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Euphorbia abyssinica, E. confinalis, E. cooperi: Convergent evolution

Don’t you think this plant looks like a candelabra-shaped, branched cactus? Its thick trunk and side branches are obviously used to store water, just as in a cactus. It has no leaves, just thorns on small bumps on its ribs. So why not classify it as a cactus?

If we were to cut through the skin of this plant, a white, milky juice would appear. It looks like liquid latex, and is poisonous. Cacti don’t have such juice, but it’s characteristic of the genus Euphorbia, the wolf’s milk plant. This genus includes 1,500 species worldwide. Some African Euphorbia in particular have developed water-storage structures that remind us of a cactus. This phenomenon, where plants from completely different families look almost the same, is known as convergent evolution. This occurs when plants develop similar strategies to cope with similar environments, resulting in a similar external appearance.

So, while in America it’s cacti that store water in their trunks to survive in a dry climate, in Africa it’s the genus Euphorbia.

When Euphorbia blooms, it’s immediately obvious that this is no cactus. Cactus flowers are funnel-shaped and brightly-colored, while those of the wolf’s milk plant are small and yellowish. They consist of a small cup containing several male flowers and a single female one. The male flowers each bear only a single anther, while the female flower consists of a round, tripartite ovary. On the upper edge of the cup, you can see a small, glistening structure: These are the nectar glands.

Audio file download
Euphorbia abyssinica, E. confinalis, E. cooperi: Convergent evolution (MP3, 721 KB)

Audio production and copyright: Soundgarden Audioguidance GmbH
Text: Ehrentraud Bayer, Botanischer Garten München-Nymphenburg

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