— Active | Passive

“Why, then, do reef corals grow into such plant-like shapes? Because, although the polyps are not themselves plants, they have plants within them. The cells in the inner layers of the polyp’s tissues contain tiny cells of an alga, complete with their grains of photosynthesising chlorophyll. And the polyp looks after its captives very well indeed, building its stony cup in a shape that ensures that they can get all the light they need."