136 



PLANT RELATIONS. 



the imsuitable insects, wliicli Kerner calls " unbidden 

 guests/' are ants, and adaptations for reducing their visits 

 to a minimum may be taken as illustrations. 



(1) Hairs. — A common device for turning back ants, 

 and other creeping insects, is a barrier of hair on the stem, 

 or in the flower cluster, or in the flower. 



(2) Glandvlar secretions.— In some cases a sticky 

 secretion is exuded from the surface of plants, which 



eifectively stops 

 the smaller creep- 

 ing insects. In 

 certain species of 

 catch-fly a sticky 

 ring girdles each 

 joint of the stem. 



(3) Isolation. — 

 The leaves of cer- 

 tain plants form 

 water reservoirs 

 about the stem. 

 To ascend such a 

 stem, therefore, a 

 creeping insect 

 must cross a series 

 of such reservoirs. 

 Teasel furnishes a 

 common illustration, the opposite leaves being united at 

 the base and forming a series of cupts. More extensive 

 water reservoirs are found in Bilhergia, sometimes called 

 "travelers tree," whose great flower clusters are pro- 

 tiMjted by large reservoirs formed by the rosettes of leaves, 

 which la-eeping insects cannot cross. 



(4) Later. — This is a milky secretion found in some 

 plants, as in milkweeds. Caoutchouc is a latex secretion 

 of certain tropical trees. Wlien latex is exposed to the 

 air it stifEens immediately, becoming sticky and flnally 



Fig. 149. A bee escaping from the pouch of Cypn- 

 pediym, and rubbing against an anther. — After 

 Gibson. 



