11th Class Biology Plant Growth and Development / पौधे की वृद्धि और विकास Seed

Development of seed : The fertilized ovule forms seed. The ovule increases greatly in size. The integuments dry up. The outer one becomes hard or leathery and forms the outer seed coat or testa while the inner one, if persist, forms the tegmen.

The nucellus is generally used up during the development of embryo but in some cases it remains outside the endosperm in the form of a thin layer, called perisperm. The endosperm may persist or completely digested during embryogenesis.

A scar is usually visible on one side of the outer seed coat. It is known as hilum and marks the point of attachment to the stalk. With these changes, the ovule changes into seed and enters a period of dormancy while the ovary ripens into a fruit.

Dicotyledonous seeds

Exalbuminous : Gram, Pea, Bean, Mustard, Mango, Groundnut, etc.

Albuminous : Castor, Poppy, Artabotrys, Custard apple (Ananas) etc.

Monocotyledonous seeds

Exalbuminous : Orchids, Alisma, Najas, Pothos, Amorphophallus, Vallisneria, etc.

Albuminous : Cereals, Millets, Palms, Lilies, etc.

Non-endospermic or Exalbuminous seeds : In exalbuminous seeds endosperm is completely consumed by the developing embryo, and the mature seeds are without endosperm. The food is stored in cotyledons.

Endospermic or Albuminous seed : In albuminous seeds, embryo not consumed all endosperm. So it persists in the mature seed. In these seeds food stored in endosperm. In monocot seed the membranous covering of :

• Radicle is called coleorrhiza.
• Plumule is called coleoptile.

Germination of seeds : The process by which the dormant embryo of the seed resumes active growth and grows into a new plant is known as germination.

Types of seed germination

Epigeal germination : In this type of germination, the cotyledons come above the surface of the soil into the air and light due to the rapid growth and elongation of the hypocotyl. The cotyledons turn green and finally dry up and fall off and seedling becomes an independent plant. Germination of seeds of Bean, Gourd, Castor, Cotton, etc. is of epigeal nature.

Hypogeal germination : In this type of germination, the cotyledons remain in the soil or just above the surface. In this case epicotyl elongates pushing the plumule upwards. The cotyledons do not turn green and gradually dry up and fall off. Common examples of hypogeal germination are the seeds of Pea, Mango, Groundnut, etc.

Viviparous germination : This is a special type of germination found in mangrove plants. The embryo grows not only out of the seed but also out of the fruit and projects from it in the form of a green seedling displaying root and hypocotyl. Due to its increasing weight the seedlings separate from the parent tree and falls into the mud or water and soon develops lateral roots. Vivipary is seen in Rhizophora and Sonneratia.

Factors for seed germination

External factors : Water, oxygen, suitable temperature.

Internal factors : Foods and growth regulators, completion of rest period, viability.

Seed dormancy : In several plants seeds germinate as soon as they have undergone maturation and provided proper conditions for germination. e.g., seeds of Bean, Pea, Maize etc. In some plants seeds are incapable of germination because of some inhibitory factors. Such seeds are unable to germinate even under suitable conditions. This is called seed dormancy.

Causes of seed dormancy

The seed dormancy may be due to many causes some of which are as follows :

(1) Impermeability of seed coats to oxygen. (e.g., Xanthium) and water. (e.g., Chenopodium and many leguminous seeds).

(2) Seed coat is mechanically hard, thus resisting the growth of embryo. e.g., Mustard, Capsella, Amaranthus.

(3) Presence of rudimantary or immature embryo. e.g., Ginkgo biloba (a gymnosperm).

(4) Some plants produce such chemical compounds that inhibit the germination of their own seeds. e.g., Tomato, (possesses inhibitor ferulic acid).