STEM TILTING AND PSEUDOCEPHALIUM ORIENTATION IN CEPHALOCEREUS-COLUMMA-TRAJANI (CACTACEAE) - A FUNCTIONAL INTERPRETATION

This paper analyzes the functional implications of stem tilting and ps eudocephalium orientation in the giant columnar cactus Cephalocereus c olumna-trajani. This species shows a consistent northern orientation o f its pseudocephalium (a nonphotosynthetic hairy structure where flowe rs are produced) and stem tilting in the same direction. Analysis of p seudocephalium orientation was made on field data gathered from subpop ulations of C. columna-trajani from slopes with different exposures. A dditionally, from morphometric characteristics measured in the field, a model cactus was constructed with the purpose of simulating radiatio n interception by different morphologies. Variations of this model cac tus allowed the simulation of irradiance on erect and tilted facti, as well as on plants with varying pseudocephalium orientation. Results o f irradiance interception by different morphologies were related to ac tual data of growth rates, flowering period, and rainfall and temperat ure patterns on the study zone. Sampled individuals of C. columna-traj ani showed a significant north-northwest pseudocephalium orientation ( angular mean = 339 degrees +/- 22 degrees). Simulations showed that ti lted cacti with pseudocephalia facing northwards increase yearly inter ception of direct solar radiation by the whole plant compared to erect cacti with or without a pseudocephalium (2 and 7% increase, respectiv ely), and with tilted cacti with the pseudocephalium facing away from the north (9-10% increase). Additionally. the observed morphology decr eases radiation interception during the hottest and driest period of t he year. From our results, pseudocephalium orientation and stem tiltin g in C. columna-trajani appear to be morphological adaptations that al low the fine-tuning of a columnar morphology to its thermal and radiat ion environment. However, the cost of tilting in this giant columnar c actus is that branching (which increases photosynthetic area and repro ductive output) appears to be almost impossible without serious risk o f stem breakage.