452 BIOLOGIC KELATIONS BETWEEN PLANTS AND ANTS. 



species is very instructive from the point of view of the genesis of these 

 foliary bursee. In tbis species, on the under surface of the liinbus near 

 its base, in the angle formed by the median nervure and the two lateral 

 primary nervures, there may be observed small cavities surrounded by 

 hair. The analogy of these organs to the acaro-cecidia (that is to say, 

 to galls caused by certain acarids) of laurels and various other plants 

 is striking. If we conceive such an organ increasing in size without 

 enlarging its orifice we will obtain exactly the foliary bursa of the above- 

 cited Melastornacese. 



The galls of laurels and some other plants, Viburnum liicidum, for 

 example, are inhabited by acarids. It is not irrelevant to recall here 

 that these acaro-domatias (to use the expression of Lundstrom) are by 

 no means pathological productions injurious to the plant. The acarids 

 that cause them render, on the contrary, eminent services by clearing 

 the plant of the spores of parasitic or saprophytic fungi found on the 

 surface of the leaves. There is, then, no improbability in supposing 

 that primitively the foliary bursse of the Melastomaceai have been 

 acarid galls in which the ants sought an asylum provisionally. Finding 

 the dwelling suitable, they installed themselves there permanently, and 

 the irritation of the plant caused by them may have occasioned the 

 eusuing hypertrophy of the bursas. In the Melastoinacea? the lodging 

 organ seems to be undoubtedly of parasitic origin. 



Let us now attempt to extract from this mass of facts some general 

 views. 



Primitively the relations between ants and plants were as simple as 

 possible — those of the eaters and the eaten. Such are at the present 

 time the relations of the harvester ants, especially of the leaf cutters, 

 to the plants which they despoil. But we should note that already the 

 plants from which the ants harvest obtain some advantages from that 

 harvesting. A number of seeds are sacrificed, but some, escaping 

 the voracity of the ants, are disseminated by them and thus truly 

 aided by those insects in their struggle with rival species for existence. 

 From this dissemation, at first accidental, comes the normal mimicry of 

 the cocoons of ants shown by some seeds. 



As the industry of the ants becomes more developed they no longer 

 content themselves with merely gathering vegetable products. They 

 undertake agriculture, and the plants cultivated by them are, by the 

 very care they receive, favored in their struggle with rival species, as 

 are cereals cultivated by man, which have no longer to struggle with 

 indigenous species. A number of ants also content themselves with 

 sugary liquids, such as honeydew and nectar. Primitively they seem to 

 have been satisfied to gather the honeydew diffused on the surface of 

 leaves; afterwards their suction, localized at special poiuts upon the 

 foliaceous organs, may have led to the formation of extra floral nec- 

 taries. These organs may have served the plant in two ways — first, by 

 attracting to its surface auts that would protect it against phytophagi; 



