38 ON PLANTS AND INSECTS. [lect. 



offer an almost impenetrable barrier to ants. Some 

 species of plants are quite smooth, excepting just below 

 the flowers. The common but beautiful Cornflower 

 (Centaurea cyanus) is quite smooth, but the involucres 

 forming the flower-head are bordered with recurved 

 teeth. In this case, neither the stem nor the leaves 

 show a trace of such prickles. 



The same consideration throws light on the large 

 number of plants which are more or less glutinous, a 

 condition generally produced, as, for instance, in the 



Fig. 34. — Knautia dipsacifolia. 



flowers of the Gooseberry and of Linncea borealis (Fig. 

 35), by the presence of glandular hairs. Kerner has 

 called attention to a very interesting illustration afforded 

 by Polygonum amphibium. In this species the stigma 

 projects about one-fifth of an inch above the flower, so 

 that if ants could obtain access, they would steal the 

 honey without fertilizing the flower ; a flying insect, 

 on the contrary, alighting on the flower, could scarcely 

 fail to touch the stigma. 



