Bark of tree is impervious to gases and water because cork cells through which bark is made up of are dead in nature and they also do not have any intercellular space in between through which water and gases can move.

Secondly, there is deposition of a compound called suberin in their walls which also makes the cork cells impervious to the gases and water.

So, due to above mentioned reasons bark of a tree is impervious to water and gases.