GROWTH AND DEVELOPMENT
In form one, we said that growth and development is one of the characteristics of living organisms. This means that all living organisms increase in size and also change their form and complexity with time.
In this topic we shall discuss:-
- The difference between growth and development.
- How to measure growth in a given organism.
- How to analyze experimental data on growth rates.
- Germination in plants.
- Metamorphosis in insects.
- The role of hormones in growth and development.
By the end of this topic you should be able to:
- Differentiate growth from development.
- Analyze experimental data on growth rate.
- Distinguish the types of germination.
- Measure one aspect of growth in a given seedling.
- Explain apical dominance.
- Distinguish between complete and incomplete metamorphosis in insects.
- Explain the role of hormones in regulating growth and development in plants.
Growth may be regarded as the permanent and irreversible increase in size of an organism. It involves a permanent increase in such measurable aspects as length, weight, width and so on.
Development on the other hand is the gradual change in form and complexity of an organism in the course of its life. It involves modifications of cells formed during cell division into different shapes to form specialized tissues.
Many changes that occur during development of an organism are not measurable.
Click the play button to view the animations
In this animation, the seedling increases in size due to cell division and elongation (growth) while it develops organs such as roots, stem and leaves due to cell differentiation (development).On the other hand, the child increases in size due to cell division and elongation (growth), while it develops organs such as eyes, ears, hands and legs due to cell differentiation (development).
Germination is the process through which a seed develops into a seedling. For the process to occur water is absorbed through the micropyle in a process called imbibition causing the seed to swell. The water taken up is used to dissolve and break down the stored food in the cotyledon into soluble food substance, which is then taken to the growing regions of the plumule and radicle. A seed embryo germinates into a seedling with a radicle developing into a root as the plumule develops into a shoot. The radicle increases in size forming roots which may assume different shapes i.e. Specialized roots. The plumule develops into the shoot system which differentiates into stems, branches, leaves and flowers.
Play the animation below to see a seed absorbing water through the micropyle, swelling and the germinating in to a seedling.
There are 2 types of germination
- Epigeal germination
- Hypogeal germination.
If all the necessary conditions for germinations are present, the radicle emerges first, grows out through the micropyle down into the soil as a primary root and other roots arise from it. The part of the embryo between the cotyledon and the radicle is called hypocotyl. The hypocotyl curves and pushes upwards protecting the delicate shoot tip, then straightens and elongates carrying the cotyledons above the soil level. This type of germination is called epigeal germination.
The illustration below shows the process of epigeal germination in a bean plant.
Play the animation below to view a germinating bean seedling. The cotyledons are raised above the soil level.
If all conditions are availed, the radicle emerges first, along its protective covering called coleorrhiza. The radicle grows down and develops some root-hairs behind its tip. Other adventitious roots arise from the base of stems producing a fibrous root system. The part of the embryo between embryo and cotyledon is called epicotyl. The epicotyl elongates above the soil and allows the plumule to grow out of its sheath (the coleoptiles) to form the 1st foliage leaves and start manufacturing food. The cotyledons remain below the ground but the stored food is broken down and transported to the growing region making it shrink and reduce in size. The cotyledon then withers away. This type of germination is known as hypogeal germination.
The illustration shows the process of hypogeal germination in maize. The cotyledons are left on the ground
Conditions necessary for germination
For germination to occur, the following conditions are necessary:
- Favorable temperature
- Seeds must be viable (viability)
MEASUREMENT OF GROWTH
Growth can be estimated by measuring some aspect of an organism such as height, weight, volume and length over a specified period of time.
Play the video below to see a person measuring the length and breadth of plant leaves.
This can be demonstrated by taking the study of growth in the length of the leaf. The results obtained are plotted into a graph and interpreted. The curve acquires the S-shaped graph called Sigmoid-Curve. The Sigmoid curve may therefore be divided into four parts.
The illustration shows a sigmoid curve obtained from measurement of growth.
Parts of a sigmoid curve
a) Lag Phase (slow growth).
Growth is slow initially and this is due to the fact that the number of cells dividing are few and has not adjusted to the surrounding environmental factors.
This is illustrated by the region between A and B.
b) Exponential phase (Log Phase)
Phase of very fast growth due to the increase in the numbers of cells dividing, the cells have adjusted to the environment and all the other factors are not limiting.
On the curve this is between B and C.
c) Deceleration phase
This is the phase during which growth becomes limited due to the following reason. Most cells are fully differentiated, Fewer cells are dividing, Shortage of oxygen and nutrients due to high demand by the increased number of cells, Limited space due to high number of cells, accumulation of metabolic waste products and limited acquisition of carbon (iv) oxide in plants
This is between C and D.
d) Plateau (stationary) phase.
