Sexual reproduction in flowering plants (New syllabus chapter 1 / old syllabus Chapter 2)
Seed developement|reproduction in flowering plants|short notes| NCERT| standard 12| Biology
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Note 13
seed is the final product of sexual reproduction in angiosperms.
Seed can be described as a fertilised ovule.
Seeds are formed inside fruits.
A seed typically consists of
- seed coat(s),
- cotyledon(s) and
- an embryo axis.
> The cotyledons of the embryo
- simple structures,
- generally thick and swollen due to storage of food reserves (as in legumes).
Mature seeds may be non-albuminous or ex-albuminous.
Non albuminous seeds have no residual endosperm - it is completely consumed during embryo development - example: pea, groundnut.
Albuminous seeds retain a part of endosperm - it is not completely used up during embryo development - example: wheat, maize, barley, castor.
In seeds of black pepper and beet, remnants of nucellus are also persistent.
This residual, persistent nucellus is the perisperm.
> seed coat
Integuments of ovules harden and form 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.
> seed maturation
As the seed matures, its water content is reduced
seeds become relatively dry (10-15 % moisture by mass).
The general metabolic activity of the embryo slows down.
The embryo may enter a state of inactivity called dormancy.
If favourable conditions - adequate moisture, oxygen and suitable temperature - are available, seeds germinate.
As ovules mature into seeds, the ovary develops into a fruit,
the transformation of ovules into seeds and ovary into fruit proceeds simultaneously.
The wall of the ovary develops into the wall of fruit called pericarp.
The fruits may be fleshy as in guava, orange, mango, etc.,
Or the fruits may be dry, as in groundnut, and mustard, etc.
Many fruits have evolved
mechanisms for dispersal of seeds. Recall the classification of fruits and
their dispersal mechanisms that you have studied in an earlier class.
Is there any relationship between number of ovules in an ovary and the number of seeds present in a fruit?
- yes, Ovules after fertilization, mature into the seed and the ovary develops into a fruit. So if all ovules present in an ovary are fertilized, the number of seeds present in the fruit will be equal to the number of ovules present in the ovary.
In most plants, by the time the fruit develops from the ovary, other floral parts degenerate and fall off.
However, in some - the thalamus also contributes to fruit formation. Example: in a few species such as apple, strawberry, cashew, etc. Such fruits are called false fruits.
Most fruits however develop only from the ovary - they are called true fruits.
Fruits are the results of fertilisation - in most of the species.
Some fruits develop without fertilisation. Such fruits are called parthenocarpic fruits. Example: Banana.
Such fruits are seedless.
Parthenocarpy can be induced through the application of growth hormone
> advantages of seeds to angiosperms
- reproductive processes such as pollination and fertilisation are independent of water, seed formation is more dependable on water.
- 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.
> advantages of seeds to human
Seed is the basis of our agriculture.
Dehydration and dormancy of mature seeds are crucial for storage of seeds
Seeds can be used as food throughout the year and also to raise crop in the next season.
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? -
It is disadvantageous if the seeds can't be stored. It is because seeds if sowed in conditions which are unfavorable to the crop, they will not be able to germinate efficiently. Even if germination occur and the conditions are unfavorable for the growth of the plant, then this would lead to post harvest losses. Therefore, it is difficult to imagine agriculture without seeds or with those seeds that can germinate instantly.
How long do the seeds remain alive after they are dispersed?
The period for which the seeds remain alive after they are dispersed 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.
Can you think of some plants in which fruits contain very large number of seeds?
Orchid fruits are one such category and each fruit contain thousands of tiny seeds.
Similar is the case in fruits of some parasitic species such as Orobanche and Striga.
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Urvi Bhanushali
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