RELATIONS OF ANTS TO VASCULAR PLANTS. 317 



in Cobcca scandcns and gains access to these organs by biting a hole 

 through the base of the petal, a habit which has also been observed in 

 bees that are confronted with flowers whose nectaries they are unable 

 to reach in any other way. But there are graver maladjustments 

 between plants and ants, maladjustments that may lead to the death of 

 the insects in great numbers or even to the extinction of their colonies. 

 I have described a number of such cases in a recent paper (i9o6/). 

 The abundant and sticky juices of Silcne, Lactuca and Helianthus 

 exuding onto the stems or petioles often entrap and kill numbers 

 of ants (Fig. 180). We owe to a similar property of the resiniferous 

 conifers of the Tertiary the preservation of the ants in the Baltic 

 and Sicilian ambers. Our North American pitcher plants (Sarra- 

 ccnia) also entrap, kill and digest enormous numbers of ants in the 

 liquid at the bottoms of their ascidia. The ants most frequently found 

 in these modified leaves are Cremastogaster pilosa, a species which will 

 sometimes even nest in the dead pitchers of a plant whose active green 

 leaves are busy killing them off in great numbers a singular commen- 

 tary on the " intelligence " of these insects, especially when we stop to 

 consider that C. pilosa is one of the ants that constructs such beautiful 

 sheds over aphids and coccids. 



Another hostile relationship between plants and ants has been 

 described in detail by Holmgren (1904). He observed that the mound 

 nests of Formica e.vsccta in the bogs of Lapland are gradually invaded 

 and eventually so completely covered with a dense carpet of moss 

 (Polytrichnm strictitin) that the ants are either driven away or de- 

 stroyed. This moss is in turn replaced by a carpet of Sphagnum, in 

 which many plants eventually take root, so that the ants are instru- 

 mental in forming the hummocks of moss and hence facilitate the 

 growth of peat-forming vegetation. In the bogs of Prussia, according 

 to Kuhlgatz ( 1902), the Mynnica nests are invaded in a similar manner 

 by P. strict urn, and I have been able to observe various stages in the 

 extinction of colonies of our North American F. exsectoidcs by an 

 allied moss, P. commune (i9o6/). This moss starts in the form of a 

 narrow zone around the base of the huge mound nests (Fig. 181) 

 and gradually grows upward till it completely envelopes their summits 

 with a dense mat and either smothers the colony outright or compels 

 it to emigrate (Fig. 182). 



