Polystemonous androecia are diverse in both number and position of sta
mens. This investigation of polystemonous Hydrangeaceae uses developme
ntal data to characterize (1) the range of developmental variations th
at account for the diverse androecial patterns and (2) how the express
ions of polystemony among Hydrangeaceae compare to those found general
ly among other angiosperms and especially in their sister family, the
Loasaceae, some of which have particularly complex androecia. All poly
stemonous Hydrangeaceae share the common element of stamen clusters in
antesepalous positions. In each of these taxa, the first stamens are
initiated opposite the medians of the sepals. Subsequently, stamens fo
rm laterally on the flanks of the initial antesepalous stamens, giving
rise to the clusters designated as antesepalous triplets. The simples
t elaborations based on those common initial developmental steps inclu
de (1) adding additional lateral flanking stamens and (2) adding a sin
gle stamen in each antepetalous position between adjacent antesepalous
groups. More complex elaborations are characteristic of (1) Carpenter
ia and Philadelphus, which form common primordia at the beginning of a
ndroecial development and, subsequently, have stamen primordia form on
them, and (2) Deinanthe, which has an elongate hypanthial region on w
hich numerous whorls of stamens are initiated. Carpenteria is unique a
mong Hydrangeaceae in having groups of stamens that are initiated cent
rifugally in antepetalous positions, and this is similar to complex el
ements found among some Loasaceae. Generally, the polystemony of Hydra
ngeaceae that is based in the formation of antesepalous triplets is ve
ry similar to that found to evolve in parallel among various clades of
rosids and asterids.