Untitled - The Alfred Russel Wallace Website

392 NOTES OF A BOTANIST CHAP.
so that even if the leaves of this Tococa were sac-
ciferous, they could not afford a permanent refuge to
ants. But all the other sub-riparial species grow
so far away from the real shore that the periodical
inundations never overwhelm them completely, but
leave at least the tops of the branches out of water ;
and it is noticeable that not only are the first leaves
of young plants of every Tococa often esaccate, but
that also the lowest leaves of each ramulus of the
adult plant have either no sac or only the slightest
rudiment of one. I suppose, then, that the primeval
Tococa the ancestor of all the existing species had
no sac at all on the leaves, but that a few ants hav-
ing sheltered in the deep narrow angles formed by
the junction of the prominent lateral ribs with the
midrib, found the axils perforable, and having thereby
reached the interior of the leaf, scooped out the
parenchyma between the two surfaces. The leaves
of any plant, when its juices are sucked away by
insects (Aphides, for example) or otherwise diverted
from their usual course on the one surface, are apt to
become bullate on the opposite surface ; hence it is
easy to understand that, when mined by ants, the
cuticular tissue of both surfaces should expand out-
wardly and contract laterally so as to form a sac,
whose further enlargement would be effected by the
continual crowding in of ants. [This process re-
peated on the plants for many generations would
induce an hereditary tendency to the production of
sac- bearing leaves.] It is natural that the ants
should select the largest leaves, as affording most
room for their operations ;
but
that one leaf of
each pair should be often larger than the other
depends on some cause anterior to any
action of