COOK, MARTHA E.* AND WILLIAM E. FRIEDMAN. Department of Environmental, Population, and Organismic Biology, University of Colorado, Boulder, CO 80309. - Development of tracheids in the basal extant vascular plant Huperzia lucidula (Lycopodiaceae): implications for the early evolution of plant water-conducting cells.
Recent paleobotanical discoveries indicate that Silurian and Devonian
tracheids differ significantly from the tracheids of extant plants.
While secondary walls of tracheids in previously studied extant seed
plants are lignified throughout, tracheid walls of early fossil plants
have two distinct wall layers, one of which is more degradation
resistant than the other. We report a previously unknown pattern of
cell wall formation in the tracheids of a living plant. In
Huperzia, one of the most primitive extant vascular plants,
tracheid cell wall deposition includes two layers of wall material
that are distinguished by their staining properties under the TEM.
These layers are further distinguished by their differing responses to
treatment with the wall degrading enzymes pectinase and cellulase.
Enzyme treatment affects the first-deposited secondary wall layer, but
not the later-deposited secondary wall layer. For this reason, we
call the second-formed layer of secondary wall material the
"resistant layer". Because the first-formed layer of
secondary wall material determines the pattern of further wall
deposition, we call it the "template layer". This pattern
of secondary wall layers in Huperzia matches precisely the
pattern of wall thickenings in the fossil G-type tracheids of
ancestral zosterophylls and lycopods, and provides critical evidence
for an explicit hypothesis of tracheid developmental evolution.
Moreover, structural congruence between tracheids of Huperzia
and fossil S- and P-type tracheids provides strong evidence for a
single origin of tracheids among vascular plants.
Key words: developmental evolution, Huperzia lucidula (Lycopodiaceae), tracheid