Category: GROWTH & DEVELOPMENT

GROWTH AND DEVELOPMENT IN PLANTS

9/6/2021

GROWTH AND DEVELOPMENT IN PLANTS

The main growth and development phase in plants begins with the germination of the mature seed. Seeds are of two kinds depending on the number of cotyledons or embryo leaves.
Practical Activity:
Requirements

Bean seeds and maize grains which have been soaked overnight.
Scalpel or razor blades,
iodine solution,
Petri-dish and
hand lens.

Procedure

Using a scalpel or razor blade make longitudinal sections (LS) of both the bean seed and the maize grain.
Observe the LS of the specimens using a hand lens.
Note any structural difference between the specimens.
Draw the LS of each specimen and label.
Put a drop of iodine solution on the cut surfaces of both specimens.
Note any differences in colouration with iodine on the surfaces of the two specimens.
On your diagrams indicate the distribution of the stain.
Account for the difference in distribution of the colouration with iodine in the two specimens.

STRUCTURE OF THE SEED

A typical seed consists of a seed coat enclosing an embryo.
The seed coat is the outer covering which, in most seeds, is made up of the two layers, an outer testa and inner one, the tegmen.

Structure of dicotyledon seed (bean grain)

The testa is thick; the tegmen is a transparent membrane tissue.
The two layers protect the seed bacteria, fungi and other organisms which may damage it.
There is a scar called hilum on one part of the seed.
This is point where the seed had been attached the seed stalk or funicle.
Near one end of hilum is a tiny pore, the micropyle.
This allows water and air into the embryo, embryo is made up of cotyledons, a plumule (embryonic (and a radicle (the embryonic root).
In some seeds the cotyledons swollen as they contain stored food for growing plumule and radicle. Such seeds, called non-endospermic seeds.
In other cases, the seeds have their food stored in endosperm.
Such seeds are called endospermic seeds. Seeds with one cotyledon are referred to as monocotyledonous while those with two are referred to dicotyledonous.
This is the major basis in differentiation between the two large categories of plants, the monocotyledonae and dicotyledonae.

Dormancy in Seeds

The embryo of a dry, fully developed seed usually passes through a period of rest after ripening period.
During this time the seed performs all its life (physiological) processes very slowly and uses up little food. This is a period of dormancy.
Even if all the favorable environmental conditions for germination are provided to the seed during this period of dormancy, the seed will not germinate.
This is due to the fact that the seed embryo may need to undergo further development before germination.
Some seeds can germinate immediately after being shed from the parent plant (e.g. most tropical plants) while others must pass through dormancy period, lasting for weeks, months or even years before the seed can germinate.
Dormancy provides the seeds with enough time for dispersal so that they can germinate in a suitable environment.
It also enables seeds to survive during adverse environmental conditions without depleting their food reserves.
The embryo has time to develop until favorable conditions are available e.g. availability of water

Factors that Cause Dormancy

Embryo may not yet be fully developed.
Presence of chemical inhibitors that inhibit germination in seeds e.g. Abscisic acid.
Very low concentrations of hormones e.g. gibberellins and enzymes reduces the ability of seeds to germinate.
Hard and impermeable seed coats prevent entry of air and water in some seeds e.g. wattle.
In some seeds the absence of certain wavelengths of light make them remain dormant e.g. in some lettuce plants.
Freezing of seeds during winter lowers their enzymatic activities rendering them dormant.

Ways of Breaking Dormancy

When the seed embryos are mature then the seed embryos can break dormancy and germinate.
Increase in concentration of hormones e.g. cytokinins and gibberellins stimulate germination.
Favourable environmental factors such as water, oxygen and suitable temperature.
Some wavelengths of light trigger the production of hormones like gibberellins leading to breaking of dormancy.
Scarification i.e. weakening of the testa is needed before seeds with hard impermeable seed coats can germinate.
This is achieved naturally by saprophytic bacteria and fungi or by passing through the gut of animals.
In agriculture the seeds of some plants are weakened by boiling, roasting and cracking e.g. wattle.

Seed Germination

The process by which the seed develops into a seedling is known as germination.
It refers to all the changes that take place when a seed becomes a seedling.
At the beginning of germination water is absorbed into the seed through the micropyle in a process known as imbibition and causes the seed to swell.
The cells of the cotyledons become turgid and active.
They begin to make use of the water to dissolve and break down the food substances stored in the cotyledons.
The soluble food is transported to the growing plumule and radicle.
The plumule grows into a shoot while the radicle grows into a root.
The radical emerges from the seed through micropyle, bursting the seed coat as it does so.

Conditions Necessary for Germination

Seeds can easily be destroyed by unfavourable conditions such as excessive heat, cold or animals.
Seeds need certain conditions to germinate and grow.
Some of these conditions are external, for example water, oxygen and suitable temperature while others are internal such as enzymes, hormones and viability of the seeds themselves.

1. Water

A non-germinating seed contains very little water.
Without water a seed cannot germinate.
Water activates the enzymes and provides the medium for enzymes to act and break down the stored food into soluble form.
Water hydrolyses and dissolves the food materials and is also the medium of transport of dissolved food substances through the various cells to the growing region of the radical and plumule.
Besides, water softens the seed coat which can subsequently burst and facilitate the emergence of the radicle.

