Jesus used, we are told, many parables when teaching the people. Now - it seems - he did not explain them at the time, but left the people to ponder on them. The apostles did get the meaning explained. They also seem on the whole to have accepted Jesus's teaching methods and those who wrote the Gospels did not write down those explanations!
Although agriculture was very different back then, these two parables about growing plants still seem to make sense today - the surface o the story seems to have an obvious meaning. Parables, however, always have layers of deeper meaning and it is well to linger with the story a while and see if we can get to the deeper meanings.
Seeds often grow secretly - and for a long time. Perhaps Jesus is telling us that the long wait for understanding, or acceptance oft he faith that many go through is important, and of value in itself. Our RCIA group has both young, fast moving souls, and souls that have been pondering this movement into the Catholic Church for decades. And the mustard seed - could be an individuals' impression of their own tiny and weak faith, or of the small numbers of believers who can by example and persistence, save an entire population!
Let parables mull on your mind, like a good wine lives on your palette, so that they grow in flavour, meaning and joy!