An embolism is where an air bubble is created in a tracheid. The tracheid files vary in tangential dimension. A tracheid cell wall usually contains the polymer lignin. The term "tracheid" was introduced by Carl Sanio in 1863, originally as "Tracheide", in German. Several groups of plants later developed pitted tracheid cells independently through convergent evolution. By the middle Devonian, the tracheid diameter of some plant lineages (Zosterophyllophytes) had plateaued.