seed : structure, parts of a seed, types

The ovules after fertilization develops into seeds.
Consist of an embryo, with or without endosperm and a seed coat.
Found inside a fruit.
Plants like Pteridophytes and Bryophytes do not produce seeds.
Gymnosperms do not have ovaries and produce naked seeds.
Angiosperms produce seeds having protective seed coat, food reserves(endosperm) and embryo.

1. SEED Submitted to, Dr. Liza Jacob Head of the dept Dept of botany St Teresa’s college Submitted by, Silpa Selvaraj Roll no: 13 II MSc Botany St Teresa’s college

2. What is a seed ? The ovules after fertilization develops into seeds. Consist of an embryo , with or without endosperm and a seed coat . Found inside a fruit. Plants like Pteridophytes and Bryophytes do not produce seeds. Gymnosperms do not have ovaries and produce naked seeds. Angiosperms produce seeds having protective seed coat, food reserves(endosperm) and embryo. The cotyledons of the plant embryo within the seed are used in classifying angiosperms. 2

3. The changes that takes place during the transformation of an ovule to a dry dormant seed; Integuments dry up and form the hard, protective seed coat. Nucellus forms the perisperm. Egg cell becomes the embryo. Endosperm nucleus develops to endosperm. Nutrient materials are converted to insoluble storage products. Cells lose water and becomes dry. Physiological activities are reduced to minimum. 3

4. Seed coat 4 The two integuments of the ovule undergo profound changes and form the seed coat. Outer integument – outer testa . Inner integument – inner tegmen . In many cases, the tegmen gets completely fused with testa . In bitegmic ovules, development of seed coat occurs in two ways; Both the integuments contribute to seed coat formation. Outer integument alone contributes to seed coat formation.

5. Embryo The fertilized egg develops into an embryo. Consist of three parts ; Radicle Plumule One or two cotyledons The cotyledons are almost leaf like. Dicot plants- two cotyledons - eg ; Bean, Pea, Tamarind etc. Monocot plants- one cotyledon - eg ; Maize, Wheat, Coconut etc. 5

6. Endosperm The developing embryo requires food material which may be stored in cotyledons or endosperm . Sometimes the food material is stored up in the cotyledons in the form of starch, protein, oil etc. Hence there is no endosperm in such seeds and they are called as non endospermous or exalbuminous seeds. Eg : Bean,Pea etc. Seeds having endosperm are called endospermous or albuminous seeds. Eg : Castor, Wheat etc. 6

7. The chemical composition of endosperm varies with plants. Castor- plenty of oil is stored up in the endosperm, so the endosperm is oily. Paddy,Wheat and other food grains – endosperm rich in starch- mealy or farinaceous endosperm. In coconut, the kernel is the endosperm- fleshy and contains plenty of oil. In Water lily and Pepper seeds, a small amount of nucellus is left behind unused, even after the seeds are fully formed. This remaining portion of nucellus is called as perisperm . Such seeds are called as perispermous seeds . 7

8. 8 SEED DICOT SEED MONOCOT SEEDS (Based on the no.of cotyledons) Endospermic or albuminous dicot seed Non endospermic or exalbuminous dicot seed Endospermic or albuminous monocot seed Non endospermic or exalbuminous monocot seed Eg : Pea, Gram Eg : Castor Eg : Wheat, Rice Eg : Orchids

9. Monocot seed Single cotyledon present. Endosperm – food storing tissue – hence called as endospermic seed/albuminous seed . Endosperm is surrounded by a protein layer called aleurone layer . Most of the monocots are endospermic except Orchids . 9

10. Dicot seed Two cotyledons present. Endosperm is absent. Food is stored in the cotyledons. Since endosperm is absent, dicot seeds are non endospermic/ exalbuminous . Exception – Castor (endospermic) Seed coat – double layered – outer testa & inner tegmen. 10

11. Perispermic seed Persistent nucellus is called as perisperm. Formed from ovule which has persistent nucellus. Eg ; Coffee , Black pepper etc. 11

12. A) Exalbuminous dicot seed of pea Pea seed is an exalbuminous dicot seed. It attaches to the fruit wall by funiculus (short stalk). At maturity, the funiculus detaches from the seed, leaving a scar called hilum . Close to the hilum, micropyle is present which serves for the absorption of water during germination. Tough seed coat is formed by the fusion of testa and tegmen. It has a longitudinal ridge called raphe . The whole of the seed enclosed within the seed coat is called kernel . 12

13. There are two fleshy cotyledons, attached to the embryonal axis. They store food materials for the embryo. The tip of the embryonal axis is called plumule/shoot primordium . The base is called radicle/root primordium . Cotyledonary node is present at the central portion of the embryonal axis. The portion between the plumule and the cotyledonary node is called epicotyl and the portion between the radicle and the cotyledonary node is called hypocotyl . 13

14. B) Albuminous dicot seed of castor Castor seed is a three chambered capsule. The seed coat is hard and shining with brown or black markings. Composed of testa and tegmen. The hilum is covered by a white spongy outgrowth of the testa called caruncle or strophiole . It is believed to help in germination by absorbing water. A distinct raphe is present. Inside the seed coat, papery white membrane is present . Formerly it was considered as tegmen but now it is shown to be perisperm . Inside it is the white, fleshy and oval endosperm, rich in oil. Thus the seed is endospermous or albuminous . 14

15. C) Albuminous monocot seed of rice Most monocot seeds are albuminous. The grain of rice is a single seeded caryopsis. Its outer covering/husk has two halves namely lemma (lower) and palea (upper). Inside the husk is a thin and brown coat called bran which is formed by the fusion of pericarp and seed coat. The large starchy endosperm forms the bulk of the grain. 15

16. It is surrounded by a proteinaceous sheath called aleurone layer . It plays an important role in seed germination. Aleurone cells contain aleurone grains, made of carbohydrates, proteins, phytin, phospholipids etc. Plumule is protected by coleoptile and radicle by coleorrhiza . Single cotyledon is present which is modified as a shield like organ called scutellum . It secretes enzymes to digest the endosperm during seed germination. The part of the embryonal axis, lying between the coleoptile and the cotyledonary node is sometimes called mesocotyl . 16

17. Germination of seeds Conditions required for germination; a) EXTERNAL Water/moisture (not excess) Oxygen Optimum temperature b) INTERNAL Viability of the seed Embryonic development Some seeds require a special condition for germination ie , low temperature. It is called vernalization. 17

18. Events during germination; Imbibition. Rupturing of seed coat. Absorption of water. Cells get hydrated. As a result oxygen uptake increases (aerobic respiration) and the concentration of gibberellic acid increases. GA stimulates the synthesis of hydrolytic enzymes (digest the reserve food materials) like proteases, amylases, lipases etc. The digested food material is supplied to the developing embryo. Once the embryo acquires the required nutrients, emergence(appearance of radicle) takes place. 18

19. Thank you 19