SEED

CHAPTER 2                                          SEXUAL REPRODUCTION IN FLOWERING PLANTS

Seed

[1]  In angiosperms, the seed is the final product of sexual reproduction.

 [2]  It is often described as a fertilised ovule.

[3]  Seeds are formed inside fruits.

[4]  A seed typically consists of seed coat(s), cotyledon(s) and an embryo axis.

[5]  The cotyledons of the embryo are simple structures, generally thick and swollen due to storage of food reserves (as in legumes).

[6]  Mature seeds may be non-albuminous or albuminous.

[7]  Non-albuminous seeds have no residual endosperm as it is completely consumed during embryo development (e.g., pea, groundnut).

[8]  Albuminous seeds retain a part of endosperm as it is not completely used up during embryo development (e.g., wheat, maize, barley, castor, sunflower).

[9]  Occasionally, in some seeds such as black pepper and beet, remnants of nucellus are also persistent. This residual, persistent nucellus is the perisperm.

[10]  Integuments of ovules harden as tough protective seed coats. The micropyle remains as a small pore in the seed coat. This facilitates entry of oxygen and water into the seed during germination.

[11]  As the seed matures, its water content is reduced and seeds become relatively dry (10-15 per cent moisture by mass).  The general metabolic activity of the embryo slows down. The embryo may enter a state of inactivity called dormancy, or if favourable conditions are available (adequate moisture, oxygen and suitable temperature), they germinate.

[12]  As ovules mature into seeds, the ovary develops into a fruit, i.e., the transformation of ovules into seeds and ovary into fruit proceeds simultaneously.

[13]  The wall of the ovary develops into the wall of fruit called pericarp.

[14]  The fruits may be fleshy as in guava, orange, mango, etc., or may be dry, as in groundnut, and mustard, etc.

[15]  Many fruits have evolved mechanisms for dispersal of seeds.

[16]  In most plants, by the time the fruit develops from the ovary, other floral parts degenerate and fall off.

[17]  In a few species such as apple, strawberry, cashew, etc., the thalamus also contributes to fruit formation. Such fruits are called false fruits.

[18]  Most fruits however develop only from the ovary and are called true fruits.

[19]  Although in most of the species, fruits are the results of fertilisation, there are a few species in which fruits develop without fertilisation. Such fruits are called parthenocarpic fruits. Banana is one such example.

[20]  Parthenocarpy can be induced through the application of growth hormones and such fruits are seedless.

[21]  Seeds offer several advantages to angiosperms

·                   Firstly, since reproductive processes such as pollination and fertilisation are independent of water, seed formation is more dependable.

·                   Also seeds have better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas.

·                   As they have sufficient food reserves, young seedlings are nourished until they are capable of photosynthesis on their own.

·                   The hard seed coat provides protection to the young embryo.

·                   Being products of sexual reproduction, they generate new genetic combinations leading to variations.

[22]  Seed is the basis of our agriculture.

[23]  Dehydration and dormancy of mature seeds are crucial for storage of seeds which can be used as food throughout the year and also to raise crop in the next season.

[24]  Can you imagine agriculture in the absence of seeds, or in the presence of seeds which germinate straight away soon after formation and cannot be stored ?

[25]  How long do the seeds remain alive after they are dispersed

·       This period again varies greatly. In a few species the seeds lose viability within a few months.

·       Seeds of a large number of species live for several years.

·       Some seeds can remain alive for hundreds of years.

·       There are several records of very old yet viable seeds.

·       The oldest is that of a lupine, Lupinus arcticus excavated from Arctic Tundra.

·       The seed germinated and flowered after an estimated record of 10,000 years of dormancy.

·       A recent record of 2000 years old viable seed is of the date palm, Phoenix dactylifera discovered during the archeological excavation at King Herod’s palace near the Dead Sea.