The below mentioned article provides a study note on the Seed Growth.
Seed germinates to produce seedling. Seedling grows first into a juvenile plant which develops only vegetative organs. Later on it begins to produce flowers, fruits and seeds.
Unlike animals, plants do not stop growing after reaching maturity. They continue to grow and bear new roots, leaves, branches, flowers, etc. While roots, stems and their branches have indefinite growth, other organs like leaves, flowers and fruits show limited or definite growth.
They appear and fall off periodically and sometimes repeatedly. There is a highly ordered succession of events. A plant consists of billions of cells arranged in specific tissues and organs. All of them are descendants of a single celled zygote.
However, by their specific differentiation and location in the plant they come to have different functions. As a result complex organisation is formed that produces roots, leaves, branches, flowers, fruits and seeds. After a period of growth, differentiation and development, each plant dies. There are a number of intrinsic (internal) and extrinsic (external) factors which control various development processes.
Seed germination is sprouting of seed due to growth and development of its embryo to form a seedling. A seed can germinate only when it has favourable internal and external conditions.
Favourable Internal Conditions:
(i) Non-dormancy or completion of dormancy,
(ii) Viability, i.e., the embryo of the seed is alive and capable of resuming growth,
(iii) Sufficient storage food, and
(iv) Maturation of embryo.
Favourable External Conditions:
They are availability of water, oxygen, optimum temperature (25°-35°C), in some cases light and particular pH. In the absence of favourable external or exogenous conditions, a non-dormant seed is said to be in quiescent state or quiescence.
In the presence of favourable external conditions a viable non-dormant seed imbibes water through micro Pyle and seed coat. The seed coat softens. The seed kernel swells up and ruptures the softened seed coat. Imbibition of water is the first step of seed germination.
Embryo becomes active. It produces hormones and enzymes for hydrolysis of stored polysaccharides, proteins and lipids. Respiration is initially anaerobic. Soon it becomes aerobic. Rate of respiration also increases. The mobilised nutrients reach the embryo cells. They grow, divide and differentiate.
Radicle grows, breaks the seed coat and comes out to form the first root. Emergence of radicle is the first sign of seed germination. There is now increased absorption of water. Rate of embryo growth increases. Plumule grows to produce the shoot system. It converts the seed into seedling which is capable of manufacturing its own food.
Growth of plumule is accompanied by rapid growth of either hypocotyl or epicotyl. Cotyledons remain inside the soil when epicotyl grows. It is called hypogeal germination (e.g., Pea). Cotyledons come out of the soil along with enclosed plumule when hypocotyl grows. It is known as epigeal germination (e.g., Bean).
It is germination of seed while it is attached to the parent plant and is being nourished by it. Vivipary occurs in mangrove plants (e.g., Rhizophora, Sonne ratio) which grow in saline marshes along the sea shore. Seeds cannot germinate in saline marshes due to high salinity. Such seeds do not undergo dormancy.
While attached to the plant, the embryo of the seeds grows out to form a seedling with long hypocotyl. Being heavy, the seedling falls down in the saline marsh in vertical position with plumule remaining out. The radicle end soon develops lateral roots for proper anchorage of the seedling.