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 con- 

 tinuous with the leaf, and Diviner to the latter a 



