If you have a friend who also has Harvest Moon 2, you can trade items from Mary's computer via your Gameboy gamelink cable. You can't complete her Plant picture book if you don't trade with someone of the opposite gender to get the seeds that you wouldn't normally get.
To activate the Trade Center, hook together your Gameboys using the gamelink cable and head to Mary's Library. One of the options she has is called "Communicate". You and your friend must both select "Communicate" in order to enter the Trade Center. Before you can continue on, you are prompted to save your game. Save your games and the two of you will be linked together to trade.
There are five different items you can trade for with your friend. You can trade Animals (cows, chickens, and sheep), Seeds, Bugs/Fish, Pets, or Combine Maps. Whatever option you choose, your friend will always get a confirmation message on his/her screen asking if it's all right to trade that item. If you want to trade something and you friend doesn't have anything equal to trade, you can give it as a "gift" without receiving anything back in return. For example say you want to trade a cow but your friend doesn't have any animals at all to trade with then the cow becomes a "gift". If your friend had a chicken then you would have to trade the chicken for the cow.
One of the benefits of animal trading is that it's the only way to get a Golden Chicken. Just trade a chicken (or give as a gift) to your friend and magically it becomes a Golden Chicken. If you want the Golden Chicken back, you have to leave the Trade Center and relink up. If you immediately trade the chicken back it won't be golden. The only restriction would be if you want to trade sheep. Your friend must have the Sheep Pen built in order to accept a sheep from you.
Each one of your animals will show up on your screen. Down at the bottom will show its name, how large it is, and if it is healthy or grumpy. The love meter for each cow and sheep will show up over next to the heart on the right side of the screen. Chickens don't have love meters so nothing will show next to the heart when a chicken is selected. To trade an animal place the arrow next to which one you want and press the A BUTTON. You'll get a confirmation asking if that's the animal you wish to trade. Choose "yes" and then the animal will be traded and the game saved. Once the trade is complete you'll return back to the Trade Center.
You need to trade Herb and Flower seeds from another gender's game in order to fill in the missing pages of Mary's Plant book. If your farmer is a boy you'll need to trade with a girl farmer game in order to get Rosemary, Camomile, Orchid, and Lavender seeds. If your farmer is a girl you'll need to trade with a boy farmer game to get Sage, Saffron, Geranium, and Primrose seeds. You can also trade regular crop seeds too like Tomato and Broccoli seeds. To trade seeds just select which seed you want to trade and wait for your friend to select his/her seed. Then just "yes" to the confirmation and the save. You can only trade one seed at a time though, so if you want four Primrose seeds from your friend you're going to have to trade four of your seeds, one at a time, for the four Primrose seeds.
If you are missing pictures out of your fish or bug picture book, you can trade with a friend to fill in the blanks. Trading bugs or fish doesn't remove the picture out of your own picture book, it just adds into theirs.
Each bug and fish is listed just as it is in the picture books (butterflies, cicadas, dragonflies, etc). The majority of the bugs are listed on the first page. The rest of the bugs and the fish are listed on the second page. You can reach the second page by pressing the A BUTTON when the arrow in the lower right hand corner is selected. Trading a fish doesn't remove any fish out of your stock in your pond back on the farm either. If you only have 12 Bluegill you'll still have 12 Bluegill after you trade one to a friend.
If you want you can also trade your pet with your friend. You would get your friend's pet in return. There's no giving away pets as "gifts" since the point is to swap pets with your friends.
One of the special items in the game is the Treasure Map, which you receive one half of. In order to complete the map, you have to combine the map pieces with a friend who also has part of the Treasure Map.
Choosing this closes the link between your friend and takes you back to Mary's Library.