modified in such a way that it has three phases of rapid growth i.e. the infant phase and adolescence phase and at the stationery phase of growth, growth reduces due (shows negative growth) to senesce associated increasing age until death.
In monocots, fish, many non-vertebrates and some reptiles, in their stationary phase, growth may slowly increases to show positive growth until the organism dies. In certain cnidarians, growth curve flattens in stationary phase indicating zero growth.
A sketch of graph showing growth change of annual plant with time (limited growth) GRAPH (Fig 22.10 page 762 BS—for bean plant dry mass)
A to B: The dry mass decreases gradually and then rapidly due to hydrolysis and respiration of the stored foods substances in the cotyledons and embryo respectively.
A to C: There is a gradual and then a rapid increase in dry mass due to production of food by the developed leaves of the seedling during photosynthesis. Also during mitotic cell division, cell elongation and differentiation to form many cells, tissues and organs modified to serve particular roles occurs.
A to D: There is gradual growth up to maximum where the plant start to flower and bear the fruits at maturity.
A to E: Growth decreases gradually (negative growth) due to fruit and seed dispersal and senesce.
Note: In limited growth, growth does not continue throughout the life time of an organism.
Absolute Growth curve and Absolute Growth rate curve
The absolute growth curve is obtained when the cumulative increase in height, length or mass (over successive time intervals) is plotted against time. The curve shows the overall growth pattern and the extent of growth. (Fig. 22.3 Page 760 BS)
The absolute growth rate curve shows how the rate of growth changes with time. It is obtained by plotting change in height or mass with time. A bell-shaped growth rate curve is always obtained indicating that at early stages, the growth rate is low and then increases gradually and then rapidly up to the maximum (inflexion point) from where growth rapidly decreases and later gradually decrease. GRAPH (fig 22.4 page 760 BS)
This graph indicates the age of organisms when growth is slower, faster and decline.
Relative/percentage growth curve
Relative growth curve is a measure of the efficiency of growth i.e. the rate of growth relative to the size of the organism. It may be obtained when the increase in growth over a successive period of time is expressed as a percentage of the growth which has already occurred and plotted against time. This curve shows that the percentage growth of the already grown tissues is high at early stages of growth and decreases rapidly and then gradually giving rise to an l-shaped percentage growth curve. GRAPH (fig 22.5 page 760 BS)
Human growth Curve
GRAPH (Fig 22.6 page 761—show absolute growth curve and relative growth curve)
The absolute human growth curve shows four phases where maximum mass is achieved in adulthood and the greatest growth during infancy and adolescence while relative growth rate curve shows that there is greatest growth during embryological development.
Patterns of Growth
There are two growth patterns i.e. the isometric and allometric patterns of growth
Isometric pattern of growth: This is when an organism grows at the same rate as the rest of other body organs. This growth pattern is not accompanied with change in the shape of the body. The proportion of the organs and the entire body remains the same. Isometric growth occurs in fish and some insects e.g. locusts with exception of their wings and genitalia.
Allometric growth pattern: Is when the organs of an organism grow at different rates from the mean growth rate of the entire body. Most organisms exhibit allometric growth e.g., mammals, plants, birds, reptile. Growth of an organism here is accompanied by change in its body shape. There is change in the proportion of the individual organs and the entire body which renders them different. A sketch graph showing Allometric growth of Human being GRAPH (Fig 22.9 page 762 BS and fig 26.1 page 412 FA)
There is rapid increase in in relative growth of the lymphoid (thymus gland) immediately after birth up to about 7 to 10 years when it reaches the maximum and reduces in growth. This is because lymphoid tissue rapidly produces T-lymphocytes which defend the body against the infection during the early stages of growth when the immune system and immunity is yet developed and the individual is highly sensitive to microbe attach.
The brain grows early and rapidly and then gradually until it attains maximum growth at around 10 years because the brain is used to control growth of other body parts and it is also used by the organism to learn different situations in the environment.
The growth rate of reproductive organs is far below that of the entire body up to adolescence stage (around 13 years) because the body is sexually none receptive and there is no secretion of sex hormones. Towards puberty, there is gradual and then rapid increase in relative growth of the genital organs, because of secretion of sex hormones into the blood stream which causes growth and development of reproductive organs, rendering them sexually receptive and active for reproductive purposes.
Note: Body growth is rapid between 0 and 3 years because of rapid mitotic cell division which corresponds to the high metabolic rate and body needs. Between 3 and around 14 years body growth is slow as the body harmonizes with environmental conditions. Between 14 and 18 the body growth increases rapidly and then gradually due to secretion of growth and reproductive hormones influences growth of different parts of the body from puberty to adult stage.
Limited and Unlimited growth
Limited growth is the increase in size and dry mass of an organism until it reaches a certain size and mass and growth ceases e.g. in birds, mammals, fruits, dicot stems, annual plants. The extent to which the organism grows is determined mainly by genes and hormones.
Unlimited growth is when an organisms grows more or less indefinitely until its large size results into its death and destruction e.g. fungi, algae, woody perennial plants, fish, reptiles. GRAPH (Fig 22.11 page 763 BS)
Many growth sigmoid curves are linked together in this form of growth curve.
Intendment growth/discontinuous growth curve of insects/arthropods.
Due to possession of a tough, hair and inelastic, chitinised and waxy cuticle, insects undergo discontinuous growth in that when the cuticle is intact, the body does not expand as it has to shed and opt a new cuticle for growth. When the new cuticle becomes fully developed, it resists and prevents expansion of the body. Some insects take in air or water immediately after moulting for the body to expand before the new cuticle becomes hardened, developed and inelastic and growth ceases for some time. GRAPH (fig 22.12 page 763 BS)