Mark 4:31

a grain of mustard seed The growth of a worldly kingdom had been already set forth under the image of a tree, and that of the kingdom of God also had been similarly compared. (See Daniel 4:10-12; Ezekiel 17:22; Ezekiel 17:24; Ezekiel 31:3-9.)

in the earth In St Matthew 13:31 a man is represented as taking and sowing it "in his field," while St Luke, Luke 13:19, says "in his garden."

less than all the seeds "Small as a grain of mustard seed" was a proverbial expression among the Jews for something exceedingly minute. The mustard-seed is not the least of all seeds in the world, but of all which the husbandman was accustomed to sow, and the "tree," when full grown, was larger than the other herbs in his garden.