While both Lithops and Pleiospilos belong to the Aizoaceae family, often referred to as the ice plant or vygie family, they are distinct genera. This is the most fundamental botanical difference. Lithops is its own genus, comprising numerous species and cultivars, all commonly called "living stones." Pleiospilos is a separate genus, with the most common species in cultivation being Pleiospilos nelii, known as "split rock" or "royal flush." Their evolutionary paths diverged, leading to different survival strategies and physical forms despite occupying similar ecological niches in their native South African habitats.
The most apparent difference lies in their leaf composition and arrangement. A Lithops plant consists of a single, fused pair of leaves. This pair forms an inverted cone shape with a fissure or "split" across the top from which new growth and flowers emerge. The plant body is typically short, with most of it residing below the soil surface to avoid predation and desiccation. In contrast, a Pleiospilos plant has two to four (sometimes more) pairs of leaves that are not fused together. These leaves are much thicker, taller, and more hemispherical, giving the plant a cluster-like appearance that sits mostly above the ground. They often have a noticeable keel, or ridge, on the lower surface.
Their internal water management and growth cycles are critically different. Lithops operate on a very strict annual cycle. They absorb water into their single leaf pair, which then shrivels and dies as a new pair develops internally, drawing resources from the old leaves. It is crucial not to water Lithops when the new leaves are emerging, as the old leaves must be reabsorbed. Pleiospilos, however, maintains multiple sets of leaves simultaneously. It is not unusual for a Pleiospilos to have two or three active pairs at once. While it also absorbs moisture into its leaves, it does not have the same single-pair dependency. Watering can be slightly more frequent, but the golden rule of "when in doubt, don't water" still applies vehemently to both.
Both genera produce daisy-like flowers, but key distinctions exist. Lithops flowers almost exclusively emerge from the fissure between its single leaf pair. They are typically white or yellow and can be very large relative to the plant's body, often covering the entire plant. Pleiospilos flowers emerge from the center of the leaf cluster, not from a specific fissure in a single pair. The flowers are generally similar in structure but are often more orange or pinkish in colour and are even more proportionally gigantic compared to the plant, sometimes appearing overwhelmingly large.
