The sapling is a user-made plant that grows trees. It's made from 2 seeds and 1 wood. Saplings sell for 5 bronze coins per piece and are black tier. Saplings are the only way so far to actually reproduce wood, so they're pretty important. In the fact that you only need 1 wood and 2 seeds, all you need is to destroy a ton of tall grass and chop down one tree to grow around 10 more (therefor making them a renewable resource).