| Riassunto: | The orientation of photosynthetic surfaces affects the interception of photosynthetically active radiation (PAR), as well as the temperatura of the photosynthetic tissues. Plante face the problem of displaying their photosynthetic surfaces in order to obtain adequate levels of PAR interception without reaching dangerously high temperatures. In the Zapotitlan Valley, Puebla (México), Opuntia pilifera cladodes exposed to direct solar radiation have inclinations chiefly between 80° and 90fl, as well as a preference for an E-W orientation. This pattern is less marked in shaded plante, in which dadode inclination is more variable and which do not present a preferred azímuth. Given the geographic location and climate of the study area, the spatial display of O. pilifera cladodes is interpreted as an adaptive solution to the conflict between PAR interception and thermoreguiation.
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