Phylogenetic analyses of rbcL sequences were used to address both syst
ematic and evolutionary questions posed by the angiosperm family Hydra
ngeaceae. Our analyses suggest the presence of a monophyletic Hydrange
aceae most closely allied with Loasaceae, a finding in agreement with
other molecular as well as morphological analyses. Molecular data indi
cate that Hydrangeaceae comprise Decumaria, Pileostegia, Schizophragma
, Hydrangea, Dichroa, Broussaisia, Platycrater, Cardiandra, Deinanthe,
Carpenteria, Philadelphus, Deutzia, Fendlerella, Whipplea, Fendlera,
Jamesia, and the enigmatic Kirengeshoma. A particularly close relation
ship of Kirengeshoma and Deutzia is indicated. Analysis of rbcL sequen
ces suggests that Fendlera and Jamesia are sister to the remainder of
the family, lending support to the hypothesis that at least some Carpe
nterieae are basal in the family and that Hydrangeaceae may have origi
nated in xeric habitats. If this phylogenetic placement of Jamesia and
Fendlera is correct, the rbcL trees also suggest that the level of ep
igyny has decreased in these genera, as well as in the Fendlerella- Wh
ipplea clade and Carpenteria when compared to the outgroup taxa, which
are wholly epigynous. Furthermore, the rbcL trees support proposed ev
olutionary trends in wood anatomy, suggesting, for example, that uplan
d tropical taxa have evolved longer vessel elements with more numerous
bars on scalariform perforation plates. The xerophytic basal members
of Hydrangeaceae, like the closely related Loasaceae, have short, narr
ow vessel elements with scalariform perforation plates bearing few bar
s. Following Jamesia and Fendlera, the remaining hydrangeoids are divi
ded into two large subclades that closely parallel the traditional div
ision of the family into Philadelpheae and Hydrangeae. Both rbcL seque
nces and morphological data suggest close relationships between: 1) Fe
ndlerella and Whipplea; 2) Decumaria, Pileostegia, and Schizophragma;
3) Carpenteria and Philadelphus; 4) Deinanthe and Cardiandra; 5) Dichr
oa, Broussaisia, and Hydrangea macrophylla. Molecular and morphologica
l data also concur in demonstrating that the large genus Hydrangea is
not a monophyletic assemblage.