Flax Seeds can be obtained by breaking tall Grass or by harvesting fully grown Flax plants. When tall grass is broken there is a chance of dropping either Flax Seeds or Wheat Seeds.
Flax Seeds can be planted on hydrated Farmland to grow Flax, a plant that produces String when harvested. Unlike Wheat, Flax can only be planted on hydrated farmland, which means the farmland must be within four blocks of a block of Water.
Flax is 2 blocks tall when fully grown, and can take up to 10 Minecraft days to grow. The Flax plant is sometimes used as a decoration block, like in this picture. Flax Seeds cannot be used in Cross Breeding. Flax Seeds can be used to construct automated EMC farms. These cost 4 EMC per seed.