Hong Kong Croton reappears in Hong Kong

First Discovery and Re-discovery

In 1997, staff of the Hong Kong Herbarium collected an unusual shrub of the Family Euphorbiaceae in Tsing Yi Island. After thorough examination by the experts from the South China Institute of Botany, the shrub was confirmed as Croton hancei (Hong kong Croton) which was lost to the botanists for over a century. (Figure 1)

Croton hancei is an endemic species and has never been found outside Hong Kong. This species was first discovered around 1850's by H. F. Hance on Hong Kong Island. Botanist George Bentham published it as a new species in Flora Hongkongensis in 1861. After that, the species had not been found until 1997. Although there were previous records of new specimens of Croton hancei, the specimens were subsequently confirmed as Sapium atrobadiomaculatum which also belonged to Euphorbiaceae. The re-discovery of Croton hancei sheds light to the study of the Hong Kong flora and the genus Croton.

Features

C. hancei is a shrub usually found under shade in dense forests. It is about 2m in height. The young branches and inflorescence are densely covered with stellate hairs which are absent in other parts of the plants (Figure 2). Leaves oblong, margins serrulate (Figure 3), net-veined, petioles short, leaves densely aggregate at the branch tips. Inflorescence raceme, 3 styles in each female flower and 2-lobed in the upper part of each style. Flowers in summer. Fruits globular. As the flowers and fruits are small, C. hancei is easily overlooked.

There is a pair of cup-shaped glands on the underside at the base of the leaf (Figure 4). The style is 2-lobed in C. hancei (Figure 5) while that in S. atrobadiomaculatum is intact.

Among the local species of Croton, only C. hancei lacks the tri-fid venation. On the other hand, stellate hairs are present only in C. hancei on young shoots and infloresence.

Conservation

The Agriculture, Fisheries and Conservation Department has commenced a series of conservation measures for C. hancei , including in-situ conservation, ex-situ conservation and active propagation (Figure 6). Living specimens are cultivated and displayed in the Shing Mun Arboretum.

Figures

Figure 1: Croton hancei in wild environment

Figure 2:Stellate hairs

Figure 3:Leaves margins serrulate

Figure 4:Pair of cup-shaped glands

Figure 5: The style is 2-lobed

Figure 6: Seedling propagation