Orientations of Terminal Cladodes of Platyopuntias

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Abstract
The orientations of terminal unshaded cladodes of 23 spp. of platyopuntias were observed in North America, South America, Australia and Israel. When the seasonality of rainfall favored cladode development in the winter and the site was located above 27.degree.N, the cladodes tended to face north-south. In all other cases without topographical blockage of incoming radiation, the tendency was to face east-west. Terminal cladodes of Opuntia phaeacantha var. discata and O. stricta tended to face north-south in Israel but east-west in the USA. Such dissimilar orientation patterns also occurred for cladodes of O. chlorotica at 2 sites in the Sonoran Desert and for O. basilaris var. basilaris developing at different seasons at a single site. Contrary to previously published observations, cladodes of O. ficus-indica and O. compressa had a significant tendency to orient. When topographical features affected the direction of prevailing photosynthetically active radiation (PAR), the preferred orientation of terminal cladodes was changed accordingly. The preferred direction always maximized the interception of PAR, which is often a limiting factor in the productivity of cacti, even in a putatively high radiation environment.