Instant Notes: Plant Biology

number of cells in an embryo sac is four, eight or sixteen in various orientations,

but in all types there is a single haploid embryo nucleus that will form the next

generation after fertilization and a second nucleus that will form the

endosperm. The others play no further part after fertilization.

Pollination occurs when pollen grains land on the stigma. If compatible, a

pollen tubeemerges through one of the apertures of the grain and grows

through the stigma and into the style. It follows the transmitting tissue in the

center of the style or the inner surface of a hollow style and its progress is

marked by deposits of the polysaccharide callose(Topics H3 and M3) behind

the growing tip. It can then grow through the ovary and into the micropyle of

the ovule (more rarely into the other end). The rate of pollen tube growth is

variable, usually around 3–4 mm h–1but may be up to 35 mm h–1.

Twosperm cellsare discharged, either into one of the synergid cells or

directly into the embryo sac, and they lose their cytoplasm before one fertilizes

the egg cell, the other the central diploid cell that will form the endosperm. The

embryo is then diploid, with its cytoplasm derived entirely from the egg, and

will grow into the new plant. The endosperm is triploidsince the haploid male

nucleus fuses with a diploid cell in the commonest form of embryo sac (see

above). Other types of embryo sac have diploid or pentaploid (5n) endosperm.

In most plants the endosperm grows after fertilization to provide the food store

for the developing seed.

Pollination and
fertilization

48 Section D – Reproductive anatomy

Micropyle

Antipodal cells

Central nucleus

Egg cell

Synergid

Fig. 2. An eight-celled embryo sac.