Growth
Growth is one of the characteristics of living things. It means permanent increase in size and formation of different parts.
In multicellular organisms, it involves formation of new tissues and organs.
Growth could occur in non-living things as well! For example a crystal of copper sulphate placed in a saturated solution of copper sulphate can increase in size.
Nature of growth:
In animals growth occurs only during a period of time in their lives and then ceases; and this growth takes place in all parts of the body. Such a type of growth is called intercalary growth. Exceptions include certain fish and lizards which grow throughout their lives.
In plants growth continues throughout their lives but in specific parts only especially at the end of the roots and stems. Such a type of growth is called apical growth.
Growth in plants takes place in the:
Apical meristems: Meristem means tissue that is capable of cell division and the tissue at the tip of the root and at the terminal end of the shoot is called apical meristem.
At the leaf bases near the node.
In the cambium of roots and stems.
Illustration to show growth in a radicle:
The root cap protects the root tip.
The root tip has meristematic cells that divide by mitosis to carry out growth.
The region of elongation is just above the root tip. In this region, the cells absorb water, enlarge and differentiate to form different tissues.
The root hairs appear above the region of elongation.
Experiment to demonstrate the region of elongation in a root:
Materials required: Bean seedling, pen with black ink, ruler, moist cardboard, pin.
Method:
Take a germinating bean seedling with a straight radicle or root about 2.5 cm long.
Mark the root every 2mm with lines in black ink.
Pin the seedling on moist cardboard with the radicle in vertical position to avoid curvatures.
Keep the seedling in a dark place with moist conditions.
Examine the root after a day or two.
Take measurements of the intervals between the black ink marks.
Observation:
The region of elongation is indicated by the area where the black ink lines are further than 2mm apart.
Conclusion: That is the region of most rapid growth.
SECONDARY GROWTH IN STEMS
Apart from growth at the apical meristems, dicotyledonous plants carry out secondary thickening to increase in circumference or girth.
The vascular cambium divides more to produce secondary xylem on its inside and phloem on the outside.
In woody plants it divides more on its inside than on its outside.
This makes more secondary xylem to be produced each season while the phloem is kept on being pushed out against the bark of the tree.
The formed secondary xylem is then impregnated with lignin to make it hard.
Interspersed in the vessels of the xylem are the tracheids which also help in hardening the wood of the tree.
At the beginning of each season the cells of the formed secondary xylem are largest because they are formed at the height of the rains; the cells formed at the end of the season are smaller because then the rains are fading.
So each season a new ring of secondary xylem is recognisably formed.
These annual rings of secondary xylem formed by division of the vascular cambium can be used to estimate the age of the tree by considering seasonal changes.
Importance of secondary thickening:
The hard xylem vessels and tracheids help in supporting the ever increasing weight of the plant.
The more secondary xylem produced helps to supplement on the extra water required by the tree due to growth.
THE SIGMOID CURVE:
Points to note when drawing graph curves:
The full heading of the graph should be written down.
The variable factor is put on the vertical axis and the constant of the factors is put on the horizontal axis.
The units used are recorded on both axes.
A suitable scale for the vertical and the horizontal axes is got by considering the range between the highest and the lowest values and diving them in regular intervals on the axes.
The axes are then marked regularly.
The points are then plotted accurately.
A smooth curve or straight line is then drawn out to join the points.
The following table shows the rate of growth in an organism:
|
Age in months |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
|
Weight in Kg |
3 |
5 |
17 |
26.5 |
29 |
30 |
29 |
28 |
27.5 |
Represented graphically, the results will be a graph of this nature:
In many organisms growth follows such a pattern like the
Greek letter sigma. Such curve is called a sigmoid curve.
At A the organism is born.
Between A and B there is little growth because cell division is taking place with very little formation of tissues (differentiation).
Between B and C there is rapid growth caused by tissues being formed. Anabolic (building up) reactions are faster than catabolic (breaking down) reactions.
Between C and D there is no growth because differentiation is complete and cell division rate equals cell destruction rate.
Between D and E there is negative growth so the cells being destroyed can not all be replaced by cell formation. This is the senescence period and the catabolic reactions are faster than the anabolic reactions.
The vertical axis of the graph shows dry weight of the bean seeds.
Dry weight is the weight taken after evaporating off all water and it is best to use when measuring growth rate because it takes into account tissue formation and excludes accumulating water.
But dry weight method destroys the specimen.
Between A and B there is reduction in drying weight because food stored in the cotyledons is used up for respiration during germination.
Between B and C there is increase in dry weight because leaves have developed and photosynthesis is taking place so that it is manufacturing food for rapid tissue formation.
GRAPH OF GROWTH IN INSECTS:
The following table shows the growth rate of a certain insect:
|
Age in days |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
|
Weight in gms |
1.5 |
1.6 |
2.1 |
2.1 |
2.6 |
2.6 |
3.1 |
3.1 |
- In insects there is what is called intermittent growth i.e. growth which is repeatedly interrupted by formation of a new cuticle so that when the cuticle becomes hard, very little or no growth occurs as shown by regions BC, DE, FG and HI.
- So when ecdysis has taken place and the cuticle is shed at A, C, E, G and I, there is rapid growth until the cuticle hardens as shown by regions AB, CD, EF and GH.
SUSPENDED GROWTH:
In plants growth is suspended when the seed is in a state of dormancy.
In the tapeworm growth is suspended during unfavorable conditions during the cyst stage of its life cycle- a stage where a round mass with a thick protective covering is formed.
In the lung fish and amphibians, growth is suspended during aestivation - a stage of inactivity undergone by an animal during the dry season to avoid water loss by evaporation.
In small mammals in the temperate regions during winter, growth is suspended by hibernation - a stage of inactivity in animals which takes place in the cold season with the aim of reducing the search for food required to keep temperature up when there is scarcity of food.
So during aestivation and hibernation growth, locomotion and nutrition do not take place. Very little respiration takes place too.
FACTORS AFFECTING GROWTH:
Availability of food.
Accumulation of waste products, which could become toxic and retard growth.
Temperature controls growth because enzymes work at an optimum temperature of 35 - 400C. Lower temperatures make enzymes inactive and higher temperatures make enzymes denatured.
In frogs cold temperature results in a life cycle of 70 days and warm temperature results in a life cycle of 45 days!
Light is especially a factor in the growth of green plants since it decreases etiolation - a condition where the plant develops only a few tiny, yellow leaves and abnormal internode elongation.
Light also affects flowering in some plants.
Internal factors also control growth rate for example hormones.