Living stones

On 14th September 1811, in South Africa's Northern Cape Province, the prolific natural history collector and travelling artist William Burchell made a succulent discovery 'on picking up from the stony ground, what was supposed a curiously shaped pebble, it proved to be a plant ? but in color and appearance bore the closest resemblance to the stones, between which it was growing'.
During the subsequent 200 years approximately 40 species have been placed in Lithops (literally 'resembling stone'), a genus native to arid areas of South Africa, Namibia and Botswana. These plants are evolutionary curiosities and commonly grown as houseplants because of their colours and extreme adaptations to desert life.
In their native regions, Lithops plants are embedded in the ground and shaped like inverted cones. A plant comprises a pair of fat, opposite leaves attached to a very short stem. The only part of these cryptically coloured plants usually visible is a small 'window' in the leaf tip through which light enters.
For most of their lives Lithops plants are barely distinguishable from the rock-strewn landscapes in which they occur, which led Burchell to speculate that this was so they could 'escape the notice of cattle and wild animals'. Only when they produce their vivid yellow or white, insect-pollinated, daisy-like flowers, from between their leaves, are they revealed to the casual observer.
Succulence, thick cuticles and minimal surface areas are typical plant adaptations to desert life. Embedding whole plants in the soil is another solution to the water retention problem but it may severely limit photosynthesis. In Lithops this problem is overcome by the light passing through the exposed 'window' being concentrated on chloroplast-containing cells lining the inside of the leaves; maximum illumination is achieved with minimum exposure to a hostile environment.
'Window' clarity may also help thermal responses, whilst burial allows the plants to take advantage of soil's thermal properties; the deeper, photosynthetic and growing parts of the plant are insulated from daily temperature extremes. Furthermore, small plants, close to soil surfaces, can use dew as a water resource.
Lithops reproduce by seed but for germination water must be available. In deserts, water supply is both limited and unpredictable. Lithops have adapted to desert life by producing hundreds of long-lived seeds which are retained inside dry, capsular fruits. In response to water the capsule valves open releasing the seeds, which quickly germinate taking advantage of the transient conditions.