Function
In the fourth generation, which includes Pokemon Pearl, Diamond and Platinum, the Tiny Mushrooms do not have any of the function from the previous generations. In Pearl, the Tiny Mushrooms are rare, and other than being popular with other trainers and collectors in the game, the Tiny Mushrooms are just items to be sold.
Selling Price
You can sell a Tiny Mushroom for 250 Pokemon Dollars. For a frame of reference, the basic Poke Ball costs 200 Pokemon Dollars, and the Ultra Ball costs 1200 Pokemon Dollars. You can sell the Tiny Mushrooms at any Poke Mart. You can get ten times more money for a Big Mushroom, but the Big Mushroom is much rarer.
Obtaining Tiny Mushroom
The Tiny Mushroom is a rare item, and the location varies in different generations. In Pearl and other third generation games you can find the Tiny Mushroom all over the game when fighting the wild pokemon Paras. You have a 50 percent chance of getting a Tiny Mushroom after fighting a wild Paras. The Tiny Mushrooms can also be found on a wild Parasect. You may be able to buy a Tiny Mushroom in Goldenrod Tunnel on Monday mornings, but it costs 3,000 Pokemon Dollars.
Changes Across Generations
In most generations the Tiny Mushrooms were just for sale. In the third generation games FireRed and LeafGreen, the Tiny Mushrooms actually served a more concrete purpose: you could give two Tiny Mushrooms to a man on Two Island to change a single move for a single Pokemon. In addition to the fifth generation, the Tiny Mushrooms appeal to certain in-game collectors in the third and fourth generations.