ASPECTS OF GROWTH
It is sometimes very difficult to determine growth in organism. Growth varies from one organism to another. Parameters normally used to measure growth in organisms include:
- Length, height or width.
- Area or volume.
a) In most growth studies, mass may be measured as wet mass and dry mass.
- Wet mass: Wet mass is the mass of the organisms under normal conditions. It is not a reliable indication of growth.
- Dry mass: Dry mass is the mass of an organism after all the water in it has been removed. Although measuring growth in terms of dry mass is an accurate and reliable method, the organisms gets killed in the process. This means, it is not possible to measure growth in the same organism. To study growth by measuring dry mass, we must carry out the study on a large number of similar organisms. Growth can then be estimated by removing a given number of organisms at a time and estimating their dry weight.
b) Size and length: Size and length can be measured at successive intervals on the same organism, e.g. height of man and length of snake.
c) Increase in number of cells: Increase in the number of cells is also a proof of growth of a population. A popular example of growth in organism is the yeast. The yeast cell is capable of budding or dividing into two, two into four and four into eight etc. The yeast culture continues to double its number as long as none of the cells dies or loses its power of division.
Regions of Fastest Growth in Plants
The regions of fastest growth in plants are the root and stem apices. In the root, the root tip is a region where the cells are dividing rapidly. The root and stem apices of a plant can be divided into the region of cell division, followed by the region of cell elongation and the region of cell maturation.
The region of cell division is also known as the apical meristem. It consists of meristematic cells, i.e ., cells capable of active division. The root tip is covered by the root cap.
In the region of cell elongation, the cells become enlarged to their maximum size by the stretching of their ways. The cells in the region of maturation attain their permanent size and become specialised to carry out certain functions.
Note: The stem apices include the terminal buds and the lateral or axillary buds.
Apical meristems bring about the growth in length (height) of the plant. In shoot, they also give rise to branches, leaves and flowers. Apical meristems bring about primary growth (the first growth) of a plant.
Determination of Fastest Growth Regions in the Root
A young germinating seedling is taken and its radicle is marked with Indian ink at interval of 2mm. The seedling is then pinned onto a cork and is placed in a bottle containing some water. The experiment is left in a dark room for about eight hours.
Then, the seedling is taken out and the distances between successive ink marks are measured. The difference between the length of each new interval and an old interval (i.e. 2mm) would give the increase in the length of that interval in eight hours. From this, the rate of growth of the root can be calculated.
Factors Affecting Growth
Factors which affect the growth of organisms are grouped into two parts which are:
- External factors which include availability of nutrients, humidity, light, temperature, pH and accumulation of metabolic product;
- Internal factors which include the hormones.
- Availability of nutrients: All living organisms require nutrients or food and water which are necessary for the normal growth and development of the body.
- Humidity: All living things also require certain level of humidity to enable them grow. Too low or too high of it will affect growth.
- Light: Most plants require the presence of sunlight to enable them carry out photosynthesis. It is from this source that they derive their energy and food. Most animals, bacteria and fungi can live in darkness and grow in their habitat.
- Temperature: All metabolic processes are accelerated at a certain level of temperature. Too low or too high a temperature can adversely retard major processes in the body.
- pH: The pH of the fluid in contact with a cell has a profound effect on all its activities. Growth can be hampered at certain level of acidity or alkalinity.
- Accumulation of metabolic products: Most metabolic products which accumulate within the body can affect growth. Excessive accumulation is toxic or harmful to the body system and will eventually retard growth.
Hormones: Hormones are internal factors which are known to affect the growth of plants and animals. Plant hormones which affect the growth of plants are auxins and gibberellins. Auxins promote or inhibit cell elongation in stems and roots. They also stimulate cell division. Gibberellins promote cell elongation and bring about growth in the stem. They also affect cell division and cell differentiation to a certain extent.
In animals, hormones are secreted by endocrine glands. The hormones mainly concerned with growth are secreted by anterior pituitary gland, the thyroid gland and the gonads. Abnormal growth in humans, like dwarfism or gigantism is due to errors in the secretion of one of these hormones.