When the ovule is receptive, the cone scales open apart slightly and a drop of
sticky liquid is exuded from the micropyle. Pollen grains are caught in this drop
which is then reabsorbed, and after pollination the cone scales may close up
again. The air bladders on the pollen grains serve to orient the pollen grain as it
approaches the micropyle.Once a pollen grain has reached the micropyle, the two male gametesare
formed and the pollen tube grows actively through the nucellus to reach the
archegonia. The male gametes are discharged into the egg along with the tube
nucleus and the sterile nucleus. Fertilization is achieved by the fusion of the
larger of the male gametes with the egg nucleus, the other three male nuclei
degenerating.
If more than one archegonium has been fertilized several embryosmay be
formed, and each fertilized egg may divide to form several embryos, so compe-
tition can occur. Eventually, one embryo outcompetes the others and absorbs
the nutrients from the female gametophyte, filling the seed when it is mature.
The embryo has several cotyledons(the food stores and first leaves; Topic D3).
In most conifers the seeds are released when the cone scales separate at
maturity. The seeds may be nutritious and can be a major food for some birds,
such as crossbills, and small mammals. Some species have a resistant integu-
ment that allows the seed to lie dormant, and a few only germinate after a fire
has passed over them. The yewhas a fleshy outgrowth from the integument
which is attractive to birds.
In comparison with most flowering plants the stages of reproduction are
slow, taking about a year from female cones being receptive to the mature seed,
but in pines fertilization does not occur until a year after the female cones
become receptive and the cycle takes 2 years.Fertilization and
the seed
300 Section R – Seed plants
Cone scaleOvules
Stalk1 mmFig. 3. Surface view of a scale from the female cone of a pine, showing two ovules at its base.