Several types of fruit trees grow thorns. Thorns on a plant protect it from the ravages of unfriendly, hungry fauna. Over the years, botanists have propagated fruit trees to discourage the growth of thorns. After all, fruit trees developed specifically for the harvest of their fruit have no need of thorns, since the orchardists who grow them have a vested interest in perpetuating, rather than ravaging, the tree.
The lemon tree has thorns. You may not think so, if you have a tree carefully propagated over generations to eliminate the unfriendly aspects and encourage larger, more flavorful fruit. However, even the most "cultivated" lemon tree may have a thorn or two, hidden among the greenery and sweet blossoms.
Among the orange trees with thorns is the Trifoliata Orange. Sure enough, this is an older variety which, according to ubcbonatincagarden.com, "is a good rootstock for 'dwarfing' other species."
Wild Apple Tree
If you happen upon an apple tree in the wilderness, unless it grows near the remains of an ancient homestead, you have probably discovered a wild apple tree. As such, the wild apple will have some survival gear which has allowed it to outlast the generations: thorns.
Also called a Hawthorn tree, the thorn apple produces many small fruits known as haws, which some prize as a jam ingredient. The Hawthorn, a showy spring bloomer, also boasts some spectacular thorns.
This small tree, sometimes considered a bush, offers its deliciously seedy fruit nestled among a protective array of thorns. The pomegranate originated in Persia, but has become increasingly popular in the western world as more people discover its health value as a source of vitamin C and antioxidants.
Wild plum trees–known as Prunus Americana–are sociable fruit trees that thrive in dense stands. Not as sociable with other species, wild plums have thorns to discourage visitors from plucking their fruits.
Wild Pear Tree
Wild pears, like wild apples, have thorns. The pear is one of the few fruit trees which, when planted and allowed to grow without any human intervention, survives quite handily on its own. With an arsenal of thorns to ward off invading harvesters, it's no wonder.
Not to be confused with a plague of locusts, the honey locust tree has a trunk and branches liberally spiked with sharp, flat thorns. This tree does produce edible fruit, although it is not as well known as citrus or apples. The fruit pulp of the honey locust is sweet like molasses and used both as a source of sugar and an ingredient in home brewing.