The dragon trees and mother-in-law’s tongues (Dracaena and Sansevieria) form a monophyletic group within Ruscaceae (Yamashita & Tamura 2000, Rudall et al. 2000, APG II 2003) of ca. 180 species found across the wet to dry forest spectrum in the Old World tropics as far East as New Guinea, with outliers in Hawaii, the Canary Islands and Madeira, the Caribbean and Central America (Govaerts 2007). The highest levels of species diversity occur in West/Central Africa (Dracaena) and Eastern/Southern Africa (Sansevieria), with a secondary centre of diversity in South (Sansevieria) and SE Asia (Dracaena). A number of species are important in both temperate and tropical horticulture; virtually very European garden centre, for example, stocks Dracaena and Sansevieria species among its houseplants. However, a number of authors over a considerable time period (e.g. Baker 1875, Brown 1914, Obermeyer et al. 1992) have postulated alternative classificatory approaches to two genera based largely on habit and degree of succulence. Bos (1998) reduced Sansevieria to synonymy with Dracaena in his treatment of Dracaenaceae, although he did not make the many new combinations required at specific level and thus uptake of the change has been limited.
In terms of morphology, the underground organs and stems of both taxa are very similar, often possessing secondary thickening. Above ground, “woody”, often branched stems are more prevalent in Dracaena but present (and absent) in both taxa. The degree of leaf succulence is the most easily observed difference between Sansevieria and Dracaena, with the former possessing flat, sword-shaped to elliptic or cylindric-attenuate xeromorphic leaves, sometimes with a hard terminal spine and the latter more mesomorphic leaves with soft caudate apex, but intermediates exist. Leaves in both genera can be pseudopetiolate. Many, but not all, Sansevieria species have white or brown leaf margins. There is a depressed central longitudinal compound vein in most Dracaena species and, for example, Sansevieria sambiranensis from NW Madagascar, but not in D. draco L. and its allies. Variegation is prevalent in both genera, hence their horticultural importance as houseplants in temperate zones.
Reproductive Morphology, Pollination and Dispersal
The inflorescences and flowers of many species are very similar. The basic unit of the inflorescence is a raceme, with fascicles of flowers being borne at nodes. Pedicels are articulated, and exude apparently nectariferous secretions towards the base. The flowers are usually white, less often pale yellow, pale pink or light green with a narrow tube and open nocturnally by recurvature of the corolla lobes. This colour and morphology, with their often heavy scent, suggests moths are the primary pollination vector. Significant exceptions are Sansevieria sambiranensis, which has coral-pink to red bracts and flowers, bee-pollinated Dracaena draco and its allies, where the tube is very short or absent, and the taxa from Hawaii related to Dracaena aurea which have yellow, bird-pollinated flowers. The main differences are said to lie in the pollen and fruit morphology. There has been limited sampling of the former, but what material has been studied suggests that there is monosulcate, reticulate pollen in Dracaena, as is prevalent in monocots, but that in Sansevieria is operculate (Ojeda et al. 1984, Ojeda & Ludlow-Wiechers 1995). The fruit in Dracaena is a berry, with a distinct exocarp and an apical scar or persistent style base (Bos 1984). Sansevieria is said to possess a sarcotesta, with no apical style base or scar and gymnospermous seed development (Nakai 1936, and several subsequent authors). Despite this, they are similar in overall appearance and probably dispersed by birds, which may help to explain their widespread distribution and frequent occurrence on isolated islands. Other characters which have been suggested to differ between the two genera are the point of filament insertion in the corolla tube, thickening of the filament, the degree to which the stigma is lobed and seed morphology (Mwachala 2005) appear less convincing.
Significant systematic treatments
The most significant recent studies of Dracaena were undertaken in Wageningen by J.J. Bos. He established major collections of literature, herbarium specimens and living plants, and published regional treatments of African Dracaena (Bos 1984, 1992, Bos & Teketay 1997). A Flora of Somalia treatment for both genera was published by Thulin (1995). Sansevieria was monographed by Brown (1915), whose research began the sizeable RBG, Kew living collection. Most recent research on the species of the genus has been by succulent plant enthusiasts and researchers such as Chahinian and Newton.