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S 1 Seed structure eeds and germination seed structure


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1 Seed structure
eeds and germination - seed structure

A


testa

cotyledons
seed is the product of sexual reproduction in flowering plants. In dicotyledonous plants the seed consists of a miniature plant, the embryo and two modified leaves, the cotyledons, swollen with food reserves.


plumule





plumule


epicotyl



radicle

cotyledon stalk

micropyle


hypocotyl





cotyledon


radicle


(a) Longitudinal section

(b) Diagram of seed structure

Fig.1 Seed structure of a dicotyledon


Testa The seed coat; encloses and protects the seed from insects and fungi. It is usually hard and dry.
Micropyle Hole which admits water when the seed starts to germinate.
Plumule The embryo shoot; two leaves and a growing point.
Cotyledons Modified leaves containing food reserves.
Radicle Embryo root.
Epicotyl The section of stem above the cotyledon stalk.
Hypocotyl The section of stem below the cotyledon stalk.
Germination is the process by which the embryo grows and develops, eventually becoming a fully mature plant. The pattern of germination is similar in most dicotyledonous seeds. When the seed is shed, it is usually dry and hard, containing very little water. In this dehydrated state it is best suited to withstand drought and extreme temperatures.

When conditions become suitable for germination, the seed takes in water through its micropyle. The tissues absorb water and swell and the testa becomes soft. The radicle grows first, pushing though the testa and entering the soil. Next, either the hypocotyl or the epicotyl, depending on the species, starts to elongate and carry the plumule upwards through the soil.

Elongation of the epicotyl brings the embryo out from between the cotyledons and through the soil, leaving the cotyledons below ground.

Elongation of the hypocotyl brings the cotyledons and the plumule above ground.

Whichever pattern of germination occurs, the energy and raw materials required for growth

c
© D.G. Mackean


ome from the food (usually starch) stored in the cotyledon.


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