4IO NOTES OF A BOTANIST 
are the primary attraction to the ants, which are 
always of one species and sting virulently. 
I find that I had myself given a short account of 
these ant-infested plants of both hemispheres in my 
volume on Natural Selection and Tropical JVattcre 
(p. 284), in which I refer to Mr. Forbes's observa- 
tions, and also to those of the late Mr. Belt on the 
Bull's-Horn Acacia, which has the thorns in a young 
state filled with a sweetish pulpy substance which at 
first serves as food for the ants, while later on they 
are supplied by honey-glands upon all the leaves. 
He also notices and figures in his Naturalist in 
Nica7^agua (p. 223) the leaves of one of the 
Melastomae with swollen petioles, and he states 
that, besides the small ants always infesting them, he 
noticed, several times, some dark-coloured Aphides. 
He also suggests that these small virulently-stinging 
ants are of use to the plants by guarding them 
from leaf-eating enemies such as caterpillars, snails, 
and even herbivorous mammals, but above all 
from the omnipresent Sauba or leaf- cutting ant, 
which he declares he observed to be much afraid of 
these small species. 
I think the facts that have now been observed in 
both the western and eastern tropics are really 
sufficient to enable us to understand the probable 
origin of the various remarkable structures that 
have been developed in many different groups of 
plants and are utilised by ants. There is clearly 
"utility" on both sides. The ants obtain dwellings, 
protection from floods, a safe shelter for their eggs 
and larvae, and a portion of their food — in some 
cases perhaps all — from the plant they inhabit; 
while the plant derives protection to its foliage, 
