The flower
The flower is the site of sexual reproduction in angiosperms. Its male part is the androecium (a whorl of stamens) and its female part is the gynoecium (one or more pistils).
The stamen and pollen
A stamen has a stalk, the filament, and a terminal anther. A typical anther is bilobed and dithecous (each lobe has two thecae), with four microsporangia in all, which develop into pollen sacs.
The wall of a microsporangium has four layers: epidermis, endothecium, middle layers and tapetum. The innermost layer, the tapetum, nourishes the developing pollen.
Microsporogenesis. Cells of the sporogenous tissue act as pollen mother cells (microspore mother cells, diploid). Each undergoes meiosis to form a tetrad of four haploid microspores, which separate and develop into pollen grains.
The pollen grain:
Its hard outer wall, the exine, is made of sporopollenin, one of the most resistant organic materials known: it withstands high temperatures, strong acids and alkalis, and no enzyme can break it down. That is why pollen survives as fossils. Gaps in the exine called germ pores allow the pollen tube to emerge.
The inner wall is the intine, made of cellulose and pectin.
A mature pollen grain has two cells: a large vegetative cell with abundant food, and a small generative cell that divides to form two male gametes. In over 60% of angiosperms, pollen is shed at the 2-celled stage.
Pollen of some plants (such as Parthenium, carrot grass) causes allergies. Pollen tablets are also used as food supplements.
The pistil and embryo sac
A pistil has a stigma (which receives pollen), a style and an ovary. Inside the ovary are one or more ovules (megasporangia).
An ovule is attached to the placenta by a stalk, the funicle. It is covered by one or two protective integuments, which leave a small opening, the micropyle. The opposite end is the chalaza. Inside is the nucellus, a mass of cells with food reserves.
Megasporogenesis. A megaspore mother cell in the nucellus divides by meiosis to give four megaspores. Usually three degenerate and one remains functional.
The embryo sac (female gametophyte). The functional megaspore's nucleus divides three times by mitosis, giving 8 nuclei, which arrange into 7 cells:
at the micropylar end, the egg apparatus: one egg cell and two synergids (the synergids have a filiform apparatus that guides the pollen tube);
at the chalazal end, three antipodal cells;
in the centre, a large central cell with two polar nuclei.
This 7-celled, 8-nucleate embryo sac is the monosporic type, the most common.
Pollination
Pollination is the transfer of pollen from an anther to a stigma.
Type | Pollen goes to | Notes |
|---|---|---|
Autogamy | stigma of the same flower | needs anthers and stigma to mature together. Cleistogamous flowers never open and are always self-pollinated (Commelina, Viola, Oxalis) |
Geitonogamy | another flower on the same plant | functionally cross-pollination, but genetically like self-pollination |
Xenogamy | a flower on a different plant | brings genetically different pollen |
Agents:
Wind: pollen is light, dry and produced in huge amounts; stamens are well exposed; stigmas are large and feathery. Common in grasses and maize (the "tassels" are the stigmas and styles).
Water: rare, in about 30 genera, mostly monocots: Vallisneria, Hydrilla, Zostera (seagrass). Pollen is protected by mucilage.
Insects and other animals: flowers are large, colourful, scented and rich in nectar. Some plants offer safe places to lay eggs in return, like the yucca and its moth, which can't complete their life cycles without each other.
Outbreeding devices
Continued self-pollination leads to inbreeding depression. Plants avoid it by:
releasing pollen and making the stigma receptive at different times,
placing anther and stigma at different positions,
self-incompatibility: a genetic mechanism that stops pollen from germinating on the stigma of the same plant,
producing unisexual flowers: male and female flowers on different plants (dioecy, as in papaya) prevents both autogamy and geitonogamy.
Pollen-pistil interaction
The pistil recognises whether the pollen is compatible. Compatible pollen germinates, and the pollen tube grows through the stigma and style to the ovary, entering the ovule usually through the micropyle and then a synergid.
Artificial hybridisation in plant breeding uses emasculation (removing anthers from a bisexual flower before they shed pollen) and bagging (covering the flower with a bag to keep out unwanted pollen).
Double fertilisation
The pollen tube releases two male gametes into a synergid.
Syngamy: one male gamete fuses with the egg, forming the diploid zygote.
Triple fusion: the other male gamete fuses with the two polar nuclei, forming the triploid primary endosperm nucleus (PEN).
Because two fusions occur in one embryo sac, this is double fertilisation, a feature unique to flowering plants.
After fertilisation
Endosperm: the PEN divides to form the endosperm, which nourishes the embryo. Endosperm development comes before embryo development. In coconut, the "water" is free-nuclear endosperm and the white kernel is cellular endosperm.
Embryo: develops from the zygote at the micropylar end. A dicot embryo has two cotyledons; a monocot has one, called the scutellum.
Seed: the ovule becomes the seed; the integuments harden into the seed coat. Seeds in which the endosperm is used up are non-albuminous (pea, groundnut); those that keep it are albuminous (wheat, maize, castor). Mature seeds lose water and become dormant.
Fruit: the ovary becomes the fruit. A true fruit develops only from the ovary; in a false fruit, other parts such as the thalamus also contribute (apple, strawberry, cashew). Parthenocarpic fruits develop without fertilisation and are seedless (banana).
Apomixis and polyembryony
Apomixis is the formation of seeds without fertilisation, a form of asexual reproduction that mimics sexual reproduction. It occurs in some grasses and Asteraceae. Hybrid seeds made apomictic would let farmers reuse seeds year after year without the hybrid characters segregating.
Polyembryony, more than one embryo in a seed, occurs in citrus and mango.
Summary
Pollen develops from pollen mother cells by meiosis; its exine of sporopollenin is extremely resistant.
The embryo sac is 7-celled and 8-nucleate: egg apparatus, three antipodals, central cell.
Pollination types: autogamy, geitonogamy, xenogamy; agents: wind, water, animals.
Double fertilisation: syngamy (2n zygote) and triple fusion (3n endosperm).
The ovule becomes the seed and the ovary becomes the fruit.