Plax is a crop added by Thaumic Bases. When harvested it will drop its own seeds and String.
Sometimes a thaumaturge just needs to get him some string - for magical candles or for magical clothes - it does not matter that much what for. And sometimes a thaumaturge just does not want to fight spiders for endless nights or shear sheep for wool to process later. You have applied the knowledge of plants and nature you've just learned to solve the problem for you. The result is a strange crop, named Plax, which will grow on farmland in 8 stages and once fully grown will produce 3 string! It functions just like a regular wheat crop - it also needs light and will only grow on farmland.