GROWTH AND DEVELOPMENT IN PLANTS

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

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

Procedure

1. Using a scalpel or razor blade make longitudinal sections (LS) of both the bean seed and the maize grain.
2. Observe the LS of the specimens using a hand lens.
3. Note any structural difference between the specimens.
4. Draw the LS of each specimen and label.
5. Put a drop of iodine solution on the cut  surfaces of both specimens.
6. Note any  differences in colouration with iodine on the surfaces of the two specimens.
7. On  your  diagrams  indicate  the distribution of the stain.
8. 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.

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