Seeds, germination and tropisms

Seed Structure

A seed develops from an ovule after fertilization. It consists of a tough coat or testa enclosing an embryo which is mad up of a plumule, a radicle and one or two cotyledons. In favourable conditions the seed can grow and become a fully independent plant, bearing flowers and seeds during its life cycle. In the embryo of the seed are all the potentialities of development and growth to a mature plant resembling other members of its species in almost every detail of leaf shape, cell distribution and flower colour and structure. The testa – The integuments round the ovule form the testa, a tough, hard coat which protects the seed from fungi, bacteria and insects. It has to be split open by the radicle before germination can proceed. The hilum – This is a scar left by the stalk which attached the ovule to the ovary wall. The micropyle - This is the opening in the integuments through which the pollen tube entered at fertilization. It remains as a tiny pore in the testa opposite the tip of the radicle and admits water to the embryo before germination. The radicle – This is the embryonic root which grows and develops into the root system of the plant. The plumule – This is a leafy part of the embryonic shoot. These leaves are attached to the embryonic stem, of which the part above the attachment of the cotyledons is called the epicotyl and the part below, the hypocotyl. The cotyledons – The grasses, cereals and narrow leaved plants such as iris and bluebell have seeds with only one cotyledon. Such plants are called monocotyledon. Other flowering plants, the dicoytledons, have two cotyledons in their seeds. These cotyledons are modified leaves attached to the epicotyl and hypocotyl by short stalks, and they often contain food reserves which are used in the early stages of germination. In most dicotyledonous plants the cotyledons are brought out of the testa and above the ground where they become green and make food by photosynthesis. The cotyledons eventually fall off; usually after the first foliage leaves have been formed. The cotyledon leaves bare no resemblance to the ordinary foliage leaf, the shape of which is first apparent when the plumule leaves open and grow.

Structure of types of seed

One important point of difference is that maize has only one cotyledon, and a separate food store, called the endosperm, that is not present in the others. The plants have been selected because they show two of the different ways in which germination can take place, and their seeds are large enough for the structure to be examined and the course of germination followed in some detail.

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Biology


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