Many fruit trees possess sharp protective growths, commonly referred to as thorns. Botanically, a true thorn is a modified stem, a spine is a modified leaf part, and a prickle is an outgrowth of the plant’s outer layer. Although these terms have distinct technical meanings, they all serve the same purpose: to deter animals from eating the plant. For general purposes, “thorn” is used for any of these sharp structures, which are common features across different families of fruit-bearing trees.
Thorny Trees of the Citrus Family
The Citrus genus is the most common group of cultivated fruit trees that frequently exhibit thorns on their stems and branches. These protective structures develop at the nodes, often appearing on new growth and fruiting wood. The presence of thorns is particularly noticeable on young trees, which are more vulnerable to browsing animals.
Many true lemon varieties, such as the ‘Meyer’ lemon, typically have sharp thorns lining their twigs, though some popular hybrids like ‘Eureka’ have been bred to be nearly thornless. Thorns are also prevalent on limes, including the Key Lime and Kaffir Lime, and on some bitter orange varieties. In many cases, a citrus tree will produce fewer and blunter thorns as it matures, suggesting the need for defense decreases once the plant is established.
A common reason for unexpected thorniness on a grafted citrus tree is the growth of suckers from the rootstock. The rootstock is often selected for its hardiness and may be a naturally thorny variety, such as Poncirus trifoliata, also known as Trifoliate Orange. If these rootstock shoots are not removed, their thorny growth can quickly overtake the desirable, less-thorny fruiting variety grafted onto it.
Thorns on Pome and Stone Fruits
Thorns are also found in the Rosaceae family, which includes pome fruits like apples and pears, and stone fruits such as plums and cherries. However, the presence of thorns is less universal in this group than in citrus, largely due to selective breeding for commercial purposes. Most modern, cultivated varieties of apples and plums are thornless, but the original, wild ancestors commonly possessed them.
Certain wild pears, like Pyrus pyraster, are naturally thorny, and these sharp defenses allow them to survive without human intervention in the wild. Quince, a pome fruit related to apples and pears, can also exhibit thorns on its branches. Furthermore, thorny rootstock is sometimes used for grafting apple and plum varieties, meaning shoots arising below the graft union can produce sharp growths.
Some varieties of hawthorn, often called “thorn apple,” are notable for their prominent thorns and small, edible fruit. These examples demonstrate that while commercial selection has favored thornless fruit trees, the genetic potential for sharp defenses remains widespread in both pome and stone fruit relatives.
The Evolutionary Purpose of Thorns
The primary biological function of thorns and similar sharp structures is to provide a physical defense against herbivores. These growths deter large browsing mammals, such as deer and livestock, from consuming the tender leaves and shoot tips of the tree. By making the plant unpalatable, thorns help ensure the tree survives to maturity and reproduces.
This defense is particularly important during the juvenile phase of the tree’s life, when its growth is low to the ground and more vulnerable to grazing animals. Compared to producing thick bark or complex chemical deterrents, creating a sharp physical barrier is a relatively low-cost investment that yields a high protective return.