Introduction to Measurement of Growth

What is Measurement of Growth?

Growth can be estimated by measuring some aspect of the organism such as height, weight, volume and length over a specified period of time. The measurements so obtained if plotted against time result into a growth curve.

​Study Question  

The following results were obtained from a study of germination and early growth of maize.
The grains were sown in soil in a greenhouse and.at two-day intervals. Samples were taken, oven dried and weighed. See table.

Plot a graph of dry mass of embryo against time after sowing.

Describe the shape of the graph.
For most organisms when the measurements are plotted they give an S-shaped graph called a sigmoid curve such as in figure. This pattern is due to the fact that growth tends to be slow at first and then speeds up and finally slows down as adult size is reached. A sigmoid curve may therefore be divided into four parts.

Lag phase (slow growth)

This is the initial phase during which little growth occurs. The growth rate is slow due to various factors namely:

1. The number of cells dividing are few.
2. The cells have not yet adjusted to the surrounding environmental factors.

Exponential phase (log phase)

This is the second phase during which growth is rapid or proceeds exponentially. During this phase the rate of growth is at its maximum and at any point, the rate of growth is proportional to the amount of material or numbers of cells of the organism already present.
This rapid growth is due to:

1. An increase in number of cells dividing, 2-4-8-16-32-64 following a geometric progression,
2. Cells having adjusted to the new environment,
3. Food and other factors are not limiting hence cells are not competing for resources,
4. The rate of cell increase being higher than the rate of cell death.

Decelerating Phase

This is the third phase during which time growth becomes limited as a result of the effect of some internal or external factors, or the interaction of both.
The slow growth is due to:

1. The fact that most cells are fully differentiated.
2. Fewer cells still dividing,
3. Environmental factors (external and internal) such as:
   1. shortage of oxygen and nutrients due to high demand by the increased number of cells.
   2. space is limited due to high number of cells.
   3. accumulation of metabolic waste products inhibits growth.
   4. limited acquisition of carbon (IV) oxide as in the case of plants.

Plateau (stationary) phase

This is the phase which marks the period where overall growth has ceased and the parameters under consideration remain constant.
This is due to the fact that:

1. The rate of cell division equals the rate of cell death.
2. Nearly all cells and tissues are fully differentiated, therefore there is no further increase in the number of cells.
3. The nature of the curve during this phase may vary depending on the nature of the parameter, the species and the internal factors.
4. In some cases, the curve continue to increase slightly until organism dies as is the case monocotyledonous plants, man invertebrates, fish and certain reptiles. indicates positive growth.
5. In some other cases the curve flattens out indicating change in growth while other growth curves may tail off indicating a period of negative growth rate.
6. This negative pattern characteristic of many mammals including humans and is a sign of physical senses associated with increasing age.

PRACTICAL ACTIVITY I: PROJECT

To measure the growth of a plant
Requirements
Small plots/boxes, meter rule and seeds of beans (or green grams, peas, maize),
Procedure

1. Place some soil in the box or prepare a small plot outside the laboratory.
2. Plant some seeds in the box and place it in a suitable place outside the laboratory (or plant the seeds in your plot).
3. Water the seeds daily.
4. Observe the box/plot daily and note the day the seedlings emerge out of the soil.
5. Measure the height of the shoot from the soil level up to the tip of the shoot. Repeat this with four other seedlings. Work out the average height of the shoots for this day.
6. Repeat procedure 5 every three days for at least three weeks.
7. Record the results in a table form.
8. On the same seedlings measure the length of one leaf from each of the five seedlings (from leaf apex to its attachment on the stem
9. Calculate the average length of the leaves and record in the table.
10. Plot a graph of the height of the shoot against time. On the same axes plot length of leaf against time.
11. Compare the two graphs drawn.

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