Martin Deasy’s series on the Hamamelidaceae continues today with its fourth entry. The fifth, and final entry, will appear on Saturday. Martin writes:
The monotypic genus Sinowilsonia is named for the prolific English plant hunter E. H. “Chinese” Wilson, immortalizing both his surname and his nickname (the prefix “sino-” means “pertaining to China”). The species was first collected for Western science from the wild in 1889 by the remarkable Irish plantsman Augustine Henry, and bears his name. The original herbarium voucher specimen he sent to Kew can be viewed digitally (or see Wilson’s 1907 collection of the same species).
Sinowilsonia henryi is a medium-sized tree native to the mixed forests of central China. Its unisexual inflorescences–effectively a type of catkin–are numerous and highly distinctive. Both male and female catkins are ca. 5cm long at pollination, but the rachis of the female inflorescence elongates markedly after fertilization, attaining a final length of 20cm or more.
The photograph shows a fertilized female inflorescence in the process of elongating. Each flower bears twin pinkish-green styles and 5 greenish sepals on a swelling pistil. Sporadic rusty-brown stellate trichomes (a familiar Hamamelidaceae character) are also visible on close inspection. Adjacent are the exhausted male catkins, beginning to dessicate having released their pollen some time earlier.
Sinowilsonia‘s reduced flowers (lacking petals and either male or female sexual parts) are typical of wind-pollinated taxa, which do not require petals to attract pollinators, but instead need to produce large volumes of pollen to maximize the chances of fertilization.
The characteristic inflorescences of Sinowilsonia (together with those of the other anemophilous Hamamelidaceae genera Sycopsis, Distylium and Parrotia) give some insight into why the Hamamelidaceae were historically closely aligned with other families of wind-pollinated catkin-bearers (the artificial grouping dubbed the Amentiferae). Systems such as that of Engler & Prantl treated them as neighbours to e.g. Fagaceae, Betulaceae, Juglandaceae, Urticaceae and Platanaceae, and despite almost immediate dissent, these relationships proved surprisingly slow to be dismantled.
More sophisticated morphological, and later molecular, taxonomy made clear that the reduced flowers found in wind-pollinated catkins were highly derived–that is, they did not represent a “primitive” ancestral state, and were therefore of little use in drawing conclusions about evolutionary relationships between taxa. Fragments of the Amentiferae are now widely scattered within the Fagales, Rosales and even Proteales. The Hamamelidaceae, meanwhile, are located firmly within order Saxifragales (the apically unfused bicarpellate pistils, and twin styles, are highly diagnostic).
In fact, within the Hamamelidaceae alone, wind pollination has evolved several times, in each case from an ancestral state of insect pollination. Thus most of Hamamelidaceae’s anemophilous genera are found within tribe Fothergilleae, whereas Sinowilsonia has evolved separately within tribe Eustigmateae.