Trees that develop sharp, rigid projections are found across diverse global environments. These structures, commonly called spikes or thorns, are not uniform; they can emerge from the trunk, the branches, or modified leaf structures. They serve a clear purpose, primarily acting as a mechanical defense against animals that would otherwise damage the plant. Understanding where these projections grow and their specific form helps to categorize and identify these protected trees. The location of the spikes often relates to the type of threat the tree faces and the environment in which it grows.
Trees Known for Spiked Trunks
Trees that cover their massive trunks in pointed outgrowths are often found in tropical or subtropical regions. These trees frequently have large, bottle-shaped trunks adapted to store water. The Silk Floss Tree (Ceiba speciosa), native to South America, is a prime example, displaying large, hard, conical bumps across its bark. These projections are technically prickles, as they arise from the outer layers of the trunk rather than being modified branches.
The prickles on the trunk serve as a deterrent to large climbing animals that might try to reach the leaves or soft bark higher up the tree. Younger Silk Floss trees often have a smooth, green bark that contains chlorophyll, allowing the trunk to perform photosynthesis, especially when the tree drops its leaves during dry periods. The dense covering of prickles helps protect this photosynthesizing bark, which is often thinner and thus more vulnerable to damage. The related Kapok Tree (Ceiba pentandra) also exhibits similar defensive projections on its trunk, especially when the tree is young.
Trees with Spiked Branches and Twigs
This category features sharp structures primarily along the branches and smaller woody stems. This adaptation is most effective against browsing herbivores, such as deer or cattle, that feed on young shoots and foliage. The Honey Locust (Gleditsia triacanthos) is known for its multi-branched thorns that emerge directly from the twigs and branches. These structures can grow several inches long, often forming dense clusters that make the branches nearly impenetrable.
The Black Locust (Robinia pseudoacacia) features smaller, paired sharp projections at the nodes of its young branches. These projections are actually modified stipules, which are small appendages at the base of the leaf stalk. The Hawthorn species (Crataegus) are classic examples of trees with true thorns, which are modified branches that terminate in a sharp point. Many citrus trees, including lemons and limes, also feature small, pointed thorns along their stems, which likely evolved to protect the valuable, nutrient-rich leaves and fruit from being eaten.
Why Do Trees Develop Spikes?
The development of sharp structures is a widespread form of mechanical defense, primarily aimed at deterring herbivory, or the eating of plant material by animals. Botanists distinguish between three main types based on their cellular origin: thorns, spines, and prickles.
- Thorns are modified stems or branches, meaning they contain vascular tissue and grow from the woody structure of the plant, making them difficult to remove.
- Spines are modified leaves, parts of leaves, or leaf attachments, such as the stipules found on the Black Locust.
- Prickles are outgrowths of the outer epidermal layer of the plant and do not contain the internal vascular bundles found in true thorns or spines.
In addition to defense, these pointed projections can play a role in water conservation. This is seen in desert plants where modified leaves reduce the surface area available for moisture loss.