The term "seed" also has a general meaning that antedates the above -- anything that can be sown, e.g. "seed" potatoes, "seeds" of corn or sunflower "seeds". In the case of sunflower and corn "seeds", what is sown is the seed enclosed in a shell or husk, whereas the potato is a tuber.

Based on this passage, what is the outer shell of a seed called? Let's give stream of consciousness first:
The relevant information to answer the above question is: In the case of sunflower and corn "seeds", what is sown is the seed enclosed in a shell or husk, whereas the potato is a tuber.
So, the final answer is a shell or husk.