GROWTH

Growth is an increase in size/mass. It is the progressive development of a living thing, especially the process by which the body reaches its point of complete physical development.

THE CONCEPT OF GROWTH

Growth is an increase in size/mass or growth. It is the progressive development of living thing, especially the process by which the body reaches its point of complete physical development.

The growth process is not a steady one; sometimes growth occurs rapidly, at other times slowly. Individual patterns of growth vary widely because of differences in heredity and environment.

When the rate of cell increase is higher than the rate of cell loss, growth is referred to as positive growth. When the rate of cell increase is lower than the rate at which cells are lost from the body, the organism decreases in size and weight. This is also referred to as negative growth. Several factors are known to affect growth example nutrients, temperature, light and hormones.

MITOSIS AND GROWTH

Mitosis is the process of cell division whereby the chromosome are duplicated and distributed equally to the daughter cell.

The process of mitosis takes place in several stages, which are described below. The diagrams illustrate the stages showing a simple cell with four chromosomes.

PROPHASE

This is the first stage of mitosis. In the early stages thread-like structures appear in the nucleus. These structures are the chromosomes. With time the chromosomes shorten and thicken. Then each of them splits longitudinally into two structure chromatids. Each of the chromatids is held together at a point calledcentromere.

Diagram of early prophase

As the chromosomes become visible, other events tae place. The nuclear membrane and the nucleus gradually disappear and a network of fibers appears in the cytoplasm. This network of fibers is referred to as spindle.

METAPHASE

Prophase is followed by metaphase stages. The nuclear membrane has disappeared completely by the cell and become arranged. The centromere of each pair of chromatids is attached to a spindle fiber.

Diagram of metaphase

ANAPHASE

During anaphase the centromere splits and the sister chromatids separate from each other (see figure below). Once the sister chromatids separate from each other, each is referred to as chromosome. It follows, therefore, that at this stage the chromosome number in the cell has doubled. Then the chromosomes begin to move towards opposite sides of the cell. The movement is in such a way that an equal number of chromosomes move to each pole of the cell.

TELOPHASE

Telophase begins when chromosomes reach the poles of the daughter cells. Many events in the telophase are the reverse of prophase. The chromosomes uncoil the nuclear membranes around daughter nuclei appear, the spindle apparatus break down and the nucleus reappears and nuclear membrane forms around each mass of chromosomes.

CYTOKINESIS

Telophase is followed by a stage called cytokinesis. This is the division of cytoplasm. In plant cells a delicate membrane called a cell plate starts to form in the middle of the cell. Finally a new cell wall forms on either side of the plate. In this way, two new daughter cells are formed. In animals cells the cell membrane pinches the cytoplasm at the middle of the cell until two daughter cells are formed. Cytokinesis is completed as telophase ends.

SIGNIFICANCE OF MITOSIS IN GROWTH

• Mitosis results in the formation of two identical daughter cells
• The daughter cells are also identical to the parent cells because each daughter cell has the same number of chromosomes as that found in the original cell
• Mitosis enables an organism to increase in size and maintain the same number of chromosomes in its body cells
• Mitosis is also important for the replacement of worn out or damaged cells example in the lining of the gut and the surface of the skin

Mitosis alone does not bring about growth, when the cell divides the two daughter cells are initially only half the size of the parent cells.

The new cells must take in more materials to form the additional cytoplasm to produce fully-grown cells.

GROWTH AND FLOWERING PLANTS

In most flowering plants growth starts when the seed begins to germinate. Germination is the growth of the seed into a seedling.

There are changes that occur during seed germination. These are:

1. Seed absorb water and enlarge
2. Later on the testa bursts and the radicle emerges. The radicle continues to elongate and gives rise to many roots
3. As the radicle elongates, the plumule is curved. At this stage, young plant is called a seedling.

Accompanying these morphological changes are chemical changes which occur inside the seed. As the seed absorbs water the foods are hydrolyzed into soluble food. The starch stored in the cotyledons or endosperm is converted to sugar by action of diastase. In some seeds, lipase catalyzes the hydrolysis of fats to fatty acid and glycerol.

It is likely that glycerol is converted into sugars since it is not detected in germinating seeds. The proteolyctic enzymes catalyze the hydrolysis of proteins to amino acids.

During germination a lot of energy is required. This energy is derived from the stored food materials.

It follows, therefore that as the seed germinates its weight decreases. This is because the stored food is being used. The decrease in weight continues until the seedling is capable of photosynthesizing.

Environmental Conditions necessary for germination

Environmental conditions necessary for germination are moisture, oxygen and suitable temperature.

The amount of each of these requirements varies greatly in different seeds. Seeds of some water plants for example, will germinate while submerged in water. The small amount of oxygen dissolved in water is enough to enable them to germinate. On the other hand, seeds of most land plants will not germinate when submerged in water. This is because they require plenty of oxygen. Seeds of desert plants require very little water to germinate. Dew is enough for them to germinate.

Epigeal and hypogeal germination

When the seedling emerges from the soil it is curved. The curved portion, the hypocotyl, pushes through the soil. As germination continues the hypocotyl straightens and carries the cotyledons and the plumule above the soil surface. This type of germination where the cotyledons are carried above the soil is calledEpigeal Germination. Examples of other seeds that exhibit epigeal germination are those of most dicotyledons plants such seeds includes castor oil seeds, groundnuts, cotton and Bambara nuts. Epigeal germination occurs also in a few monocotyledonous seeds such as onions and lilies.

Germination of maize grain follows a different pattern from that of a bean seed. The plumule pushes its way out of the soil while the cotyledon remains underground. The plumule does not form a hook as in bean seeds. This type of germination in which the cotyledons remains underground is called hypogeal germination. Other examples of grains exhibitions hypogeal germination are wheat, sorghum and millet. A few dicotyledonous seed such as kidney beans and broad beans exhibit hypogeal germination.

Germination can occur only in a seed, which is viable. A viable seed is one in which the embryo is alive. The length of time of a seed can remain viable carries in different species. Many seeds remain viable up to 50 years if properly stored.