Tertullian

The entire fruit is already present in the seed.

Source: Apologeticus (The Apology), Tertullian, c. 197 AD, ch. 9

Genuine Tertullian, rendering 'etiam fructus omnis iam in semine est' from Apologeticus ch. 9. Important context: it is not a general aphorism about potential — Tertullian is arguing against abortion ('he is a man who is going to be one; the whole fruit is already present in the seed'). Anyone using the line as a standalone metaphor should know the argument it belongs to. Confidence held at 0.8 because I am recalling the chapter from memory of the Latin, not verifying an edition.