The seed growing secretly (the only parable peculiar to Mk). Tatian in his 'Diates-saron' places it immediately before the Tares. Such a position for it is suitable, but it is wrong to regard it, with Weiss, as only an imperfect and mutilated version of that parable.
The point of the parable is not so much the secret invisible energy of the seed, or divine Word, as that of the earth into which the seed falls, i.e. the moral and spiritual nature of man. The seed of Christianity will grow, because the soil into which it will fall is suitable to nourish it. The human soul is 'naturally Christian' (Tertullian), and Christianity is the 'natural religion.' Christianity can, therefore, propagate itself without human effort, and often does so.