we measured the differences in stiffness between extrafloral nectaries of v. faba and the tissue that surrounds them . we mounted 20 stipules from 20 , 18-day - old plants on adhesive tape that was attached ( sticky side up ) to the surface of a piece of flat styrofoam . we did a penetrometer test to measure the initial modulus of rigidity of the spot being tested , using a texture analyzer ( ta.xt2i , stable micro systems , surrey , uk ) fitted with a 10 stainless steel cone moving at 0.5  mm / s . in each stipule , one measurement was done in the center of the extrafloral nectary and 3 measurements were done in random locations on the stipule . for each stipule , an average stipule stiffness  was calculated from these 3 measurements and this value was compared with the measured stiffness of the extrafloral nectary on that stipule .
abstractextrafloral nectaries play an important role in plant defense against herbivores by providing nectar rewards that attract ants and other carnivorous insects 
 . however , extrafloral nectaries can themselves be targets of herbivory , in addition to being exploited by nectar - robbing insects that do not provide defensive services . 
 we recently found that the extrafloral nectaries of vicia faba plants , as well as immediately adjacent tissues , exhibit high concentrations of chemical toxins , apparently as a defense against herbivory . here 
 we report that the nectary tissues of this plant also exhibit high levels of structural stiffness compared to surrounding tissues , likely due to cell wall lignification and the concentration of calcium oxalate crystals in nectary tissues , which may provide an additional deterrent to herbivore feeding on nectary tissues .