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THE SECRET LIFE OF FLOWERS : Animation description

Three animations of increasing complexity are available on a VHS video tape or DVD covering the reproductive life cycle of a typical flower: 
1. Simple. Short, no terminology and easy to understand. (Running time, 17m: 11s) 
2. Intermediate. Full length, with summaries and simple terminology. (Running time, 21m: 20s) 
3. Advanced. Full length with scientific terminology. (Running time, 18m: 50s)

All plants have what is known as an 'alternation of generations'. The familiar flowering plant (sporophyte) produces spores which develop into a very small gamete-producing tissue (gametophyte). Fertilisation results in the formation of another flowering plant sporophyte, completing the alternation of generations.

The spore-producing stamen and the male cells (male gametophyte)

The stamen consists of a stalk (filament) and a terminal sac (anther). Inside the anther there are four spore sacs (microsporangia). A special jacket of cells (tapetum), surrounds each microsporangium and nourishes it. The microsporangium contains diploid fertile cells called sporocytes. Each sporocyte divides by reduction division (meiosis) to produce a group of four haploid spores (microspores). 
These acquire a tough protective layer of special wall material called sporopollenin. One or more portions of the spore wall lack this deposit and remain thin walled. This is where the future pollen tube will emerge. At the same time that sporopollenin is being deposited, the spore cell within divides. The smaller of the resulting cells, the generative cell, is surrounded by the larger cell, the tube cell. These two cells represent the male tissue (male gametophyte). The entire structure consisting of the thickened spore wall plus the male tissue is called the pollen grain. Aided by a special layer of cells, the endothecium, the anther now opens, exposing the pollen grains.

The spore-producing ovary and the female tissue (female gametophyte)

Flowers contain one or more carpels. In many monocots, there are three fused carpels. Each carpel consists of a basal ovary, a slender stalk (style) and an enlarged distal region, the stigma. 
In the young ovary, spore sacs called megasporangia form. Each contains a single diploid, fertile spore-producing cell (sporocyte). A jacket (integument), grows over each sporangium leaving a small pore or micropyle at one end. 
A sporocyte undergoes meiosis to produce four haploid nuclei. In most flowering plants walls are then formed around each nucleus, forming four spore cells (megaspores). However, three of these degenerate and the surviving megaspore enlarges, while its nucleus divides three times to form eight daughter nuclei. Walls now form around the nuclei, but one cell, the central cell, contains two nuclei that are called the polar nuclei. Of the remaining six cells, three are at the micropylar end of the ovule. One of these becomes the egg cell and the two adjacent cells (synergids) are believed to play a role in attracting the pollen tube. The other three cells are at the opposite end of the ovule and are called the antipodal cells. This seven-celled tissue is the female gametophyte, also called the embryo sac because the embryo develops inside this structure.

Pollination and Fertilisation

Depending upon the type of flower, pollen is transferred to the stigma by wind, water, or animals (insects, birds etc.). This is known as pollination. The pollen grain absorbs fluid from the stigma, and a pollen tube containing the tube cell emerges . The generative cell soon divides to form two sperm cells which do not contain flagella. Growth of the tube carries the sperm down into the style toward the ovary. Inside the pollen tube, cytoplasmic streaming takes place, and wall growth is concentrated at the very tip of the tube. The sperm are carried down inside this tube as plugs of wall material (callose) seal off older portions. Eventually, the pollen tube reaches an ovule, enters the micropyle and penetrates one of the two synergid cells adjacent to the egg. The synergids then degenerate. One sperm moves into the egg cell, fertilising it to form a diploid zygote. The other sperm moves into the central cell, joining with its two nuclei to form a cell with a triploid nucleus (primary endosperm cell). The fertilisation of two cells is unique to the flowering plants and is known as double fertilisation.

Seed and fruit formation

The nucleus of the fertilised central cell (primary endosperm cell), begins to divide and form a large number of nucei filling much of the embryo sac. This is the young endosperm tissue. The zygote forms an embryo located at the tip of a filament of cells called the suspensor. Elongation of the suspensor pushes the embryo down inside the ovule (embryo sac). In the monocots, a single seed leaf (cotyledon) forms, with a small shoot apex located to one side. In most monocots, the surrounding endosperm transfers nutrients to the growing embryo, but after embryo growth ceases nutrients accumulate in the endosperm for the future requirements of germination. However, in most dicotyledons, the surrounding endosperm becomes absorbed by the growing embryo and nutrients are stored in the two enlarged cotyledons. In both types of ovules, the jacket (integument) becomes a hard, tough protective seed coat. The ovule, containing an embryo is now called a seed. 
At the same time that the ovule is changing into a seed, the developing embryo is releasing hormones which stimulate the surrounding ovary to expand and form the fruit.

For further information

The following books include a discussion of the flower life cycle:

  • Gifford, E.M. and Foster, A. S., Morphology and Evolution of Vascular Plants, 3rd ed., Freeman Publishers, 1989
  • Moore, R., Clark, W. D., and Vodopich, D. S., Botany, 2nd ed., WCB/McGraw-Hill Publishers, 1998.
  • Raven, P., Evert, R. and Eichhorn, S., Biology of Plants, 6th ed., Freeman-Worth Publishers, 1999.
  • Roost, T.L., Barbour, M.G., Stocking, C. R., and Murphy, T.M., Plant Biology, Wadsworth Publishing 1998