Trees have developed various adaptations, with sharp, protective structures being among the most noticeable. These growths deter threats, demonstrating their evolution for self-protection.
Common Trees with Thorns
The Hawthorn (Crataegus), a widespread small tree or shrub, has long, sharp thorns, 1 to 3 inches, on its twigs and branches. These thorns, sometimes branched, are useful for forming impenetrable hedges.
The Honey Locust (Gleditsia triacanthos) has often branched thorns that grow directly from the trunk and limbs, reaching 12 inches. Reddish-brown and rigid, they emerge in clusters and harden with age. Many cultivated Honey Locust trees are thornless, unlike wild varieties.
The Black Locust (Robinia pseudoacacia) has sharp, paired thorns at the base of its leaves on young branches and trunks. These thorns are 1 to 2 inches long. The tree’s rapid growth and durable wood are notable, alongside its defensive structures.
The Osage Orange (Maclura pomifera) is identified by its dense, thorny branches bearing stout, straight thorns 1 inch long. Historically, its thorns made it a popular choice for planting as living fences to contain livestock. This tree also produces large, green, brain-like fruit, earning it the nickname “hedge apple.”
Many citrus trees, including lemon, lime, and orange varieties, have small but sharp thorns on their branches. These thorns are more prevalent on younger trees or on the rootstock of grafted plants. While some modern cultivars are thornless, thorns are common in various citrus species.
Why Trees Develop Thorns
The primary function of thorns in trees is a physical defense against herbivores. These sharp structures deter animals, such as deer, cattle, and insects, from consuming the tree’s leaves, bark, or fruit. This increases their chances of survival and reproduction.
This adaptation results from evolutionary pressures. Trees with more effective defenses against browsing animals were more likely to thrive and pass on their genetic traits. In some environments, thorns also play a role in water conservation by reducing the surface area where moisture can be lost, particularly in arid climates.
Thorns minimize damage from predation, allowing the tree to allocate more energy to growth and reproduction rather than constant repair. Thorns also help protect vulnerable young trees, which are more susceptible to herbivore damage. While not all trees have thorns, they represent a successful survival strategy in many ecosystems.
Distinguishing Thorns, Spines, and Prickles
While often used interchangeably in everyday language, botanists distinguish between thorns, spines, and prickles based on their origin and structure. True thorns are modified stems or branches that contain vascular tissue, which transports water and nutrients. They typically arise from an axillary bud, the point where a leaf or branch normally grows from the stem. Hawthorn and Honey Locust trees are classic examples of plants with true thorns.
Spines, in contrast, are modified leaves, stipules, or parts of leaves. Like thorns, they also contain vascular tissue. Cacti are known for their spines, modified leaves that reduce water loss and protect against herbivores. Some holly species also have spines along their leaf margins.
Prickles differ from both thorns and spines as they are outgrowths of the plant’s epidermis (outer skin layer) and do not contain vascular tissue. This makes them easier to remove compared to thorns or spines. Roses, often mistakenly said to have thorns, actually bear prickles on their stems.