Notes on the Bible, by Albert Barnes, , at sacred-texts.com
Compare Eze 37:16 ff.
Thou shalt write Aaron's name upon the rod of Levi - The Levites had taken part in the late outbreak. It was therefore necessary to vindicate the supremacy of the house of Aaron over them; and accordingly his name was written on the rod of Levi, although being the son of Kohath, the second son of Levi (Exo 6:16 ff), he would not be the natural head of the tribe.
Before the testimony - See Num 17:10 note.
The whole number of rods was twelve exclusive of Aaron's, as the Vulgate expressly states.
Yielded almonds - "Ripened almonds," i. e. "brought forth ripe almonds." The name almond in Hebrew denotes the "waking-tree," the "waking-fruit;" and is applied to this tree, because it blossoms early in the season. It serves here, as in Jer 1:11-12, to set forth the speed and certainty with which, at God's will, His purposes are accomplished. So again the blossoming and bearing of Aaron's rod, naturally impotent when severed from the parent tree, may signify the profitableness, because of God's appointment and blessing, of the various means of grace (e. g. the priesthood, the sacraments), which of themselves and apart from Him could have no such efficacy. Compare Isa 4:2; Isa 11:1; Isa 53:2; Jer 33:5; Zac 6:12.
The testimony - i. e. the two tables of the Law; compare Exo 25:16 note. No doubt the rod lay in front of the tables within the ark. In the days of Solomon Kg1 8:9 there was nothing in the ark save the Two tables. Aaron's rod was probably lost when the ark was taken by the Philistines.
A new section should begin with these verses. They are connected retrospectively with Num. 16; and form the immediate introduction to Num. 18. The people were terror-stricken by the fate of the company of Korah and by the plague. Presumption passed by reaction into despair. Was there any approach for them to the tabernacle of the Lord? Was there any escape from death, except by keeping aloof from His presence? The answers are supplied by the ordinances which testified that the God of judgment was still a God of grace and of love.