Haworthia is an interesting genus that consists of about 60 species (as recognized by B. Bayer) endemic to southern Africa.
These small succulents come in a wide array of shapes and colours, and there is a large amount of variation at the species level, as well. Such novelty, along with small stature and relative ease of growth, has made Haworthia very popular with gardeners and succulent enthusiasts. Haworthia taxonomy remains relatively loosely-defined, and recent phylogenic studies suggest that Haworthia should actually be split into three genera (Haworthia, Haworthiopsis, and Tulista). Generally speaking, the currently recognized members of this genus are small succulent plants that usually have firm, fleshy and stem-less leaves.
Some Haworthia species, such as today’s Haworthia truncata, have epidermal windows that allow sunshine to penetrate into the leaf. This allows most of the plant to remain underground. These windows, composed of transparent epidermal tissues on the adaxial surface of the leaves, form the pouty lips in Bruce’s photo. In cultivation, Haworthia truncata is most easily grown above-ground, but in its native hot and dry environment, it is usually found with only the windows exposed. Light enters these windows and refracts and scatters through clear, water-storage tissue (hydrenchyma) to reach green photosynthetic tissue (chlorenchyma) that lines the bottom and sides of the leaf. Light windows allow many South African species to grow below-ground. These windows are also employed by above-ground succulents to increase their photosynthetic area.
The common Afrikaans name for Haworthia truncata is ‘perderande’, meaning horse’s teeth. Perderande is an apt name as not only do the individual leaves resemble a horse’s tooth, but the plant has a fan-like, distichous form, so that above-ground, one sees double rows of the tightly-packed, truncated leaves. This fan shape is unusual in Haworthia, as the leaves typically grow in rosettes, but common in species of the closely-related Aloe. The flowers of Haworthia truncata don’t contribute much to people’s fascination with this plant; the inflorescence is borne on a wiry spike bearing inconspicuous white tubular flowers.
If you are interested in delving more deeply into Haworthia taxonomy, there is an excellent source to assist you. Bruce Bayer seems to have made it his personal mission to clear up the “confusion and ignorance” (his words, not mine) surrounding the classification of this fascinating genus, and his website, Haworthia Updates, makes his work available to all.