Primus pilus
Magister Australis
The plant was always rumoured to be one of the buckthorns, the Rhamnaceae, and so was named Ziziphus spina-christi.If a 'crown of thorns' anything like the ones usually portrayed actually was used,
it would have caused profuse bleeding, and probably rapid death.
There are lots of blood vessels around the scalp and temples,
and any serious bleeding in that region is likely to be life-threatening.
We had a good discussion about this at
http://www.cruxforums.com/xf/threads/the-holy-crown.3894/#post-130350
I think the suggestion that the crown used to mock Jesus
may have been of acanthus leaves is a good one.
They would have been painful, but not dangerous like acacia spines.
The fruit is edible and it is quite common through the Middle East. The invasive variety we have in Australia has thorns though they are often quite short.