I wonder why people began imagining that space invaders were green. Perhaps it was because humans had already appropriated most of the other colors: black, white, khaki, red, yellow, and blue. The ancient Brits painted their naked bodies blue with woad, so they say, but that was before football fans started painting themselves in the colors of their teams. Now you can see green people rooting for Brazil, but they have a yellow stripe also.

Be that as it may, the color of the invaders I am talking about is due to chlorophyll; they are plants, not humanoids, and they do not come from outer space, but from one part of the globe to invade the space of another. Classic examples are the Nile cabbage (Pistia stratiotes), responsible for blocking navigation over long stretches of the White Nile in the southern Sudan, has turned up in the Amazon....

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