The Satchel of Cenarius is a bag that will only hold herbs. This was the largest herb-bag in the game before TBC.
This item is crafted with Tailoring (300); taught by [Pattern: Satchel of Cenarius], a Cenarion Circle reputation reward.
This bag has been surpassed by the larger bags [Mycah's Botanical Bag] and [Emerald Bag]. This means that, for tailors, you no longer have to farm rep for Cenarion Circle if you want to make the largest herb bag in-game.