136 



PLANT RELATIONS. 



the unsuitable insects, which 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) Glandular secretions. — In some cases a sticky 

 secretion is exuded from the surface of plants, which 



effectively 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 cups. More extensive 

 water reservoirs are found in Bilbergia and Ravenala 

 (" traveler's tree "), whose flower clusters are protected by 

 reservoirs formed by the rosettes of leaves, which creeping 

 insects cannot cross. 



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

 plants, as in milkweeds. Caoutchouc is a latex secretion 

 of certain tropical trees. When latex is exposed to the 

 air it stiffens immediately, becoming sticky and finally 



''^Ci 



Fig. 142. A bee escaping from the pouch of Cypri- 

 pedium, and rubbing against an anther. — After 

 Gibson. 



