Growing enchantment

Growing enchantments are enchantments that start out without the ability to do anything but "grow". A "growing" enchantment accumulates counters each turn, building up to grant the player a boon proportional to the number of counters.

Although other spells with a similar mechanic have appeared before or since, growing enchantments were a widespread theme in the Urza block.

Rulings

 * In a sense, these enchantments behave like delayed-effect sorceries. You cast one, then you wait. The more turns you wait before sacrificing it to get its effect, the bigger that effect will be.
 * Putting a counter on the enchantment when its upkeep trigger resolves is always optional.
 * The sacrifice ability's effect checks the enchantment's last existence on the battlefield to see how many counters were on it.
 * There are also two artifacts in the set that behave this way: Barrin's Codex, which accumulates page counters, and Lotus Blossom, which accumulates petal counters. Powder Keg was released in the next set with a destructive effect.
 * Thirteen years later, Golden Urn and the Shrine cycle from New Phyrexia follow the same theme, though the Shrines have the bonus ability to gain counters when a spell of an appropriate color is cast.

From Urza's Saga

 * Discordant Dirge
 * Lilting Refrain
 * Midsummer Revel
 * Recantation
 * Rumbling Crescendo</c>
 * Serra's Hymn</c>
 * Serra's Liturgy</c>
 * Torch Song</c>
 * Vile Requiem</c>
 * War Dance</c>

From Urza's Destiny

 * Archery Training</c>
 * Festering Wound</c>
 * Incendiary</c>
 * Momentum</c>
 * Private Research</c>