When people think of a flower with sharp defenses, the rose often comes to mind. This common association leads many to use the word “thorn” generically for any sharp outgrowth on a plant. Botanists, however, distinguish between three distinct types of sharp plant armaments: thorns, spines, and prickles, based on their tissue origin. This article will explore the anatomy of these defenses, provide examples of flowering plants that possess them, examine their evolutionary significance, and offer practical advice for safe handling.
Defining the Difference Between Thorns, Spines, and Prickles
These sharp structures are classified based on the specific plant tissue from which they develop. True thorns are modified branches or stems, originating from the plant’s shoot system, often arising from a bud in the axil of a leaf. Because they are extensions of the stem, thorns possess a deep connection to the plant’s vascular system, containing xylem and phloem. This connection makes them woody, rigid, and difficult to remove without tearing the tissue. Thorns may be simple or branched, sometimes even bearing leaves, further indicating their origin as a modified shoot.
Spines, in contrast, are modified leaves, stipules, or parts of leaves, such as extensions of leaf veins. They also contain vascular tissue, connecting them structurally to the plant’s core. Cacti are the most recognizable example, where the entire leaf structure has been converted into a spine. Other flowering plants, like certain species of Acacia, produce spines from modified stipules, which are small appendages at the base of the leaf stalk.
Prickles are different from both thorns and spines because they are outgrowths of the epidermis, the plant’s outer layer, and the underlying cortex. They are superficial because they do not contain vascular tissue, meaning they are not connected to the plant’s internal transport system. This lack of a deep connection allows prickles to be snapped off relatively easily without causing major damage to the stem. The sharp protrusions on roses and brambles like blackberries and raspberries are anatomically classified as prickles, not true thorns.
Common Flowering Plants with Protective Structures
Plants that possess true thorns include species in the genus Crataegus (hawthorn) and members of the citrus family, such as lemon and orange trees. The thorns on these plants are positioned where a branch would normally grow. The honey locust (Gleditsia triacanthos) is also known for producing exceptionally large, often branched, true thorns that can measure several inches long.
Plants armed with spines include the diverse family Cactaceae, where the structures protect the succulent stem. The barberry (Berberis) features spines, often appearing in pairs. Some species of holly (Ilex) also have leaves with sharp, spiny margins, which are technically considered spines.
The most common examples of plants with prickles are roses (Rosa species) and brambles in the genus Rubus, which includes raspberries and blackberries. Prickles can be coarse and numerous, often covering the entire stem in a seemingly random pattern. The Devil’s walkingstick (Aralia spinosa) is another plant known for its formidable prickles.
The Evolutionary Purpose of Plant Defenses
The primary function of thorns, spines, and prickles is physical defense against herbivores. These pointed structures deter larger browsing animals, such as deer, from consuming the plant’s foliage and vulnerable growing points. By making the plant unpalatable or difficult to access, the plant conserves the energy it would otherwise spend replacing damaged tissue.
Plant defenses are not exclusively aimed at large animals; they can also impede smaller threats. Some research suggests these sharp structures are effective deterrents against insect herbivores, which struggle to navigate the spiky surfaces. In arid environments, spines on plants like cacti have evolved additional roles. They can help shade the plant’s surface, protecting it from extreme temperatures, and reduce water loss by minimizing the surface area exposed to the sun.
These structures can also serve mechanical functions, such as aiding in climbing for vining plants. The recurved prickles found on climbing roses and greenbriers allow the plant to hook onto supporting structures, providing physical support. The evolution of these defenses represents an example of convergent evolution, where different plant families independently developed similar sharp structures to solve the common problem of survival.
Safe Handling and Pruning
Working with plants that possess sharp structures requires adopting specific safety measures to prevent injury. Gardeners should always wear appropriate protective gear, including thick leather gloves with long gauntlets to shield the forearms. Long sleeves and durable pants are necessary to protect the skin from accidental contact with stems and branches.
When pruning, the use of long-reach pruners can help keep hands and body away from dense, prickly growth, reducing the risk of being poked. Cuts should be made cleanly and precisely, often back to a healthy bud or a branch joint, to encourage proper healing and growth. For plants like roses, a tool called a thorn stripper can be used to remove prickles from cut stems, making them safer to handle and arrange.
Disposal of the trimmed material must also be handled with care to avoid creating a hazard elsewhere. Thorny or prickly cuttings should never be tossed loosely into paths or compost piles where they can injure people or pets. The best practice is to place cuttings into a durable container or bundle them tightly before disposal, ensuring the sharp points are contained.