Usually a green, sour fruit is just a fruit that’s not ripe yet. If you left a lime on the tree longer would it eventually change color and become sweet, like an orange? Actually, limes do become much more yellow and less sour as they ripen. Under ripe limes are darker green and quite bitter. There are other clues to their ripeness too. Ripe limes are heavy with juice and more aromatic, with a fragrant “limey” smell. Most fruits we eat use similar cues to advertise ripeness. Why would plants want parts of them to be eaten? Fruiting plants have evolved a partnership with certain animals in order to increase the success of their seeds. Tasty fruit pulp is the reward plants offer to animals in exchange for dispersing their seeds.