The most useful application of "specialization" in the Major Trends papers refers to differentiation in the roles of tracheary elements from tracheids which support the plant and conduct water into separate support (fiber-tracheids, libriform
fibers) and conductive cells (vessel elements).
The last few cell rows of libriform
wood fibres at the border of the annual rings are more strongly lignified than the weakly lignified libriform
wood fibres of the early wood.
For imperforate elements, is the Baileyan pattern of true tracheids transforming to fiber-tracheids and then to libriform
fibers the only transformation series, or is there evidence of other evolutionary patterns?
However, maple contains areas of libriform
fibers with large diameters that may influence the local SG and this may affect lateral and principal forces due to a density gradient.