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ANTS AND PLANT-STRUCTURE 389
which is adjacent to and inundated by the rivers
and lakes, but sometimes deep in the virgin forest,
wherever the land is so low that the water of rains
may accumulate thereon to a slight depth. All the
species have the unmistakable aspect of their order
-the ribbed opposite leaves, the polypetalous flowers
with beaked porose anthers, etc. ; but
they are dis-
tinguished at sight from most others of the order
by the large, thin, lanceolate or ovate acuminate,
leaves, very sparsely set with long hairs, and having
a hollow sac or a pair of sacs at the base either of
all the leaves, or (more frequently) of only one of
each pair when that one is much larger than the
other. The leaves in the majority of the species
have but three ribs ; a few species, however, have
five- or even seven-ribbed leaves ; but, in all, the
origin of the innermost pair of ribs is an inch or so
up the midrib from the base of the leaf; and it is
this portion of the leaf, from the insertion of the
inner ribs downwards, which is occupied by the sac.
The latter sometimes takes up only a part of the
breadth of the leaf, when it is technically considered
to be seated on the leaf ; (Epiphysca) in other
cases the sac in its lower half absorbs the whole
breadth of the leaf, when it seems to be seated half
on the leaf, half on the petiole (Anaphysca) ;
or,
lastly, throughout its length it absorbs the whole
breadth of the leaf, and then seems seated entirely
on the petiole (Hypophysca). That it is really
formed in all cases at the expense of the lamina,
and not of the petiole, is proved by the occasional
occurrence of imperfectly -developed sacs in the
hypophyscous form, bordered by a narrow wing continuous
with the leaf, and Diviner to the latter a