276 ADAFfATIONS AMONG ALPINE PLANTS 



do not emit scent in the daytime, and thus do not 

 attract many visitors. 



The inflated calyx of Silene cucuhalus (p. 164) 

 probably serves to protect the honey from robber 

 bees, who try to steal it by biting through the base of 

 the corolla. 



Some subtropical and tropical plants have remark- 

 able methods of protecting themselves from unbidden 

 guests whether visitors to the flowers or leaf- 

 destroyers. They actually hire other insects to 

 protect them, and produce nectaries outside the 

 flowers, as an inducement to their protectors to stay 

 and fight their battles for them. Kerner states that 

 the flower-heads of Centaurea alpina, and some other 

 Composites in Southern Europe, maintain a colony of 

 ants by means of special nectaries, on the involucral 

 bracts of the flower-heads, on which they feed. These 

 ants act as a body-guard against certain beetles or 

 other unbidden guests, which eat the flower-buds. 

 The ants are able to drive away the beetles, ejecting 

 formic acid if necessary, and they thus perform a 

 good service to the plant. Several other tropical 

 plants go much further. They form a group called 

 myrmecophilous plants. They not only feed, but 

 actually house standing armies of fighting ants, which 

 keep off leaf-cutting insects and other unwelcome or 

 injurious visitors. 



We now turn to the other class of unbidden 

 guests, the crawling insects. A very simple device, 

 which is sometimes adopted, to keep such insects 



