School of Sciences, Lautoka Campus BIO509 Botany Lecture 19: Pollination

POLLINATION Transfer of pollen from an anther to a stigma. When successful, pollen grain produces a pollen tube which grows through the style to the ovary where it releases two sperm into the embryo sac. Pollination is brought about by insects, wind, water, birds, bats, and other mammals. Flowers are specially adapted to attract and accommodate the pollinating agents.

Adaptations among flowers and its pollinators are complex and precise. Self pollination and self fertilization may occur in some flowers. Cross pollination and fertilization occurs in most flowers.

Some mechanisms to promote self-pollination Flowers may not open. Pollen grains may be shed before the flowers are open. Stigma and stamen are positioned closely and may mature at the same time. Stigma and stamen may be hidden by floral organs after the flowers open.

Some mechanisms to encourage cross pollination Flowers are open. Stigma and stamen are positioned far apart. Different periods of maturity in pollen and stigma. Self sterility or self incompatibility. Presence of monoecious or dioecious flowers.

Fertilization and development of seed The pollen tube grows down the style to the ovary where it penetrates the micropyle of an ovule & discharges two sperm cells into the embryo sac. One sperm cell fertilizes the egg forming a diploid zygote which develops into a sporophyte embryo.

The other sperm cell fuses with the two polar nuclei in the large central cell of the embryo sac forming a triploid cell (3x) that develops into endosperm (nutritive tissue rich in starch & other food reserves). This event is known as Double fertilization.

Double fertilization is unique to angiosperms.

Endosperm was a major development in angiosperm evolution. Because of extra copies of genes (triploid) growth hormones and nutrients used by the developing embryo can be produced very rapidly. Superior form of nutrition for the growing embryo. Major nutrition source for humans as well – rice, wheat and maize. http://1.bp.blogspot.com/_ctMJzr589O4/S6xRkztP3rI/AAAAAAAAFWM/f8TBLvxlYjE/s1600/wheat-kernel.jpg http://www.google.com/imgres?imgurl=http://mauimike6.files.wordpress.com/2009/09/endocarp_endosperm-flesh-9.jpg%25253Fw%25253D300%252526h%25253D199&imgrefurl=http://zedtrade.com.br/endospe

Life cycle of an angiosperm

Comparison of Gymnosperm & Angiosperm Life Cycles

Reproduction simpler than Gymnosperms Enclosure of ovule within carpel. more protection during development more protection of seed during dispersal Carpel forms a barrier around ovule but facilitates passage of pollen tube to ovule to allow fertilisation. Reduction of the female gametophyte to only 8 nuclei - requires just three mitotic divisions. Time between pollination and fertilisation is much faster in Angiosperms than in conifers

Is there any difference in the pollen grains between angiosperms and gymnosperms?

Embryo development - terminal cell – gives rise to the embryo. The zygote divides to produce 2 cells: - terminal cell – gives rise to the embryo. - basal cell – produces the suspensor which anchors the embryo to its parent and functions in the transfer of nutrients from parent to embryo.

Cotyledons develop first on the young embryo then the embryo lengthens. The shoot apical meristem develops between the cotyledons. Root apical meristem develops where the embryo attaches to the suspensor. Seed maturation – the seed dehydrates & the embryo becomes dormant.

As the ovules mature into seeds, the ovary develops into a fruit. Seeds germinate when environmental conditions are favorable the seed coat ruptures and the embryo emerges as a seedling, using food stored in the endosperm & cotyledons

Apomixis Apomixis - asexual type of reproduction in which the plant embryos grow from egg cells without being fertilized by pollen. There is no fusion of gametes. Plants developing from such seeds are genetically equivalent to a vegetatively propagated plant.

Parthenocarpy Development of fruits without fertilization. Develop from ovaries having unfertilized eggs. Fruits are appear normal but are seedless. Navel oranges, supermarket bananas, seedless grapes are some examples.

Angiosperm diversity Basal angiosperms – three clades that consist of plants belonging to the oldest lineages.

Magnolids – share some primitive traits with the basal angiosperms but are more closely related to monocots & eudicots.

Parallel venation Net venation

Questions??