There is only one tree species with poisonous thorns, the black locust, that is native to North America.Though there are many toxic plants and many more plants with thorns, very few plants combine the two features.
Most commonly, plant toxins are a deterrent to herbivores that might consume foliage but are not toxic simply to the touch or even with skin penetration. When a plant does develop toxic thorns, it is because the plant itself is highly toxic overall.
The black locust tree (Robinia pseudoacacia) is a deciduous tree that can grow to eighty feet in height. It is only mildly toxic unless eaten. A scratch from a black locust may become red, irritated and slow to heal. Consuming black locust in large quantities causes severe illness, though the flowers are edible and the honey produced from them is considered excellent.
In other parts of the world, there are a few more plants that have poisonous thorns. A Madagascar native and common houseplant known as the crown of thorns (Euphorbia milii) secretes a latex sap when damaged. If forced onto or underneath the skin, it can cause a poison-oak-type reaction. Another plant with a poisonous effect is the sensitive plant (Mimosa pudica), a common weed in tropical and subtropical areas. The effects are mild and include hair loss and depressed growth in mammals.