The starting point in the life of many types of plants begins from a seed.Transformation and development of seeds into a full grown plant is an extraordinary process which necessitates cell division,cell enlargement growth,the formation of such parts as roots, stems,leaves,flowers and a long series of complex changes.The final appearance of plants is also affted by the gentic pattern of the fertilized ovule and environmental factors.
Seed is a live plant which has not grown and acts as a bridge among generations of plants. Each seed is made up of embryo,tissues of storing materials,enzymes and a seed coat. When seeds are in suitable environment from the point of view of moisture , oxygen, temperature and light they begin to germinate. During the germination process, it first absorbs water and as a result of the production and the activities of special enzymes inside the seed undissolved nutrients change into dissolvable ones. The materials thus produced are transferred into embryo meristem and provides the necessary energy for the growth of the plant and eventually causes the crack in the seed coat and the emergence of a seedling.
Embryo is in fact a tiny plant which has a main axis and two growth points at its each end.On one end there are the embryonic roots called radicle and on the other end there are the embryonic stems called plumule. The embryo also contains one or more hidden leaves called cotyledon.Generally,the speed at which radicle grows is more than that of plumule,therefore,radicle is the first part to come out of the broken seed coat. This is a big advantage for the survival of seedlings . Main roots appear later than radicles.
Roots help the growth of plants through supporting the plant in soil,absorbtion of water and minerals and transferring them to the stem and in some cases by storing nutrients.
Plumules also have an apical meristem at their ends the cells of which divide quickly, grow upward and are pushed out of soil a generative layer forms in stems between xylem and phloem tissues which through fast division produce cells in two directions,these cells are eventually turned into phloem and xylem.
Leaves are also lateral projections produced from the apical meristems.Leaves have two main parts : leaf stalk or petiol and leaf blade or flat part.
Leaves play their role in the life of the plant through photosynthesis,respiration, evaporation and transpiration.Photosynthesis is a process during which green plants produce glucose and release oxygen in the presence of sunlight by using the CO2 existing in the air and water.A portion of the glucose produced is used to make complex organic compounds such as starch,celloluse, lipids,proteins,vitamins and hormones,and another portion is used in the process of plant respiration,thus the necessary energy for other plant activities are produced. Outstanding transformations in the life cycle of plants occur when they get from vegetative phase to reproduction phase.In this phase the apical meristems produce the earliest designs of flowers consisting of sepals , petals, stamens and carpels.Plants reproduction takes place with the appearce of flowers.
Reproductive parts produce gametes of male (pollen( and female) ovule(.The two male and female reproductive cells in plants meet and unite,and following fertilization and egg cell formation,fruit and seed are produced. They,then grow and spread,which in fact when suitable environmental conditions are met, a new plant life cycle begins.