The S2 layer in the tracheid walls of Picea abies wood:: Inhomogeneity in lignin distribution and cell wall microstructure

Transmission electron microscopy (TEM) of the walls of Picea abies axial tracheids showed the distribution of lignin in the S2 layer to be inhomogenous. At relatively low magnifications, some parts of the outer and inner S2 layer appeared more electron dense than the mid region in the tracheids which were in contact with or in proximity to a ray. At similar magnifications, the presence of radial and tangential features was observed in the S2 layer of the tracheids which were in contact with or close to rays as well as in those which occurred elsewhere. Higher magnification views showed the S2 layer to be differentiated into electron lucent and dense regions in both radial and tangential directions. A comparison of the counts made of lignin particles in these regions suggested that the differentiation of the S2 wall into lucent and dense regions resulted from inhomogenous distribution of lignin observable at a nano level.