392 NOTES OF A BOTANIST 
so that even if the leaves of this Tococa were sac- 
ciferous, they could not aftbrd 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 
