Figure 7.20 Tylose formation in mature Eucalyptus nitens stems (photos and microscopy by Dr Lawrie Wilson). Left: In the sapwood/heartwood transition zone, tylose plugs are formed by vessel-associated ray parenchyma cells, growing through pit pairs into the vessel lumen, eventually blocking them off from the transpiration stream. Right: Tylose plugs can also form in sapwood vessels in response to wounding. Here, disconnection of an affected vessel from the transpiration stream prevents air from entering the system and cause embolism.