Proposals for quantifying two characteristics of tracheid pitting arrangement in gymnosperm woods

Abstract

Arrangement of tracheid radial pits is frequently superficially mentioned in extant gymnosperm wood anatomy descriptions. However, pitting arrangement is a key character for delimiting fossil-genera and fossil-species in gymnosperm woods. We propose two new indices for measuring and quantifying the radial pitting arrangement in fossil and extant woods. The first one, contiguity percentage index (Cp) indicates if the pit rows are contiguous (=in contact or touching) with other rows in the above and below pit borders, and has values from 0 to 100%. The second one, seriation index (Si) is an average of the seriation of the pits and is given as a numerical value equal or greater than 1, when pits are exclusively ordered in uniseriate columns the wood has a value of 1. These indices are useful for quantifying the pitting arrangement and for helping to characterize the type of radial pitting (araucarian/mixed/abietinean), particularly in fossil woods. These new indices also allow to use these two characters as continuous instead of discrete, making them more suitable for certain taxonomic or statistic analysis.