Thorns arise from shoots in plants such as bougainvillea, hawthorn and citrus. Irish, who is also professor of ecology and evolutionary biology, was fascinated by thorns that grow on old honey locust

trees, commonly found along the streets of New Haven and other urban areas.

Did plants evolve to have thorns?

"We think that spines evolved against mammalian herbivores," said Kariyat. ... Several plant species that sprouted on land during the Devonian period more than 400 million years ago had spines in spite of there being no large herbivores around to worry about.

How did plants get thorns?

Although it has been shown that in some plant families, such as cacti, spines arose primarily as a mechanism to reduce water loss from leaves, most plants that have spines use them to protect themselves from hungry animals. ... It turns out that spines are derived from leaf tissue and thorns from stem tissue.