GROWTH

Growth is defined as an irreversible increase in volume, size, and number of parts, length and weight of                an organism It is an organic process which takes sometimes to accomplish.

The three processes involve in growth are

1. Cell division:- is a process by which cell increases in number and is achieved by cell division called mitosis. The synthesis of new protoplasm leading to the doubling of the chromosomes number in a process called replication before cell actually divide into two, with each daughter cell having the same chromosome as parent cell.
2. Cell enlargement:- This is a process after cell division in which the daughter cell absorb nutrients from their surrounding which it uses to increase in mass and size. Part of the nutrient is used to generate energy while the remaining is assimilated resulting in enlargement of the cells.
3. Cell differentiation– This process takes place after cell enlargement in which each cell develop into a special type of cell by changing its shape and structure in order to carryout growth, a specialized or particular function.

MITOSIS

Mitosis is a division of cell which produces two identical cells with the same number of chromosomes and characteristics as those of the parent cell. Mitosis is a cell division that lead to growth and it occurs in somatic cell(body cells) such as skin, bone marrow, lymph nodes and injured places and meristimatic tissues of plant. Mitosis occurs in five stages namely interphase, prophase, metaphase, anaphase and telephase.

• Interphase- This is a resting stage of the cell: At this stage, the cell has normal appearance of non-dividing cell condition. Chromosomes are not clearly visible.
• Prophase- The chromosomes become visible as chromatin threads. The chromosomes become shorter, thicker and clearly visible. Each chromosome now forms two district chromatids joined by a centromeres.

Nucleolus is gradually disappearing and formation of spindle fibres commence.

• Metaphase– At metaphase the chromosomes (now parried) called chromatids arranged themselves along the equator or middle of the spindle. The chromatids are attached to the spindle by centrometre.
• Anaphase- The chromatids of each chromosome separate. The start migrating to the poles of the cell by elongation of the spindle axis. The chromatids eventually reach the pole.
• Telophase (cytokinesis)- The cell starts dividing into two by line of division at the equator. The chromosome loses their thick appearance and the nuclear material, nucleus and nuclear membrane reformed. The spindle structure disappears and the cell split completely into two daughter cells having the same condition as interphase. The
• division of the cell at telophase into daughter cells is called cytokinesis

 

Aspect of Growth

Growth does not occur any how in all parts of the plant. It takes place in certain tissue and places in a plant bodies. The tissue where growth takes place plant is called meristem or meristematic tissue. A meristem is a tissue that retains their ability to divide by mitosis and turn out new cells. Such meristematic tissues include root apex, stem or shoot apex base or internodes and vascular cambium.

Growth which occurs in the apical stem and root meristem is called apical growth while growth which occurs       at meristem base of internodes is called auxiliary growth

Regulation of growth by Hormones

Hormones or chemical or biochemical substances produced in small quantity by cell of the body of plant and animal that has profound effect on other part of their body where they are needed for some form of growth

Plant hormones

They are produced only at the growing parts such as apical meristem of shoot or root i.e. shoot tip or root tip and transported to other part by diffusion.

Examples of plant hormones are auxin, gibberellin, cytokinin, abscissic acid and florigens. Plant hormones help plant in the following ways.

1. It promotes stem elongation
2. It makes plant to respond to tropism

iii.        It causes root and stem apical division

1. It causes fruit growth and ripening
2. It causes flower bud and lateral root initiation

Animal Hormones

In animal, hormones are produced in ductless glands called endocrine system and release straight to the blood stream which transport them to the target organ. Animal hormones include the following

GLANDS                              SITE                                        HORMONE

1. Pituitary gland              base of head                              Pituitrin
2. Thyroid gland Neck                                          Thyroxine
3. Parathyroid Neck                                          Parathormone
4. Pancreas Loop of duodenum                    Insulin
5. Adrenal gland Top of each kidney                   Adrenaline
6. Testes Testes                                        Testosterone
7. Ovaries Ovaries                                                     Progesterone

 

 FUNCTION OF ANIMAL HORMONE

1. It controls body metabolism
2. It promotes growth of the animal
3. It induces or stimulates reproduction

 

IRRITABILITY

Irritability is the ability of an organism to perceive and respond to changes in internal and external environment or stimuli. Stimulus is a change in condition which produces a change in the activities of the organism or part of its body. A stimulus can either be external or internal. External stimuli are those environmental factors that evoke response. Organisms respond to stimulus in three ways namely tactic, nastic and trophic response.

Tactic Response or taxis is a directional type of response or movement in which the whole organism moves from one place to another in response to external stimulus such as light, temperature, water or certain chemical. The response is thus said to directional and positive if the organism moves towards the stimulus or it is negative if it moves away from the stimulus.

– Phototaxis is the response to variation in light intensity

– Chemotaxis is the response to variation in concentration of chemical substance

Nastic Response or Nastism: This is a type of response in which a part of a plant moves in response to non-directional stimulus such as changes in light intensity, temperature and humidity. Nastic responses are usually described according to the stimulus evoking them.

– Nyctinasty is a response to changing in day and night condition (temperature) or light e.g. the opening of the petals of sun flower in the light and closes in the dark

– The closing of the morning glory flower when light intensity is low

– Haptonasty is the response due to contact or touch

– The infolding of the leaflet of mimosa plant when touched.

The closing of leaflet of flamboyant tree.

Tropic response or tropism: This is a type of response in which a part of a plant moves in response to directional stimulus.

Tropism are growth movement named according to the stimulus e.g. phototropism, hydrotropism, chemotropism, haptotropism or thigmotropism, geotropism.

Phototropism is the growth movement in response to light, thus the direction of growth movement depends on the direction of light. The shoot of plant is positively phototropic.

Geotropism: This is a response to gravitational force the shoot of plant is negatively geotropic while the root is positively geotropic.

Chemotropism is the response of plant to concentration of chemical substance e.g. roots respond positively to the presence of salt, particularly salt of calcium but negatively to alkaline or acid.

Haptrotropism /(Thigmotropism) is a response of plant to contact and is characteristic of tendrils and other organs by which the plant secures support. Tendrils show positive response to touch by twinkling around a support while roots on the other hand show a negative response to touch by growing away from stones.

MOVEMENT

Movement is the ability of living organisms to move from one place to another.

Reasons for Movement

1. To search for food

ii          To escape from danger

iii.        To respond to stimulus either positively or negatively

1. For the sake of reproduction

CYCLOSIS

Cyclosis or cytoplasmic streaming: This is the mass rotational streaming or movement of the cytoplasm and its contents in cells with the vacuole. In some cases, the streaming may be restricted to a particular region or the cell while at times the whole cytoplasm is subjected to cyclical movement of the cell.

Organisms                    Organelles for movement                    Mechanism of movement

Amoeba                          Pseudopodia                                           Cytoplasmic streaming (cyclosis)

Paramecium                    Cilia                                                       Beating the cilia against water

Euglena                           Flagellum                                               Lashing movement of flagellum

Hydra                                   Tentacles                                                Swimming, swaying, hoping and

Somersaulting

Earthworm                             Chaetae                                      Crawling

Fishes                                     Fins                                           Swimming

Toads and frog Limbs                                                                Hopping

 

ASSIGNMENT

1. Explain (i)   Cell division

(ii)  Cell enlargement

(iii) Cell differentiation

2. State the organelles or locomotion in the following organisms

(i) Amoeba

(ii) Paramecium

• Toad
• (iv) Earthworm

 

See also

ECOLOGICAL

MODE OF NUTRITION

ANIMAL NUTRITION

NUTRIENT CYCLING IN NATURE

PLANT NUTRITION