Mar 4:31 [It is] like a grain of mustard seed, which, when it is sown in the earth, is less than all the seeds that be in the earth:

Christ is differentiating here between the size of a piece of knowledge and its potential for yielding fruit. In his estimation, the basic idea of the universal rule, that is, the rules of God's reign, are not large or complex. The term he uses for "grain" is actually a metaphor in Greek for "a grain of sense."

When you put this particular grain of sense into your relationships, which is Christ's symbol for term translated as "the earth" here, it doesn't matter that it seems so small, new, and perhaps naive. (The word used here for "small," also means "young.") This small, new idea has the potential within them to mature and grow over time.

The wordplay in the verse using "grain" for "grain of sense" and "small" to also mean "young" only works in Greek. From my admittedly limited research into the Aramaic, they do not seem to have any of the similar meanings in that language. Was this wordplay original with Christ or was it the invention of the Gospel writers, who, in recording this verse, choose to use the same words in the same way either independently or working from the same text of sayings?

"Grain" is not from the Greek word for seed (sperma) used later in the verse, but from kokkos (kokkos), which means "a grain" and "a seed," a metaphor for a "grain of sense."

"Mustard seed," though translated as seed, is sinapi, (sinapi) which means simply "mustard."

"Sown" and "sow" are from speirô (speiro), which means "to sow a seed," "to beget offspring," "to scatter like a seed," and "to sow a field."

"Less" is from mikros, which means "small," "little," and "young."

"Seed" is from sperma (sperma), which means "seed," "sperm," "origin," "race," "descent," and "offspring."

"Ground" is from gê (ge), which means "the element of earth," "land (country)," "arable land," "the ground," and "the world" as the opposite of the sky.