In God's mind (mens dei) there is an eternally preestablished relationship between the word (verbum) "tree" (lignum) in Exodus 15:25 and a real "tree" (res) that is understood to be a sign (signum) pointing to the cross (Vulgate: in ligno) in Acts 5:30. When, in Exodus 15:25, the Lord pointed (wayyorehu) Moses to the tree (lignum) that would make the water sweet, He was pointing to a real thing (res) that itself, as a thing (res), pointed to the cross (res). That cross (res), according to Augustine, points to the "sweetness" of salvation and eternal life.
John H. Sailhamer
The Meaning of the Pentateuch
Revelation, Composition and Interpretation