People tend to call any sharp appendages on a stem thorns. In botanical language, however, there are three different classifications of these sharp points.  Thorns are modified branches and spines are modified leaf structures. Thorns and spines are often found at the nodes on a stem where you would find leaves and branches emerging.

Prickles are what roses have, according to the American Rose Society -

• Prickle – A spine-like superficial outgrowth of the stem. Roses have prickles, not ‘thorns.’

They have an irregular arrangement between the nodes on a stem. Unlike thorns and spines, prickles contain no vascular material and so are easier to remove. There is some scientific disagreement about the origination and development of prickles, though a common idea is that they form from the plant's epidermis. Another possibility is that they form from glandular trichomes, specialized hairs arising from the epidermis. Recent study suggests that they may develop from the rose's ground meristem, some of the basic growing tissue of the plant found beneath the protoderm that forms the plant's epidermis.

These woody outgrowths are usually sickle-shaped, enabling rose plants to hang onto other vegetation when growing over it and making it difficult for herbivores to crawl up the stem to the leaves. The densely packed prickles on Rosa rugosa and Rosa pimpinellifolia, sandy soil roses, are straight.

Examples of plants with spines (modified leaf structures) include holly and cactus. Honey locust trees and gooseberries have thorns.

In nature, the purpose of thorns, spines and prickles is often to protect plants from would be predators.

For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service