Occurrence of phi thickenings is correlated with gymnosperm systematics

Abstract

Phi thickenings are lignified wall thickenings in the radial and tangential walls of the root cortex of some plant species and are so named because they resemble the Greek letter phi in cross section. Although their occurrence has long been known in gymnosperms, a systematic survey of their presence has never been undertaken. Such a survey may be useful to future studies of the functions of these structures. Roots of 22 species of gymnosperms representing all of the major groups were hand sectioned, stained with phloroglucinol (a lignin stain), and examined for the presence of phi thickenings using light microscopy. Detailed observations on Thuja occidentalis (Cupressaceae) were made to provide a baseline for comparison in this study. Phi thickenings were absent in the species examined in the Cycadaceae, Gnetaceae, Pinaceae, and Podocarpaceae and were present in the Ginkgoaceae, Araucariaceae, Taxaceae, and Cupressaceae (s.l.). These results, when combined with previous literature reports, indicate that the occurrence of phi thickenings is consistent within a family. Their occurrence is also correlated with current hypotheses of phylogenetic relationships within the gymnosperms, which may indicate their usefulness as a systematic character.Key words: phi thickenings, root anatomy, gymnosperms.