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There are over 1,300 species of Acacia
Acacia are any of the species of trees and shrubs that belong to the taxonomic genus Acacia. There are over 1,300 species with over 950 found in Australia, and others found in the Americas, Africa, and Asia. They are called Acacia's because of the Greek word for thorn, Acanth-, because a lot of Acacia's have spiny, sharp parts.  Acacia is the largest genus of vascular plants in Australia , and Australia's national floral emblem is the Golden Wattle. Australia also celebrates Wattle Day every September, September 1st.
The Acacia was discovered in 1773 by Carolus Linnaeus. They sometimes form symbiotic relationships with ants, wherein they protect them within their spines and hollowed out areas, and the ants feed off of the sap. The ants in turn trim back competing plants.
All acacias have leaves, which are usually compound pinnate. In some acacias, the leaf stalk is modified and called a phyllode, or a modified petiole. However, in the instance that an Acacia does not have phyllodes, it will have small, flat extensions from the stems called cladodes. A cladode is a modified stem structure which is used for photosynthesis. All acacias are dicots, meaning the seed typically contains two embryonic leaves or cotyledons. The leaves of an Acacia tree are compound pinnate. Acacia have small flowers with five petals, six if you include the stamen. 
All acacia are monoecious, meaning they asexually reproduce. They either have globular flowers, or they are cylindrical spikes. The problem with Acacias is, the seeds are so durable, that they need to be worn down before they can even receive water or begin to grow. 
There are over 1,300 species of Acacia, and around 900-1,000 are found in Australia. They are found almost everywhere, because they are so durable. They can grow in almost any climate, but preferably warmer climates, after being in the cold for too long they will die. Also, most need full sunlight during the day. The reason most Acacia are found in the desert is because of it's highly durable seeds. It's seeds are so durable, that they need to be scarified, or worn down, before they can even begin to grow, or receive water, so the windy, harsh desert climate allows for those seeds to be worn down and be softened, then once summer is over, and the seeds have been blown all over from the harsh desert winds, and softened by the squalls of rain and heat, they will grow into Acacia. The farthest south Acacia are probably found on the tip of South Africa, near Argentina. Acacia albida, Acacia tortilis and Acacia iraqensis can be found growing wild in the Sinai desert and the Jordan valley, in Israel. 
Acacia are used for many things in modern culture, as well as have been used for past cultures. For instance, in past cultures, indigenous peoples of Australia would use the bark and wood from the Acacia tree to construct boomerangs, shields, etc., as well as for primitive medical purposes. Acacia's are used in modern culture for medicinal purposes for aiding with premature ejaculation, weight loss, and used as an astringent . In modern culture, we use Acacia the most for gum arabic. Gum arabic is not only edible, but also has huge potential. Gum arabic derived from Acacia is used from everything from soft drinks and pops, to hard candies, to marshmallows, to stamps, to cigarette papers, to watercolor paintings, to shoe polish, to pyrotechnic compositions!  Other modern uses of Acacia are: tannin, fine furniture, ornamental uses, like botanical gardens, and perfume. 
- Wikipedia Genus: Acacia
- USDA Genus: Acacia
- Australian National Botanical Garden Genus: Acacia
- Desert Tropicals Genus: Acacia
- Australian Government Botanical Pages Acacia
- Australian Government Botanical Pages #2 Acacia, #2
- Acacia Trees