GROWTH AND BIOMINERALIZATION OF CELTIS-OCCIDENTALIS (ULMACEAE) PERICARPS

A study of the growth and biomineralization of extant pericarps of Cel tis ocidentalis was conducted to clarify the chemical nature of the ab undant fossil endocarps of Celtis, and to track changes in elemental c oncentrations with time in a drupe with a highly mineralized endocarp. Fruits were collected at 7-10 days intervals through 1 growing season (155 days). A comparison of size and weight changes showed cyclic gro wth patterns comprising three distinct periods. Pericarp differentiati on into three discrete layers was observable after 46 days of fruit gr owth. Simultaneous formation of columnar cells of the mesocarp and gen eration of a silica framework accompanied calcium carbonate deposition within the endocarp. Energy dispersive x-ray spectrometry showed an i ncreasing accumulation of calcium within the endocarp from 206 to 904 counts per sec (cps), while silicon increased from 4 to 133 cps. X-ray diffraction analysis showed aragonite and opal within mature endocarp s. Increasingly positive stable carbon isotope ratios (delta(13)C) fro m -26.6 parts per thousand to -21.0 parts per thousand were obtained f rom the endocarp while those for the exo/mesocarp became slightly more negative as the growing season progressed. This is the first report o f the growth and mineralization of drupaceous fruits which accumulate high concentrations of silica and calcium carbonate. Furthermore, the dramatic changes in mineralization, microstructure and stable carbon i sotope ratios during maturation must be considered in understanding mo dern plants and interpreting the fossil record.