Aposematic (warning) Coloration Associated with Thorns in Higher Plants

Aposematic coloration, a well-known phenomenon in animals, has been given little attention in plants. Here I discuss two types of conspicuousness of thorns which are typical of many plant species: (1) colorful thorns, and (2) white spots, or white and colorful stripes, associated with thorns in leaves and stems. Both types of aposematic coloration predominate the spine system of taxa rich with spiny species—Cacti, the genera Agave, Aloe and Euphorbia. The phenomena have been recorded here in over a thousand species originating in several continents of both the Old and New World. I propose that this is a case of vegetal aposematic coloration analogous to such coloration of poisonous animals, and which communicates between plants and herbivores.