1907.] Wheeler, Fungus-gronnng Ants of North America. 685 



workers. The latter have another function, namely that of weeding the 

 garden and keeping down the growth of spores belonging to alien fungi. 

 INIoeller emphasizes the remarkable fact that the gardens are pure cultures 

 although the hairv, rough-bodied workers must be continually bringing into 

 the nest all sorts of spores and bacteria. It is probable also, that the minims 

 are instrumental in producing the "kohlrabi heads" as these arc not devel- 

 oped when the mycelium is grown in artificial culture media apart from the 

 influence of the ants. He summarizes the results of this portion of his 

 studies in the following words: "All the fungus-gardens of the Atta species 

 I have investigated, are pervaded with the same kind of mycelium, which 

 produces the 'kohlrabi clusters' as long as the ants are cultivating the 

 gardens. Under the influence of the ants neither free aerial h}-phfe nor 

 any form of fruit are ever developed. The mycelium proliferates through 

 the garden to the complete exclusion of any alien fungus, and the fungus 

 garden of a nest represents in its entirety a pure culture of a single fungus. 

 The fungus has two different forms of conidia which arise in the garden 

 when it is removed from the influence of the ants. The h^-phse have a 

 very pronounced tendency to produce swellings or diverticula, which show 

 several more or less peculiar and clearly differentiated variations. One of 

 these which has presumably reached its present form through the influence 

 of cultivation and selection on the part of the ants, is represented by the 

 'kohlrabi heads'." 



A number of experiments were undertaken by Mceller for the purpose 

 of ascertaining the behavior of the fungus in the absence of the ants. Under 

 these conditions he found that the mycelium produces aerial hyphae, the 

 "kohlrabi clusters" and "heads" disappear and soon the fungus breaks 

 up into masses of bead-like conidia. "As long as the ants are active in their 

 garden, there is never either in it or in its immediate vicinity the slightest 

 trace of an alien fungus, and, under these circumstances, the mycelium per- 

 vading the garden never produces aerial hyphse or conidia." If, however, 

 a few of the ants happen to be left in the garden, the development of aerial 

 hyphae is retarded, and though ^Nlceller did not observe the process directly, 

 he is certain that these hyphae must be bitten off by the ants as soon as they 

 make their appearance. "A relatively very small number of workers suf- 

 fices to restrain the growth of the aerial hyphse. But if the number is too 

 small, the aerial filaments begin to appear sporadically. The ants are 

 unable to move about in the dense growth of sprouting filaments and have 

 to beat a retreat before the rapidly rising hyphal forest. This, however, as 

 soon as it has acquired a little headway, proliferates mightily, and it is an 

 amazing sight to behold the poor insects, tirelessly active till the last mo- 

 ment, fleeing before their OAvn food-plant. If some of the larvae and pupte 



