STRUCTURE OF A SEED
What is the structure of a 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 legmen.
- The testa is thick; the tegmen is a transparent membrane tissue.
- The two layers protect the seed bacteria, fungi and other organisms whk may damage it.
- There is a scar called hilurn on one part of the seed.
- This is point where the seed had been attached the seed stalk or funicle.
- Near one end of 1 hilum is a tiny pore, the micropyle.
- This allows water and air into the embryo, embryo is made up of one or two seed leavi or cotyledons, a plumule (embryonic sh( and a radicle (the embryonic root).
- The of the radicle is opposite the micropyle.
- In some seeds the cotyledons swollen as they contain stored food for growing plumule and radicle. Such seeds, called non-endospermic seeds.
- In ot cases, the seeds have their food stored in: endosperm.
- Such seeds are called endospermic seeds. Seeds with one cotyk are referred to as monocotyledonous and those with two are referred to dicotyledonous.
- This is the major basis in differentiation between the two large cb of plants, the monocotyledonae aa 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 favourable 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 favourable 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.
- 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.