The
land vegetation of planet Earth is divided into six
floral kingdoms. These are areas with
plant species found throughout it. Many of these species are found only in that
kingdom. i.e. each kingdom contains characteristic
endemic plant species. The edges of these zones will of course be fuzzy, but the division can be made.
The kingdoms are:
1 Boreal Kingdom: 42% of land area, covering Europe, on the North American west coast and Central Asia. Sometimes called the Holoarctic kingdom.
2 Paleotropical Kingdom: 35% of land area, in Central Africa
3 Neotropical Kingdom: 14% of earth's land area, in South America
4 Australian Kingdom: 8% of the earth's land area, on Australia
5 Holantarctic Kingdom: 1% of the earth's land area, on the tip of South America
6 Capensis: 0.04% of the earth's land area, at the south end of Africa, in the Western Cape province of South Africa. Also called the Cape Floral Kingdom, or Fynbos)
Floral kingdoms are places where plant species have evolved in different ways, isolated by geographic and climatic barriers. No doubt the kingdoms were different in prior ages of the Earth, when the continents where differently arranged.
As noted in Guns, Germs, and Steel, the combination of geographic barriers (i.e. the sea) to the east and west and climactic changes to the north and south is effective to box in a floral kingdom.
For instance, the vast boreal kingdom spreads east-west in the same climactic band across the joined land masses of the northern hemisphere, while the african and new world rainforests are distinct, isolated from each other by the atlantic.
The Australian kingdom is an extreme example of isolation by geography.
The smallest kingdoms on the southern tips of South America and Africa are kingdoms left stranded in an isolated, diminshing climatic zone as the landmass slowly moves north under it.
research done on the interweb