The evolution of secondary growth — Although secondary
vascular tissues are thought to have evolved independently
in lycophytes and several euphyllophyte lineages ( Cichan and
Taylor, 1990 ; Rothwell et al., 2008 ; Boyce, 2010 ), the evolution
of secondary growth is incompletely understood. Diffi culties
arise from a lack of resolution of phylogenetic relationships
between major vascular plant lineages ( Rothwell and Nixon,
2006 ). This is due in part to the lack of phylogenetic resolution
within the basal plexus of Late Silurian–Early Devonian vascular
plants, as well as to a gap in understanding of evolutionary
relationships between basal paraphyletic groups with simple
organography (protracheophytes and basal tracheophytes, basal
lycophytes, basal euphyllophytes; Fig. 1 ), and the groups derived
from them, which had evolved stem-leaf-root organography
by the Middle Devonian.