How does an achene differ from a caryopsis or grain?

Both the achene and the caryopsis are types of dry, indehiscent, one-seeded fruits, but they differ significantly in how the seed relates to the fruit wall, or pericarp.

• Achene: In an achene, the single seed remains free within the fruit cavity, attached to the pericarp at only a single point. Because the seed is not fused to the fruit wall, it can be easily separated from it.12
• Caryopsis (Grain): A caryopsis is a specialized type of achene characteristic of cereal grasses. In this structure, the pericarp is completely united or fused with the seed coat. This fusion makes it difficult to separate the fruit wall from the seed without the use of special milling processes.34

While all caryopses are technically a type of achene, the term caryopsis is reserved for those instances where the seed and fruit wall are inseparable.3