2. Oxygen

Germinating seeds require energy for cell division and growth.
This energy is obtained from the oxidation of food substances stored in the seed through respiration thus making oxygen an important factor in seed germination.
Seed in water logged soil or seed buried deep into the soil will not germinate due to lack of oxygen.

3. Hormones

Several hormones play a vital role in germination since they act as growth stimulators.
These include gibberellins and cytokinins.
These hormones also counteract the effect of germination inhibitors.

4. Viability

Only seeds whose embryos are alive and healthy will be able to germinate and grow.
Seeds stored for long periods usually lose their viability due to depletion of their food reserves and destruction of their embryo by pests and diseases.

5. Temperature

Most seeds require suitable temperature before they can germinate.
Seeds will not germinate below 0°C or above 47° C.
The optimum temperature for seeds to germinate is 30°C.
At higher temperature the protoplasm is killed and the enzymes in the seed are denatured.
At very low temperatures the enzymes become inactive.
Therefore, the protoplasm and the enzymes work best within the optimum temperature range.
The rate of germination increases with temperature until it reaches an optimum.
This varies from plant to plant.

6. Enzymes

Enzymes play a vital role during germination in the breakdown and subsequent oxidation of food.
Food is stored in seeds in form of carbohydrates, fats and proteins which are in insoluble form.
The insoluble food is converted into a soluble form by the enzymes.
Carbohydrates are broken down into glucose by the diastase enzyme, fats into fatty acids and glycerol by lipase, and proteins into amino acids by protease.
Enzymes are also necessary for the conversion of hydrolysed products to new plant tissues.

To download complete set of this notes, questions and revision kit, See pricing ...

download

2 Comments

Concept of Growth and Development - K.C.S.E Biology Notes

8/6/2021

1 Comment

Growth is a characteristic feature of all living organisms.
Most multi cellular organisms start life as a single cell and gradually grow into complex organisms with many cells. This involves multiplication of cells through the process of cell division.
This quantitative permanent increase in size of an organism is referred to as growth.

For growth to take place the following aspects occur

Cells of organisms assimilate nutrients hence increase in mass.
Cell division (mitosis) that lead to increase in the number of cells.
Cell expansion that leads to enlargement an increase in the volume and size of the organism. It is therefore possible to measure growth using such parameters as mass, volume, length, height, surface area.
On the other hand development is the qualitative aspect of growth which involves differentiation of cells and formation of various tissues in the body of the organism in order for tissues to be able to perform special functions.
It is not possible to measure all aspects of development quantitative.
Therefore development can be assessed in terms of increase in complexity of organism e.g. development of leaves, flowers and roots.
A mature human being has millions of cells in the body yet he or she started from; single cell, that is, a fertilised egg.
During sexual reproduction mammals an ovum fuses with a sperm to form a zygote.
The zygote divides rapidly without increasing in size, first into 2, 4, 8, 16,32, 64 and so on, till it forms a mass cells called morula.
These first cell division is called cleavages.
The morula develops a hollow part, resulting into a structure known as a blastula (blastocyst).
Later, blastocyst cells differentiate into an inner layer (endoderm) and the outer layer (ectoderm).
The two-layered embryo implants into the uterine wall and, by obtaining nutrients from the maternal blood, starts to grow and develop.

EMBRYONIC DEVELOPMENT

As the embryo grows and develops, changes occur in cell sizes and cell -types.
Such changes are referred to as growth and development respectively.
These processes lead to morphological and physiological changes in the developing young organism resulting into an adult that is more complex and efficient.
In the early stages, all the cells of the embryo look alike, but as the development process continues the cells begin to differentiate and become specialized into different tissues to perform different functions.
Growth involves the synthesis of new material and protoplasm.
This requires a continuous supply of food, oxygen, water, warmth and means of removing waste products.
In animals, growth takes place all over the body but the rates differ in the various parts of the body and at different times.
In plants however, growth and cell division mostly take place at the root tip just behind the root cap and stem apex.
This is referred to as apical growth which leads to the lengthening of the plant.
However, plants do not only grow upwards and downwards but sideways as well.
This growth leads to an increase in width (girth) by the activity of cambium cells.
The increase in girth is termed as secondary growth.

SUBSCRIBE THE DOWNLOAD ALL NOTES

1 Comment

GROWTH AND DEVELOPMENT IN PLANTS

GROWTH AND DEVELOPMENT IN PLANTS

STRUCTURE OF THE SEED

Dormancy in Seeds

Factors that Cause Dormancy

Ways of Breaking Dormancy

Seed Germination

Conditions Necessary for Germination

Concept of Growth and Development - K.C.S.E Biology Notes

Archives

Categories

GROWTH AND DEVELOPMENT IN PLANTS

GROWTH AND DEVELOPMENT IN PLANTS

​STRUCTURE OF THE SEED

Dormancy in Seeds

Factors that Cause Dormancy

Ways of Breaking Dormancy

​Seed Germination

​Conditions Necessary for Germination

Concept of Growth and Development - K.C.S.E Biology Notes

Archives

Categories

STRUCTURE OF THE SEED

Seed Germination

Conditions Necessary for Germination