Reproduction in Flowering Plants
Structures and functions of the principal parts of an insect-pollinated dicotyledenous flower and a grass
Flowering plants can be divided into two groups: the monocotyledones (monocots) and the dicotyledones (dicots)
The names are derived from the first structures that appear following germination of the seed. For the monocots this is a single long leaf while the dicots produce two leaves (usually quite different from the mature leaves produced later)
Mature monocots typically have long, thin leaves with parallel veins. The grasses are an example
Mature dicots show a range of leaf forms but with the veins in a network
The flowers of the two groups show common structures but with different organizations
- flowers bear the reproductive structures
- typically male and female structures are on the same flower
- androecium - the male structure - contains the stamens
- stamen - made up of:
- filament - a stalk which bears the anthers
- anther - a lobed structure which contains four pollen sacs
- pollen sac - contains microspore mother cells which after a series of cell divisions will produce the male gametes.
- gynaecium - the female structure - contains the carpels
- carpel - made up of:
- stigma - the structure on which the pollen grain lands
- style - a tube down which the male gamete moves
- ovary - the structure at the base of the style in which the ovules develop
Surrounding the reproductive structures are accessory structures such as petals, sepals etc. The nature of these varies according to the pollination strategy employed by the plant. For example large, colourful petals are typical of insect-pollinated plants while in wind-pollinated plants there are no petals leaving the anthers fully exposed to the wind.
Insect-pollinated dicot structures
| 1 - filament |  |
| 2 - anther | |
| 3 - stigma | |
| 4 - style | |
| 5 - petal | |
| 6 - ovary | |
| 7 - sepal | |
| 8 - flower stalk | |
| 9 - stamen | |
| 10 - carpel | |
| 11 - perianth |
Image from http://www.usna.usda.gov/Gardens/glossary.html
Wind-pollinated monocot structures
Image from www.bbc.co.uk/.../world_of_plants/ growing_plants_rev6.shtml
Development of pollen
Inside an anther are four pollen sacs. They can be seen in the diagram below of a young anther
Image from www.nsci.plu.edu/~jmain/b359web/ pages/reproduc.htm
Image from http://zygote.swarthmore.edu/phyto1.html
The central region of each pollen sac is made up of microspore mother cells.
Surrounding the mother cells is the tapetum which provides nutrition to the mother cells
Each microspore mother cell undergoes meiosis to produce a tetrad of haploid cells
Image from botit.botany.wisc.edu/.../ Microspore_tetrad_MC.html
Each haploid cell undergoes mitosis to become a pollen grain (microspore)
The two cells produced by this mitotic division are:
- generative nucleus
- pollen tube nucleus
Image from http://zygote.swarthmore.edu/phyto1.html
The pollen tube nucleus will produce the pollen tube
The generative nucleus is the gametophyte - it will undergo a mitotic nucleus to produce two male gametes (exactly when this occurs depends on the species)
Surrounding the nuclei are two walls; the inner intine and the outer exine
When the pollen grains mature the anther will burst releasing the pollen grains
Dehisced (pollen grains released) anther
_anther.jpg)
Image from www.nsci.plu.edu/~jmain/b359web/ pages/reproduc.htm
Development of ovule
The ovary is contained within the carpel
From a structure known as the placenta an outgrowth called the nucellus develops
Inside the nucellus is a megaspore mother cell
The megaspore mother cell undergoes meiosis to produce four haploid megaspores
One of these four cells undergoes three mitotic divisions to produce an eight-celled embryo sac. The embryo sac is the female gametophyte
Two layers of cells, called the integuments, grow around the embryo sac. They do not completely enclose the embryo sac - a small gap (the micropyle) is left.
The eight cells arrange themselves with:
- three cells at the micorpylar end - these are the egg apparatus. The middle cell is the female gamete (the egg cell) while the other two are the synergids
- two cells in the middle - the polar nuclei
- three cells at the other end - the antipodal cells (these have no known function)
Image from http://zygote.swarthmore.edu/phyto1.html