Another outstanding issue, common to many biological problems is why do particular traits vary so much between individual and species? One of the more debated topics around growth stress generation is whether or not the generation mechanisms for stress in reaction wood are extreme versions of the same mechanisms in normal wood. The G-layer is not found in normal wood, however not all tension wood producing species produce G-layers. Lignin swelling could potentially fit this criteria for normal and compression wood, however modification of Boyds theory would be needed address the dependence of a MFA as some wood with lower than 40 degree MFA still produces compressive forces. There has been reported to be little lignin within the G-layer, which is suspected to be responsible or at least partly responsible for tension generation. Boyds theory combined with excessive mild compression wood formation in core wood still allows for the same tension generation mechanisms to be used by older cambiums, as long as the MFA is suited to the task.