Question

(a) Explain the events after pollination leading to the formation of a seed in angiosperms.
(b) Mention the ploidy levels of the cells of different parts of an albuminous seed.

Answer 1

(a) The following events take place between pollination and formation of a seed in angiosperms:
    (i) Double fertilisation
    (ii) Endosperm formation
    (iii) Embryo development  
    (iv) Seed formation

(i) Double fertilisation:

• When pollen grains fall on the stigma, the pollen tube enters one of the synergids and releases two male gametes.
• One of the male gametes moves towards the egg cell and fuses with it to complete the syngamy to form a zygote.

• The other male gamete fuses with the two polar nuclei and forms the triploid primary endosperm nucleus (PEN). This is termed as triple fusion.

• After triple fusion, the central cell becomes the primary endosperm cell (PEC).

• The primary endosperm nucleus gives rise to the endosperm, while the zygote develops into an embryo.

(ii) Endosperm formation:

• The primary endosperm nucleus divides repeatedly to give rise to free nuclei. This is called free nuclear endosperm.

• Cell wall formation occurs next, resulting in the formation of a cellular endosperm.

• The endosperm may be either consumed fully by the growing embryo (as in peas and beans) or retained in the mature seed (as in coconut and castor).

(iii) Embryo development:

• The embryo develops at the micropylar end of the embryo sac where the zygote is situated.

• The zygote first gives rise to the pro-embryo and then to the globular, heart-shaped mature embryo.

• A typical dicot embryo consists of an embryonal axis and two cotyledons.

• The portion of the embryonal axis above the level of cotyledons is called epicotyl. It contains the plumule (shoot tip). The portion below the axis is called hypocotyl. It contains the radicle (root tip).

• There is only one cotyledon in a monocot embryo. In grass, it is known as the scutellum and is situated at one side of the embryonal axis. At its lower end, the embryonal axis has the radicle and the root cap enclosed in the coleorrhiza.
• The epicotyl lies above the level of the scutellum and has the shoot apex and leaf primordia enclosed in hollow structures called coleoptiles.

(iv) Seed formation:

• Seeds are the fertilised ovules that are developed inside a fruit.

• Each seed consists of a seed coat, an embryonal axis and cotyledons.

• Seeds may be albuminous (endosperm present as in the case of wheat and maize) or non-albuminous (endosperm absent, as it is consumed by the growing embryo as in the case of peas and beans).

(b) The ploidy of the endosperm of an albuminous seed is 3n and that of an embryo is 2n.