BIOLOGIC RELATIONS BETWEEN PLANTS AND ANTS. 445 



eral such enlargements are contiguous on the same root, a cbaplet is 

 thus formed with more or less regular beads. 



It is admitted that these tubercular roots may offer an asylum for 

 ants. But the study of these tropical plants is as yet very incomplete. 

 We can not even affirm that the tubercles are hollow. Many appear, 

 when dry, to be full of spongy tissue, loosely arranged toward the 

 center. We might be inclined to believe, with Beccari, that later this 

 tissue is destroyed by ants, who thus hollow out a regular cavity 

 within these tuberculous roots. But it should be noted that certain 

 species of this genus have tubercular moniliform roots that are entirely 

 solid, and only in certain specimens, even in species with hollow tuber- 

 cles, do we find perforations allowing a communication between the 

 inside and the exterior, which might, indeed, have easily been the work 

 of ants. 



The Pachycentrias do not seem to be provided with extra floral nec- 

 taries capable of attracting ants. The insects are, then, attracted to 

 the plants only by the chance that they may be able to install them- 

 selves in the tubercle. But it should be noted that a type closely 

 related to the Pachycentrias, Pogonanthera robusta, has a limbus pro- 

 longed at its base into two auricles, decurrent upon the petiole, that 

 appear to be nectariferous. These nectaries attract ants, but the roots 

 of these epiphytic plants are not, like those of the Pachycentrias, tuber- 

 culiform, but normal and incapable of affording lodgment to the ants 

 attracted by the nectaries. This fact may, perhaps, give us a clew to 

 the way in which biological relations were first established between 

 the ants and the Pachycentrias. The ancestors of these latter plants 

 were, without doubt, like the Pogonantheras, provided with extra-nup- 

 tial nectaries frequented by ants. These ancestors gave rise on the one 

 hand to types that preserved the primitive features, as in Pogonanthera, 

 and on the other to types better adapted to myrmecophilism, as in 

 Pachycentria. The ants, impelled by their hereditary habits to visit 

 those plants provided with foliary nectaries, continued to visit them 

 even when those nectaries were undergoing atrophy. Profiting by the 

 tendency of these plants to form tuberculoiis roots, they have progress- 

 ively transformed these tubercles into ant nests, causing, by the irrita- 

 tion of their presence, a more marked hypertrophy of those organs. In 

 a word, the Pogonantheras, utilizing the protection the ants afford 

 against plant-eating animals, may have found it a real advantage to 

 give those insects a mere asylum instead of offering them nutritive 

 matters in the form of , nectar. It is evidently an economy to the plant 

 to offer simply a lodging to its defenders instead of both food and lodg 

 ing, as does the Acacia comigera, for example, and other plants that 

 both feed and shelter the ants. If we accept this interpretation, which 

 has only the value of an hypothesis, we would be led to regard the 

 Pogonantheras as having economical myrmecophilous features. In the 

 Pogonantheras, as in the myrmecophilous Rubiacese, the ants take up 



