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.