The overall growth has ceased and the parameters are constant. This is due to the fact that the rate of cell division is equal to the rate of cell death, and that nearly all the cells have differentiated.
This refers to region D-E.
The animation shows a sigmoid curve.
ROLE OF GROWTH HORMONES IN PLANTS
Plant hormones are chemical substances produced in small quantities within the plant body but play a major role in growth and development.
They are mainly produced at the shoot tips and a few at the root tips.
Major plant hormones include:-
- Abscisic acid
This is a group of hormones produced at the shoot tip and root tip and works in association with other plant hormones to bring about various growth responses. Auxins have various effects on growth and development in plants.
- Enhance tropic responses such as phototropism and geotropism.
- Stimulates cell division and cell elongation in stems and roots
- Stimulates growth of adventitious roots from the stem
- Stimulates development of an ovary into a fruit without fertilization i.e. parthenocarpy.
- Enhance apical dominance.
The photographs below show the following; -
a) Tendrils of a passion fruit
Another group of plant hormones that are important to plant growth are the Gibberellins. The most noticeable effect of Gibberellins is the stimulation of rapid growth in dwarf varieties of plants. For example, If cabbages which are naturally dwarf are treated with gibberellins, they can grow to high heights due to rapid cell division and elongation.
This photograph shows bolted cabbage
Another group of active growth substances are the Cytokinins. In the presence of auxins, they stimulate cell division thus bringing about growth of roots, leaves and buds.
Cytokinins also promote the formation of adventitious roots from the stems and stimulate lateral bud development in shoots.
The photograph shows sugar cane with adventitious roots
The other hormone produced in gaseous form and released in small quantities is the hormone Ethylene. Its major effects are ripening of fruits and leaf fall.
This hormone influences shedding of leaves and brings about reduction in plant activities during dry seasons. Its concentration is high in such plant organs like fruits, buds, tubers and seeds. It is also highly concentrated at the bases of leaves. It has the following effects:
- Causes leaf fall (abscission) and fruit fall.
- Promotes seed dormancy.
- Inhibits seed germination.
- Inhibits sprouting of buds from stems.
- Retards stem elongation.
- Causes closing of stomata at high concentrations.
This hormone promotes flowering in plants
The illustration below shows a bud opening to form a flower.
The inhibition of growth of lateral buds in a plant by the presence of a growing apical bud is called apical dominance. We have learnt that auxins, particularly IAA promote stem and root elongation. However, they are also capable of affecting other aspects of plant growth and development. High concentrations of auxins inhibit the sprouting of lateral buds thus hindering growth of many branches. Since auxins are more concentrated near the shoot apex, their inhibitory effects is greater on the branches higher up than the branches nearer to the roots. Hence lower branches are often longer than those nearer the shoot apex, creating a cone-shape of a tree. The failure of lateral buds to develop (grow to greater lengths) in the presence of the apical buds is due to the top-down diffusion of auxins in concentrations higher than those that should promote lateral bud development. This phenomenon is the one exploited in pruning coffee, tea, hedges etc to increase the number of side branches in order to form dense vegetation for increased yields.
Photograph 1 shows cone shaped trees due to apical dominance.
The animation shows a trimmed hedge has a lot of lateral growth due removal of the shoot tips while the hedge which is not trimmed has little lateral growth due to apical dominance.
This refers to all the developmental changes that take place from the time a fertilized egg is laid until the adult stage is attained in insects .
There are two types of metamorphosis. These are: -
a) complete metamorphosis
b) Incomplete metamorphosis
Occurs in most insects including the bee, housefly, wasp, moth and butterfly. It involves fertilized eggs being laid and hatching into larvae.
In housefly, the larvae are called maggots while in butterfly and moth it is known as caterpillar. The larvae grow fast and shed its cuticle several times to become the pupa.
Pupa is a non-feeding stage which later develops into an adult which resembles the original mother insect.
The animation shows complete metamorphosis in a housefly.
This occurs in insects such as the cockroach, grasshopper and locusts.
In this type of development, the egg hatches into a nymph which resembles the adult insect, except that it is sexually immature and has no wings.The nymph feeds and undergoes several moulting to form an adult insect.Moulting is influenced by a hormone ecdysone and therefore the process of moulting can be referred to as ecdysis.
The animation below shows incomplete metamorphosis in a grasshopper.
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