Euphorbias
Many plants of the spurge family have reduced fleshy leaves, in particular the vast Euphorbia genus of approximately 1,600 subtropical and warm-temperate species. These cactuslike plants, comprising most of the species commonly called spurge, have spiny, jointed stems and are among the most common Old World desert succulents. The euphorbias and the cacti illustrate the biological phenomenon of convergent evolution, in which unrelated groups of organisms, subject to the same environmental pressures, gradually develop similar structures. The euphorbias exhibit another family trait: “naked flowers” (i.e., flowers lacking petals and sometimes sepals) that are enclosed in a bract envelope, from which they emerge during the flowering period to permit pollination.
Many species are cultivated for their brilliant, showy bracts as well as for their frequently colorful foliage. These include snow-on-the-mountain (E. marginata), native to the United States; the cypress spurge (E. cyparissias), a favored cemetery plant that was introduced from Europe and naturalized; the scarlet-bracted greenhouse plant crown-of-thorns (E. splendens), native to Madagascar; and the poinsettia (for J. R. Poinsett), an ornamental shrub native to Central America. The poinsettia (E. pulcherrima), whose several species are sometimes considered a separate genus (Poinsettia), is a popular Christmas decoration with its large rosettes of usually bright-red bracts.
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