Milkweed is named for the milky sap that oozes from its leaves and stems when they’re broken. It contains bitter-tasting alkaloids and toxins called cardenolides, which affect heart function in some animals. (It can also cause skin irritation in some humans.) This sap is meant to protect the plant from being gobbled up by herbivores. Most grazers, however, simply avoid the plant because of its bitter taste.