Annual Report of the Board of Regents of the Smithsonian Institution for the Year ending June 30, 1896 pp. 421-422 (1898)
The biologic relations between plants and ants
Dr. Heim
Associate of the Faculty of Medicine at Paris

Along the edges of the leaves of the Rosa Banksiae are found perifoliary nectaries that attract great numbers of a large black ant (Camponotus pubescens). The presence of these ants preserves the rose from the attacks of a hymenopterous insect (Hylotoma rosae). We owe an interesting experiment upon this subject to Beccari. On a branch of Rosa Banksiae attacked by ants he placed a branch of another rose bush attacked by the larvae of Hylotoma. Incommoded by the ants, these larvae took refuge upon the youngest buds, unprovided as yet with nectaries, and consequently not visited by ants. It is to be remarked that the Banks rosebushes, which are rarely or never attacked by Hylotomas, are destitute of prickles. We may probably admit that there is a correlation between the presence on plants of thorns or prickles and that of leaf-eating insects. Is it not due to the protection given by ants and other sting-bearing hymenoptera that the Banks rosebushes attain the great age that some of them are known to do? We may cite as an instance one of these bushes planted in 1803 by Bopland in the garden of the marine hospital at Toulon, which has a stem a meter in diameter at the base and bears each year from fifty to sixty thousand flowers.

The leaves of peach, apricot, and cherry trees may, as there is reason to suppose, be derived from compound leaves. The nectaries which they carry on the petioles should then have the significance of aborted leaflets filled with sweet stores.

The extranuptial nectaries belong not only to phanerogams, they are also found in the vascular cryptogams. We find extranuptial nectaries at the base of sterile pinnules in Pteris aquilina and Acrostichmn scandens. In Acrostichum Horsfieldii we find at the base of the sterile leaflets, and also frequently at the base of the fertile leaflets, small auricles that seem to be nectariferous.

Francis Darwin, who discovered the nectaries in the fronds of Pteris aquilina, does not believe in the defense afforded by the ants against phytophagous insects. In favor of this theory, however, is the fact that the secretion of nectar takes place only in young fronds whose tissue, yet tender, is an easy prey for the leaf eaters. It should also be noted that Pteris aquilina is a cosmopolitan plant. It may not attract insects in England, and yet do so in other regions. Besides, in France, an Halictus has been seen to visit the fronds of this fern. Ferns are not exempt from attacks by plant-eating insects. Beccari saw a Cyrtomium plicatum, cultivated in a court, with all its fronds covered by a green caterpillar. Not far from this fern were found stems of Pteris aquilina, which had been reduced to small fragments by an insect. Beccari supposes that the same larva attacked simultaneously the fronds of the two ferns.

It is not only the normal organs of plants which may offer a sugary secretion prized by the ants. Certain galls may be considered as true foliary nectaries of parasitic origin. “The galls of Andricus testaceipes (Aphilotrix Sieboldi),” says Adler, “are greatly exposed to the attacks of various parasites of the genera Torymus and Synergus. It is interesting to observe how the gall has indirectly evolved a means of protection. Its red, sappy envelope secretes a sticky fluid eagerly sought after by ants, and that they may enjoy this nectar undisturbed, they build with sand and earth a perfect dome over the galls, and in this way provide the inhabitants with the best protection against their enemies.