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ANTS AND PLANT-STRUCTURE 393
ants, for it is a very common thing all through the
order of Melastomes. In species which have the
leaves of each pair nearly equal, it is usual to see
some of the smaller ones saccate and others alto-
gether esaccate on the same plant. [I have often
examined half-grown plants and have seen that sacs
begin to be developed (by inheritance] long before
any ants touch them, but that when the sacs are
taken possession of by ants they speedily became
much enlarged.]
Seeing, then, how the sacs on the leaves have
originated, and what purpose they serve, it is plain
that a species of Tococa, like T. planifolia, inhabit-
ing the very river's brink, and liable to be com-
pletely submerged for several months of every year,
could never serve as a permanent residence for ants,
nor consequently have any character impressed on
it by their merely temporary sojourn ; even if their
instinct did not teach them to avoid it altogether,
as they actually seem to do ;
whereas the species of
Tococa growing far enough inland to maintain their
heads above water even at the height of flood are
thereby fitted to be permanently inhabited, and are
consequently never destitute of saccate leaves, nor at
any season of the year clear of ants ; as I have
r .ison to know from the many desperate struggles
I have had with those pugnacious little creatures
when breaking up their homes for the sake of
specimens.
In one species (hb. 3477) with seven -ribbed
leaves, growing by the Rio Negro near the mouth
of the Casiquiari, the leaves on some plants have a
small distorted sac at the base inhabited by ants,
and on others are nearly all esaccate ;
and
I noted