ANTS AND PLANT-STRUCTURE 399 



of tropical America, the knee of the petiole may 

 sometimes be seen hollowed and enlarged by ants ; 

 [but the action of these insects has not been 

 maintained with sufficient constancy to render the 

 swelling a permanent character in any species of 

 Cassia I have met with]. 



Ants congregate on the pods of some Cassias 

 and other plants which have seeds in sweet pulp ; 

 and on those parts of any plant where* they find 

 suitable food, in the shape of mucilaginous exuda- 

 tions, etc. ; but they mostly sojourn there just so 

 long as that food lasts, and no longer ; or otherwise 

 they merely visit the plants for the sake of collecting 

 their products and carrying them off at once to a 

 permanent storehouse elsewhere. 



3. Of Inflated Branches 



Ants' nests in swellings of the branches are 

 found chiefly in soft-wooded trees of humble growth, 

 which have verticillate or quasi-verticillate branches 

 and leaves, and especially where the branches put 

 forth at the extremity a whorl or fascicle of three 

 or more ramuli ; then, either at each leaf- node or 

 at least at the apex of the penultimate (and some- 

 limes of the ultimate) branches, will probably be found 

 an ant-house, in the shape of a hollow swelling of 

 the branch ; communication between the houses 

 being kept up, sometimes by the hollowed interior 

 of the branches, but nearly always by a covered 

 way along their outside. 



The genus Cordia (Boraginaceoe) affords many 

 examples of this structure. One of the rather 

 artificial sections into which Cordia is divided in the 



