Natural deception

One evolutionary process of peculiar fascination is that termed "mimicry": for example, a plant looking so much like a stone that it escapes detection by resemblance to its background. Mimicry is developed to a high degree in the animal kingdom, a colourful example being harmless species of tropical butterflies that escape extermination by developing similar patterns to those of butterflies that are distasteful to their enemies. "Mimicry" and "imitation" are bad words to use because they imply a conscious act of camouflage where no such motivation is involved. Any chance mutation that makes the undefended easily mistaken for the defended confuses predators and increases the chances of survival. We see only the final, perfected "mimic", not the countless unsuccessful intermediates.

One familiar example of mimicry—1 continue to use the word because I know of none better— in European plants is the dead-nettle (Lamium spp), which gets its name from its superficial likeness to the stinging-nettle (Urtica spp) but lacks stinging hairs. The two plants commonly grow together, although a glance at the flowers shows them to be quite unrelated.

The best cases of mimicry among succulents come within the leaf-succulent Mesembryanthemaceae. Many of the stemless genera are inconspicuous and seem to fade into their backgrounds: Didymaotus (11.18) against granite and white, incrusted Titanopsis (11.20) on quartz chippings. The most remarkable of all are Lithops (5.10) the popular

"pebble plants" and "living stones". In habitat you can stare right at them and still not see them; then, when you finally do, they are everywhere. Professor Desmond Cole of Johannesburg, the leading authority and a past master at finding the unfindable, recognizes 37 species of Lit hops, some widespread (L.lesliei), some extremely local. In the course of field trips with him, I saw a little of the special mystique for Lithops spotting — not only in the known habitat, but in finding that habitat in the wide open spaces where landmarks and signposts are nowhere to be seen. Each of the known Lithops harmonizes to some extent with its background: in no known instance does the plant contrast in the way that a photographer would set it up in order to show off the subject.The colour and patterning of Lithops are extraordinarily diverse (11.27, 28), from plain grey or brown to intricately mottled, and one finds them associated with a corresponding range of rock formations and soils, from white quartz down to the cracked mud of dried-out river beds. And where several rock formations occur side by side, we find Lithops confined to places where they blend with the background; elsewhere

Growers often present their Lithops in "natural" displays in troughs, going to great trouble to collect smooth round pebbles that match each leaf as closely as possible. This is not at all how they appear in nature, where disruptive camouflage is more usual. For instance, a Lithops with a grey body mottled with brown will probably be found among two sorts of rock: a grey slate or quartz and brown chippings, of granite, perhaps.

Before we jump to conclusions on the protective nature of Lithops markings, we need to know the answers to a number of questions. Do grazing animals have similar vision to our own? Are they as easily deceived as we are? And does Lithops rely on otherattributes, such as a distasteful sap, to avoid extermination?

Other succulents besides the Mesembryanthemaceae resemble their natural backgrounds to some degree. In South West Africa I have almost tripped over large, robust plants of Hoodia because the grey of their stems and the blackish tubercles so closely match the rocks against which they grow (5.9). In America Ariocarpus species (16.33) are called "living rock cacti" from their resemblance to the surrounding terrain. The tiny white growths of some species of Anacampseros (12.3) are thought to escape detection by their resemblance to bird droppings.

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