Which of these does not belong with the other three? Choose the odd one out: Ginger, Tomato, Potato, Onion.

Ginger, potato and onion are all lifted out of the soil: ginger is an underground stem, the potato a swollen tuber and the onion a bulb of packed leaves. The tomato hangs above the ground on the plant and is the fruit itself. All four are heaped side by side on the same vegetable stall, which is what hides the split; the group is held together by growing below the soil.