136 



PLANT STUDIES 



the uusuitablo insects, wliieli Kenier calls " unbiilden 

 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 a,nts, 

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

 or in the iiowcr cluster, or in the flower. 



(:.') Ghtiiduliir scrrcfioHS. — In some cases a sticky 

 secretion is exuded fr(jm the surface of plants, which 



eifoctively stops 

 the smaller creep- 

 ing insects. In 

 certain species of 

 catch-lly a sticky 

 ring girdles each 

 joint of the stem. 



[o) Isolation. — 

 The leaves of cer- 

 tain plants form 

 ■\i'atcr reservoirs 

 about the stem. 

 To ascend such a 

 stem, therefi.ire, a 

 creejiing insect 

 must cross a. series 

 lit' such I'csei'voirs. 

 Teasel furnishes a 

 common illustration, the (i])posite leaves being united at 

 the l)ase and forming a sei'i(>s of cups. ]\Iore extensive 

 water reservoirs are found in BiJbrrijia, sometimes called 

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

 tected by large reservoirs formed by the rosettes of leaves, 

 which cr(H'[)ing insects cannot cross. 



(4) Lati'.r. — 'Idiis is a milky secretion fouiul in some 

 jilants, as in niillvweeds. t'adutchouc is a latex secretion 

 of certain tropical trees. When latex is ex])osed to the 

 air it stilfens immediately, becoming sticky and linally 



Fin. 14'i. A tii'C cer;i])ini; frmn flie pouch of Ctjj 

 jri'd'nnn. iiiid rnbljiiig a^aill^L an aiitlRT, — \i 

 CJlI'.SON. 



