XI THE SPECIAL COLOURS OF PLANTS 303 



in times of drought or scarcity, will devour everything in the 

 shape of a fleshy stem or tuber. 



True mimicry is very rare in plants, though adaptation to 

 like conditions often produces in foliage and habit a similarity 

 that is deceiving. Euphorbias grooving in deserts often closely 

 resemble cacti. Seaside plants and high alpine plants of 

 diiferent orders are often much alike; and innumerable 

 resemblances of this kind are recorded in the names of 

 plants, as Veronica epacridea (the veronica like an epacris), 

 Limnanthemum nymphieoides (the limnanthemum like a 

 nymphsea), the resembling species in each case belonging to 

 totally distinct families. But in these cases, and in most others 

 that have been observed, the essential features of true mimicry 

 are absent, inasmuch as the one plant cannot be supposed to 

 derive any benefit from its close resemblance to the other, 

 and this is still more certain from the fact that the two 

 species usually inhabit difl'erent localities. A few cases exist, 

 however, in which there does seem to be the necessary 

 accordance and utility. Mr. Mansel Weale mentions a labiate 

 plant (Ajuga ophrydis), the only species of the genus Ajuga in 

 South Africa, which is strikingly like an orchid of the same 

 country ; while a balsam (Impatiens capensis), also a solitary 

 species of the genus in that country, is equally like an orchid, 

 growing in the same locality and visited by the same insects. 

 As both these genera of plants are speciaHsed for insect 

 fertilisation, and both of the plants in question are isolated 

 species of their respective genera, we may suppose that, 

 when they first reached South Africa they were neglected 

 by the insects of the country ; but, being both remotely like 

 orchids in form of flower, those varieties that approached 

 nearest to the familiar species of the country were visited 

 by insects and cross-fertilised, and thus a closer resemblance 

 would at length be brought about. Another case of close 

 general resemblance, is that of our common white dead- 

 nettle (Lamium album) to the stinging-nettle (Urtica dioica) ; 

 and Sir John Lubbock thinks that this is a case of true 

 mimicry, the dead-nettle being benefited by being mistaken 

 by grazing animals for the stinging-nettle.^ 



^ Flowers, Fruits, and Leaves, p. 128 (Fig. 79). 



