XX 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 growing in deserts often closely 
resemble cacti. Seaside plants and high alpine plants of 
different ordei's 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 nymphxeoides (the limnanthemum like a 
nymphaea), 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 different 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 specialised 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 . 1 
1 Flowers, Fruits, and Leaves, p. 128 (Fig. 79). 
