Secondary xylem parenchyma - From classical terminology to functional traits

Terminology plays a crucial role in describing and understanding the morphologicaland anatomical variation in living organisms. The huge diversity in plant species andtheir many morphological and physiological adaptations to a wide range of ecosystemsis reflected in an enormous anatomical variation of woody tissues. This is perhapssomewhat surprising given that angiosperm wood consists of essentially three cell typesonly: imperforate tracheary elements (fibres and tracheids), vessel elements, and livingparenchyma cells. However, variation in the dimensions and the arrangement of thesecells provide a challenge to anyone who aims to describe and understand quantitativeand qualitative differences between wood samples. The challenge lies not only in theconsistent application and interpretation of terms (Lens et al. 2012), but also in howwe deal with a dynamic continuum (i.e., fuzzy morphology sensu Agnes Arber & RolfSattler) that includes intergradations, intermediate forms, analogous and homologousfeatures (Sattler & Rutishauser 1997).

While the International Association of Wood Anatomy (IAWA) lists (IAWA Com-mittee 1933, 1964, 1989, 2004) have been successful for identification and classificationof angiosperm and gymnosperm wood, there is a lack of an anatomical glossary thatgoes beyond identification, covering the broad fields related to wood anatomy such asfunctional and ecological xylem anatomy, evolutionary and developmental woodanatomy, dendrochronology, etc. It is clear that achieving such a general glossary willnever be perfect and will require a collaborative effort from many experts in variouswood-related disciplines.

This opinion paper attempts to provide a critical review of terminology for woodparenchyma. It is especially concerned with the overlap between the descriptive termsused in systematic wood anatomical treatments and terms with functional implications.Most terminology for ray and axial parenchyma (RAP) has been defined based on themicroscopy of transverse and longitudinal sections for wood identification purposes.A number of terms, especially for axial parenchyma, have changed in their usage orgone out of fashion; some of these are discussed here.