Flower structure

Pollination

Angiosperm life cycle

Fruits

Seeds

 

 

Growth of the embryo and seedling in a dicot

Following fertilization, the ovule develops into the seed.

The zygote divides and differentiates into the embryo, the endosperm nucleus divides to form nutritive tissue, the integuments become the testa and the ovary wall becomes the fruit.

On the left are the early stages from zygote through globular.

During the heart stage you can see the formation of the 2 cotyledons. The suspensor pushes the growing embryo into the endosperm; eventually the suspensor will disintegrate

The cotyledons develop and fill the entire embryo sac replacing the endosperm as the nutrient source.

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While the seed is developing, the ovary wall must grow to allow the expansion of the seed.

The ovary wall becomes the pericarp and the outer layer of this forms the 'skin' of the fruit.

B. Growth of a dicot seed:

 

The seed absorbs water, and swells.

Radicle is the embryonic root. It is the first structure to emerge from the seed:

After about three days, depending on the temperature, the radical grows and bursts through the testa. It grows down between the soil particles, its tip protected by a root cap

Root hairs appear in the region where elongation has ceased. Water and salts from the soil are absorbed by the root hairs on the radical and pass to the rest of the seedling. Later, lateral roots develop from the radical.

Hypocotyl is the stem-like portion of the embryo below the point of attachment but above the embryonic root

.Once the radical is firmly anchored in the soil, the hypocotyl starts to grow. The rapid growth of the hypocotyl pulls the cotyledons out of the testa ( seed coat) and through the soil.

The plumule is still between the cotyledons and thus protected from damage during its passage through the soil. Sometimes the testa, still partly enclosing the cotyledons, is brought above the soil and pushed off later as the cotyledons separate. Once above the soil, the hypocotyl straightens and the cotyledons separate, exposing the plumule

. The cotyledons become green and, presumably, photosynthesize before falling off

Mean while, the epicotyl

(the epicotyl is the stem like materials above the point of attachment of the cotyledons) has extended and the plumule leaves have expanded and begun to photosynthesize.

The germination of a seedling which brings its cotyledons above the ground is called epigeal. In the early stages of germination, the food reserves in the cotyledons, have been digested by enzymes to support the actively growing meristematic regions where new cells are being made.