disperses. It is this protein that is involved in the recognition interaction of
sporophytic self-incompatibility (Topic H3), and responsible for the allergic
reaction of hay fever.The shape and size of pollen grains varies enormously. The shape varies from
spherical to narrowly oblong, tetrahedral or even dumb-bell shaped and water-
pollinated grains of the marine grass, Zostera, resemble spaghetti and can reach
over 1 mm in length. In animal or wind-dispersed grains the size varies from
5 μm across in, e.g. forget-me-nots, Myosotis, to over 200μm in some members
of the cucumber family, but is usually around 40μm; wind-dispersed grains are
all around 40μm.
The pollen grain usually has one or more aperturesin the exine through
which the pollen tube will emerge. This aperture may be elongated forming a
furrow, or more-or-less rounded forming a pore. It seems that a single furrow
was the first form to evolve, with pores and multiple apertures appearing later
in the fossil record. A combination of the number and orientation of pores or
furrows and the sculpturing of the exine is diagnostic of plant families, genera
or sometimes species (Fig. 1). The exine of pollen that is dispersed by the wind is
fairly smooth and in some families, such as the grasses, it varies little. By
contrast, in animal-dispersed pollen the sculpturing varies hugely from circular
holes to projections of many sizes and shapes and the beautiful window-like
form of some dandelion-like composites (Fig. 1).Embryo sac The embryo sac is contained within the ovule (Topic D1) and the most common
form consists of eight haploid cells (Fig. 2). Three of these remain at the
micropyle end to form the eggand its two associated synergidcells; three
others remain at the other end to form antipodalcells. The two remaining cells,
one from each end, fuse to form a diploid central nucleus that will become the
endospermafter fertilization. The embryo sac derives from divisions of a single
haploid cell, itself the result of a meiotic division in which three of the four cells
abort.
The embryo sac described above occurs in about 70% of flowering plants but
there are at least 10 other types of embryo sac known, some characteristic of
certain families, others within one family or even on the same plant. The total
Variation in
pollen grains
D2 – Pollen and ovules 47
Fig. 1. Pollen grains showing the variety of apertures and sculpturing.