In Mark 4:13 Jesus rebukes his disciples for their failure to understand the parable of the sower: "Don't you understand this parable?"
And then he follows that with a second question: "How then will you understand any parable?"
Is the implication of the second question basically, "This is one of the easiest parables to understand. If you don't get this one, you're not going to understand any of the other ones either."
Or is the implication that understanding this parable is somehow important as a key for unlocking the meaning of Jesus' other parables?