This genus is characterized by its shrub or small tree habit, leaves that are generally 3-4-verticillate, rather short interpetiolar stipules that often have a deciduous upper part and a persistent base, distinctive inflorescences with one to several subglobose heads, four-merous flowers with slender white corollas, capitate stigmas that are exserted on well developed styles, and dry, somewhat small, schizocarpous fruits that are also borne in subglobose heads. Cephalanthus can often be recognized in sterile condition by the lenticellate stems, whorled shortly petiolate leaves with pubescent domatia in the undersurface axils of the secondary veins, ring of white pilose trichomes that persists on the stem after the stipules have fallen, and blackened glands on the apex and margins of the stipules. The plants are obligately deciduous in many parts of their range, but may be evergreen in some tropical areas (this is unknown). Details of leaf morphology and anatomy were studied and extensively illustrated by Romero et al. (2019), who found various types of crystals, and the anatomy of the flowers and fruits were detailed by Romero et al. (2022). The globose inflorescences of Cephalanthus are also distinctive, especially when the flowers are open and the long styles are exserted. The fruits can appear capsular at first glance, but the infructescence heads are composed of numerous individual fruits that separate into mericarps rather than dehiscing or fragmenting capsules. Such inflorescences are found in a number of species in Asia and Madagascar, but are uncommon in the Americas. The flowers of Cephalanthus often have 1-2 blackened stipitate glands in the sinuses between the calyx and corolla lobes, and these glands are quite distinctive but apparently the development (i.e., presence) of these may vary among plants or populations, or the glands are sometimes persistent but other times caducous. Romero et al. (2023) noted that the floewrs of Cephalanthus glabratus are morphologically bisexual but functionally unisexual, on dioecious plants (R.M. Salas, pers. comm.). Cephalanthus occidentalis and perhaps some other species if this genus are cultivated as ornamentals. Cephalanthus occidentalis characteristically grows often naturally in swamps and seasonally flooded areas, and it is planted for bank stabilization in areas where water erosion is a problem (as is Cephalanthus tetrandrus in Asia).
Romero et al. (2020) were able to distinguish the species of Cephalanthus based just on leaf characters, and presented a key to all of the species. Cephalanthus is found on almost all continents but in similar, rather ecologically specific ranges. These species are found in warm, generally humid, temperate regions in North America, southern South America, southern Africa, and eastern Asia, and each of the species ranges from there into the nearby tropical region but its range does not penetrate deeply into the tropics in any area except southeastern Asia. A similarly though entirely subtropical to tropical range is found in Paederia; the biogeographic history of their distributions has not been investigated in detail. The Asian plants of Cephalanthus are now treated as Cephalanthus tetrandrus (Roxb.) Ridsdale & Bakh.f. (Ridsdale 1976), but were considered conspecific with the North American species Cephalanthus occidentalis for some time (e.g., Lorence et al. 2012), and even when separated, these two species are so similar they may represent an eastern Asian-eastern North American disjunct. Cephalanthus has been recently reported to have 7 species (Lorence et al., 2012), but only 6 were recognized by Ridsdale (1976). Six species were again recognized in Cephalanathus by Romero et al. (2023), who presented a key to these, full descriptions and illusrtrations, and distribution maps.
Cephalanthus occidentalis is the most widespread and commonly collected species of this genus, and of the American species. It is found across much of eastern and southwestern US through northern Cental America. Cephalanthus occidentalis is, not surprisingly, also very variable morphologically. A number of infraspecific taxa have been separated based on leaf size and shape and details of the pubescence, but these morphological forms are not demonstrably distinct and do not correlate clearly with ecological or geographic distributions now that this species is well documented (ridsdale, 1976; Yatskievych & Taylor in Yatskievych, 2013). Cephalanthus occidentalis is similar to and apparently sometimes sympatric with Cephalanthus salicifolius of Mexico and northern Central America, which is also found along waterways but additionally grows very seasonally wet and perhaps sometimes dry places. These are distinguished mainly by details of leaf shape and pubescence, and their separation may deserve further evaluation.
Cephalanthus tetrandrus is similarly common and widespread in eastern Asia; these plants were separated into two species, this and Cephalanthus glabrifolius, by Romero et al. (2023) based on details of leaf pubescence, presence vs. absence of glands, and sizes of floral parts. The two species they found there are mostly syjmpatric, and a number of specimens checked here were difficult to classify. This updated species taxonomy is presented here, but given the morphological variation in flowers in this group, not only among plants but developmetnally, further observations including field work and phylogeographic are desirable to better understand these two species.
The floral biology of these species is rather unusual, with secondary pollen presentation; this is found in various Rubiaceae, but apparently less common in the America than in the Paleotropics. The floral biology of Cephalanthus occidentalis was detailed by Imbert & Richards (1993), who found that the flowers last for several days and the introrse anthers open while the flower is still a closed bud, and the pollen is placed on the stigma rather than on the style. The receptive part of the subglobose stigma is a small terminal group of papillae, which develop receptivity a day or two after the flower opens. Before these are receptive they are dry and smooth, and the pollen is apparently easily dispersed off this structure. When the stigma becomes humid and receptive, some pollen from the same flower may still be present and can germinate there but the pollen tubes are inhibited and Cephalanthus occidentalis is self-incompatable. However, these observations do not correspond entirely with observations of all populations of Cephalanthus occidentalis (pers. obs., western Illinois), which appears to be perhaps partially protogynous, or of Cephalanthus glabratus (R.M. Salas, pers. comm.), which may be sometimes dioecious; the floral biology of these species and the genus overall is far from understood.
Cephalanthus natalensis Oliv. is generally similar to other species included in this genus, but differs in its clambering to scandent habit, apparent lack of blackened glands on the stipules and flowers, brightly colored green to red corollas, secondary pollen presentation on a thickened, bright white style rather than directly on the stigma, and indehiscent, drupaceous, pink to red succulent fruits that contain two pyrenes. Based on detailed morphological study, Romero et al. (2023) separated this into the new monotypic genus Sylvainia.
The characters and identity of Cephalanthus have been somewhat confused at times, because the genus was diagnosed by its subcapitate inflorescences together with dry fruits and/or four-merous flowers. The genus identity and characters are clearer now (Ridsdale, 1976; Razafimandimbison & Bremer 2002), but some characters that are frequently cited for Cephalanthus may be errors that persist in the literature while the corresponding species have been transferred to Breonia A. Rich., Ixora L., and some other genera. One such feature may be the common report that the flowers are sometimes 5-merous: this condition is reported as an occasional variant on an inflorescence for some species (e.g., FZ), and such variation in merosity is common in Rubiaceae, but no consistently 5-merous species are currently included in Cephalanthus.
Cephalanthus was included by Ridsdale (1976) in the monotypic tribe Cephalantheae, and monographed in good detail with respect to the Asian plants. This genus was later grouped with others with a similar inflorescence arrangement in a broadened, more clearly delimited tribe Naucleeae (Razafimandimbison & Bremer (2002).
Author: C.M. Taylor.
The content of this web page was last revised on 21 March 2023
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml