this Acacia grows in southern cities of Algeria, it is a thorny tree, evergreen, more than 4m high, Leaves are alternate, bipinately compound, a pair of white thorns, about 10cm long or more. identication of the acacia species is difficult in the absence of specialized flora. thanks for your aide
It could be due to different environmental conditions, or it could be a different species altogether.
There were more than 1,300 species of Acacia before the genus Acacia was divided into 5 separate genera, one of which is Vachellia.
This article does a much better job of explaining it than I can, and also lists the other new genera if you want to do more research: http://en.wikipedia.org/wiki/Acacia.
Also, on large mature trees, the parts of the crown of the tree that are around the edges are thornier than the parts in the centre of the crown. It is a significant biological burden on the health of the tree to make so many thorns, rather than flowers for propagation and leaves for food production. Evolutionary processes have favoured the survival of the plants that only make thorns where they need them most, rather than all over the plant.
Elephants and Giraffe are the major browsers on African "Acacia" species - the tree only "needs" thorns until its leaves are above the head height of Giraffes and its main trunk and branches become too thick for the Elephants to break them down. Relatively few plants survive to reach this size - hence the classic African savannah with grassy plains and scattered trees. (Please note that I am simplifying madly here - many other things influence the density of trees as well, but I just wanted to mention the stuff about the thorns and why they grw where they grow.)