How does Black Tentacles work?

[Mana Chicken]

When I cast black tentacles, it says for each round the enemy is grappled it takes an additional 1d6 + 4 damage. Does that mean it just does 1d6 +4 every round, or the first round it does 1d6 + 4, second round it does 2d6 + 8, and so on and so forth?

[jbadams]

1d6 + 4 each round.

If it were the second it would say that the damage was cumulative.