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, sometimes called 

 " traveler's tree," whose great flower clusters are pro- 

 tected by large 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 



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

 pedium, and rubbing against an anther.— After 

 Gibson. 



