Jesus had already shown that the seed is the word of God in The Parable of the Sower, and that by understanding that parable, His disciples would be able to understand other parables. Notice this parable, in which He does not give an interpretation, but we can figure out its meaning, because of what He said about The Parable of the Sower,

And He said, “The kingdom of God is as if a man should scatter seed on the ground, and should sleep by night and rise by day, and the seed should sprout and grow, he himself does not know how. For the earth yields crops by itself: first the blade, then the head, after that the full grain in the head. But when the grain ripens, immediately he puts in the sickle, because the harvest has come” (Mark 4.26–29).

Even if we do not know how natural seed works, we still use it and gain a harvest. So it is with the seed of the kingdom. The seed is the word of God. We do not know how it works on the hearts of men and women, but it does, converting them from their old ways into the ways of Christ, and those people continue to grow and produce a harvest.