Thorns

Every rose has its thorns, right? Well, no. They don’t.

Roses do not have thorns. Roses have prickles. Citrus trees have thorns.

Lemon tree thorns

Thorns, prickles, and other spiky bits

Thorns are a type of spinose structure made out of a modified leaf, stem, root, or bud. Many people use the terms bristles, prickles, spines, and thorns interchangeably. Botanically, these terms mean very different things:

• bristles - stiff plant hairs, or trichomes
• prickles - epidermis tissue anywhere on the plant, without vascular bundles (roses)
• spines - any part of leaf tissue that contains vascular bundles, such as the stipule or petiole
• thorns - modified stems, which may or may not be branched, and may or may not have leaves, and may or may not grown from a bud (citrus)

So, where bristles are stiff hairs and prickles are hard, spiked skin (neither of which contain plant veins), spines, being modified leaves, and thorns, modified stems, do contain plant veins.

Thorny problems

Plants use thorns as a mechanical defense against herbivores (and gardeners). Cacti are far less likely to be eaten when they are covered with hard thorns. And the pollinators who specialize in pollinating these particular types of plants seem to be unaffected by the presence of thorns. In some cases, thorns are also used to shade certain plant varieties, or to provide a layer of insulation.

Home, sweet thorn

Some thorns are hollow. These tiny chambers are called domatia. Plants, such as certain acacia species, produce domatium to provide shelter for beneficial arthropods (insects, spiders, and crustaceans). Similar to galls, which are produced by the resident, rather than the landlord, domatium are the plant’s side of a mutually beneficial relationship, most commonly with ants or mites. Occasionally, thrips may also move into these tiny apartments, but they are generally unhelpful to the plant. The plants that create these thorny thresholds are called myrmecophytes.

While I do not expect any of you to stop calling rose prickles thorns, why not impress your friends with your new-found knowledge?