Instant Notes: Plant Biology

Section D – Reproductive anatomy

D2 Pollen and ovules

A pollen grain is a single cell derived by meiosis (Topic B5) from pollen mother

cells in the anther. It lays down two coats, an outer exineand an inner intine.

The intine is laid down first and is made of cellulose and pectins. The exine

matures soon afterwards and is made of extremely hard polymers of

carotenoids known as sporopollenin. It has numerous tiny cavities in its surface

and is often beautifully sculptured. Pollen exine is one of the most resistant of

plant structures and can survive long after the rest of a plant (including the

inner parts of the pollen grain) decays. Inside the pollen grain the haploid cell

divides to produce two nuclei: a large vegetative nucleuswhich will form the

pollen tube, and a generative nucleus. The generative nucleus divides again

either before dispersal or after it has germinated on a stigma to produce two

sperms that travel down the pollen tube.

Before the first division of meiosis, the pollen mother cellsof the anther are

interconnected by cytoplasmic strands and this may enable all the pollen grains

to develop synchronously. Meiosis in these gives a group of four haploid cells

(Topic B6) and these four pollen grains may mature and be dispersed together

or separately. As the pollen grains mature, proteins from the tapetum of the

anther impregnate the cavities of the wall and remain there when the pollen

The structure of
pollen

Key Notes

Pollen grains consist of one haploid cell surrounded by an inner wall,

intine, of cellulose and an outer wall, exine, of resistant sporopollenin.

The nucleus divides to produce the pollen tube and two sperms. The

tapetum of the anther provides nutrients for the developing pollen, and

proteins from it impregnate the exine.

There is great variation in size, shape, type and arrangement of apertures

and exine sculpturing in pollen. This is diagnostic to plant family, genus

and sometimes species. Wind-pollinated plants vary less than animal-

pollinated plants.

The commonest type of embryo sac consists of eight cells: an egg with

two synergids, three antipodal cells and two that fuse to form a diploid

central cell. Other types are known with four or sixteen cells in the

embryo sac.

Pollen grains germinate and the pollen tube grows through the stigma

and style to reach the ovule. Two sperm cells are discharged: one

fertilizing the egg, the other the diploid central nucleus. The embryo is

then diploid, but the central nucleus that grows to form the endosperm is

usually triploid.

Related topics The flower (D1) Self incompatibility (H3)

The seed (D3)

Embryo sac

The structure of

pollen

Variation in pollen

grains

Pollination and

fertilization