Jesus explains the farmer and the darnel weeds. What do the mustard seed, the yeast, the treasure and the pearl mean?

Matthew gives no explanation of them, so here is only what the chapter says. Jesus explains the farmer's parable ("Hear, then, the parable of the farmer", 13:18), and, when his disciples ask him in the house, the darnel weeds (13:36-43). Of the dragnet he says, "So it will be in the end of the world. The angels will come and separate the wicked from among the righteous" (13:49). The others he gives in their own words: "The Kingdom of Heaven is like a grain of mustard seed" (13:31), "The Kingdom of Heaven is like yeast" (13:33), "the Kingdom of Heaven is like treasure hidden in the field" (13:44), and "the Kingdom of Heaven is like a man who is a merchant seeking fine pearls" (13:45). Matthew says, "Jesus spoke all these things in parables to the multitudes; and without a parable, he didn’t speak to them" (13:34). At the end Jesus asks, "Have you understood all these things?" and they answer, "Yes, Lord." (13:51). Matthew doesn't say what they understood. Favor's Mark 4 leaves the mustard seed in Mark's words, and these notes don't explain any of the four.