SANCHEZ-KEN, J. GABRIEL*, ROBERT H. MARCH, and LYNN G. CLARK. Iowa State University, Department of Botany, 353 Bessey Hall, Ames IA, 50011-1020. - Evolution of the mesophyll in the grasses (Poaceae).
Specialization of the mesophyll in grasses is well known, especially
with reference to its correlation with photosynthetic pathway. In
cross-sectional view, the mesophyll has been classified as radiate or
non-radiate, and several cell forms have been identified, including
tabular cells, isodiametric cells, fusoid cells, and various lobed
cells (e.g., arm cells). With the availability of a robust phylogeny
of the grass family, we undertook a survey of the major clades in
order to understand better the evolution of mesophyll in grasses. We
included taxa of both forest and open habitats whenever possible and
cross and longitudinal sections were examined. The results were mapped
on the Grass Phylogeny Working Group phylogeny, but character
distribution on an alternative topology was also explored. Our results
suggest that arm cells, one of the cellular forms in which
lobing/invagination shows up in cross-sectional view, is polymorphic
in the Anomochlooideae, but is characteristic of Pharoideae,
Puelioideae and Bambusoideae, and then is lost in the Pooideae and the
PACCAD Clade. Fusoid cells show a similar evolutionary pattern.
Rosette cells, previously lumped with arm cells, are interpreted as a
potential synapomorphy for the Bistigmatic Clade (Puelioideae +
[BEP+PACCAD]). Rosette cells are common in the mesophyll in
Ehrhartoideae but usually occur in combination with arm cells in
Bambusoideae. The presence of rosette cells in a few members of the
Pooideae and the PACCAD clade may represent retention of this
character. Longitudinally elongated lobed cells (LEL cells) previously
known from a few taxa are here found to be a potential synapomorphy
for the Pooideae + PACCAD Clade, under the alternative topology. We
also report on some additional observations in the PACCAD Clade.
Key words: anatomy, arm cells, mesophyll, Poaceae, rosette cells