Partial alopecia for the Permo-Pennsylvanian seed-fern Macroneuropteris scheuchzeri

• M. scheuchzeri ‘bristle-like hairs’ are the content of mesophyll secretory canals.
• M. scheuchzeri concept and systematic position do not need to be reviewed.
• The secretory canals had similar functions as the resin canals of modern conifers.
• Secretory canal density is not a taxonomic criterion of prime importance.
• It strengthens the hypothesis that M. scheuchzeri was confined to wetland habitats.

Macroneuropteris scheuchzeri is a long-known taxon which, in recent years, has been the subject of renewed attention from various viewpoints. Particularly noticeable among the diverse characters used for the recognition of the species is the presence of ‘bristle-like hairs’, usually one to 3 mm long, apparently mainly located on the abaxial side of the pinnules. The “hair-status” of these elements was very recently challenged and refuted on the basis of results from the application of new techniques. Recently, dynamic molecular self-assembly was proposed for explaining the origin and emergent nature of these ‘extracuticular deposits’. However, based on these and our own observations we conclude that the elements routinely considered to be hairs for the past century and a half in the diagnostic circumscription of the taxon are neither hairs, nor outer emergent deposits. They represent the content of short secretory canals, very likely resin canals, principally located in the lower layer of the mesophyll. This is a good example showing that ‘old’ and ‘new’ techniques are complementary.