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 



