412 



LECTURE XVI. 



B. 



FlG. 46 (after Darwin). Petiole of Solanum jasminoides clasping a stick. 

 A, transverse section of free petiole ; B t transverse section of a petiole some 

 weeks after it had clasped a stick. 



Treub has observed the increased growth in the thickness of the hooks 

 in consequence of contact in the following plants : Uncaria (Rubiaceae), 

 Ancistrocladus (Dipterocarpeae), Artabotrys (Anonacede), Luvunga (Au- 

 rantiacese), Olax (Olacineae), Hugonia (Linaceae), and Strychnos (Logani- 

 aceae). The following figure exhibits the difference in thickness between 

 a hook which has and one which has not come into contact with a sup- 

 port. 



FIG. 47 (after Treub). Hooks of Uncaria ovalifolia. 



