136 
PLANT STUDIES 
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 
<J int f the stem * 
(3) Isolation.- 
The leaves of cer- 
tain plants form 
water reservoirs 
about the stem. 
To ascend such a 
stern, 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. 
