Articulated cork in Calotropis procera (Asclepiadaceae)
The cork of the small tree. Calotropis procera, which grows in a very hot district of Israel, is remarkable for its thickness and brittleness. The cork of the stems and large branches is composed of longitudinal ridges that extend over several internodes. The cork ridges have deep fissures around the circumference at almost every node. I propose that these nodal fissures in the cork serve as joints for two functions: (1) to prevent breakage of the fragile cork layer when branches bend under wind stress, and (2) to allow thermal expansion while avoiding tissue cracking on extremely hot days, similar to the joints left between steel rails in railroads and between concrete beams in bridges.