Seed Structure and function

2. SEED
• A true seed is defined as a fertilized mature ovule that possesses embryonic plant,
stored material, and a protective coat or coats.
• Seed is the reproductive structure characteristic of all phanerogams.

3. • There are hundreds of variations in the seed size, shape, colour and surface.
• The seeds range in size from tiny dust particles, as found in some orchids, to large
double-coconuts.

4. • The seed surface may be smooth, wrinkled, striate, ribbed, furrowed, reticulate,
tuberculate, alveolate, hairy, and pulpy or having patterns like finger prints.

6. Structure of Seed:
A mature seed contains an
 embryo (with a radicle and plumule)
 reserve food materials and
 seed coat.

7. • The seeds are attached to the fruit wall by a small stalk, the funiculus.
• elongated scar representing the point of attachment of seed to its stalk is distinctly
seen, this is the hilum.

8. • Close to the hilum situated at one end of it there is a minute pore, micropyle.
• During seed germination, water is absorbed mainly through this pore, and the
radicle comes out through it.

9. • Continuous with the hilum there is sort of ridge in the seed coat, the raphe.

10. • The seed is covered by two distinct seed coats
• The outer whitish one is the testa,
• the inner thin, membranous covering is the tegmen.
• The seed coats give necessary protection to the embryo which lies within.

11. Seed Coat Function
• Protecting the seed embryo from threats like insects
• Providing a physical barrier between the seed and its surroundings
• Creating a water-tight barrier
• Sensing and communicating environmental conditions to the inner portions of
the seed
• Prompting changes in seed metabolism due to environmental conditions
• Maintaining carbon dioxide and oxygen levels within the seed
• Preventing crushing
• The seed coat is also useful in helping the seed determine when the conditions
are right for germination.

12. • The whitish fleshy body, as seen after removing the seed-coats is the embryo.
• It consists of two fleshy cotyledons and a short axis to which the cotyledons
remain attached.
• The position of the axis lying outside the cotyledons, bent inward and directed
towards the micropyle is the radicle and the other portion of the axis lying in
between the two cotyledons is the plumule.

13. • The plumule is crowned by some minute young leaves. The radicle gives rise to
the root, the plumule to the shoot and the cotyledons store up food material.
• Since the reserve food material is stored in the massive cotyledons and the seed
lacks a special nutritive tissue, the endosperm.

14. • Non-endospermous or exalbuminous Seeds- The seeds which lack endosperm at
maturity are called non-endospermous or exalbuminous.
• Gram, pea, bean and groundnut have non-endospermic seeds.

15. • Endospermous or Albuminous Seed
• plants such as castor bean (Ricinus communis), coconut (Cocos nucifera) and
cereals, food is stored in the endosperm.
• Such seeds where endosperm persists and nourishes the seedling during the initial
stages are called endospermous or albuminous.

17. On the basis of the number of cotyledons in the embryo the angiosperms have been
divided into two large groups:
• 1. Dicotyledons, having embryos with two cotyledons, and
• 2. Monocotyledons, with only one cotyledon.

19. • Besides the basic structures (endosperm, embryo and seed-coat) certain special
structures may arise during seed development.
• In castor bean a fleshy whitish tissue, the caruncle, develops at one end of the
seed. It is derived from the integument.

20. • The juicy edible part of the litchi fruit (aril) is an outgrowth of the funiculus that
develops after fertilization.

21. • The cotton fibres are the elongated epidermal cells of the seed-coat.
• These fibres are single-celled and thin walled.