Why do you sow a seed instead of the whole plant you expect to harvest?
You sow the seed rather than the whole plant because you sow the bare grain—the individual kernel—not the body that shall be. That which is sown is not quickened, or made alive, except it die, and then the stalk and grain come. Paul uses this as an illustration of resurrection.
Les explains that Paul is talking about the mundane things of agriculture—sowing wheat, rye, corn, or whatever else produces a crop. When you plant something, you do not plant the whole stalk of wheat; you plant the seed. The seed is not quickened unless it dies, and you do not expect the seed itself to suddenly come up above the ground. Instead, first the stalk comes, then the grain. Paul says you do not sow what you expect to see—the whole stalk—but the bare grain, the individual kernel, whether wheat or some other grain. He uses this total agricultural illustration to picture our own resurrection experience.
And that which thou sowest, thou sowest not that body that shall be, but bare grain, it may chance of wheat, or of some other grain: