Growth and Biomineralization of Celtis Occidentalis (Ulmaceae) Pericarps
Cowan, Marlina R, Gabel, Mark L, Jahren, A Hope, Tieszen, Larry L, The American Midland Naturalist
ABSTRACT.-A study of the growth and biomineralization of extant pericarps of Celtis occidentalis was conducted to clarify the chemical nature of the abundant fossil endocarps of Celtis, and to track changes in elemental concentrations 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 growth patterns comprising three distinct periods. Pericarp differentiation into three discrete layers was observable after 46 days of fruit growth. Simultaneous formation of columnar cells of the mesocarp and generation of a silica framework accompanied calcium carbonate deposition within the endocarp. Energy dispersive x-ray spectrometry showed an increasing 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 endocarps. Increasingly positive stable carbon isotope ratios (813C) from -26.6%c to -21.0%o were obtained from the endocarp while those for the exo/mesocarp became slightly more negative as the growing season progressed. This is the first report of 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 isotope ratios during maturation must be considered in understanding modern plants and interpreting the fossil record.
Cels L., often known as hackberry, includes approximately 70 species worldwide and about five in North America (Preston, 1989). In the United States, the genus extends from the East Coast to the Rocky Mountains and in scattered areas of the far West. Although Celtis fruits (Fig. 1) have been the subject of several past studies (Yanovsky et al., 1932; Fulbright et al., 1986), observations of their growth and mineralization are lacking. Among drupaceous fruits cultivated commercially, including peaches, apricots, plums and cherries, researchers have discovered characteristic patterns of development. Lilleland (1930, 1932) reported cyclic growth patterns in apricots (Prunus armeniaca L.) and peaches (P persica Batsch) which he described as "rapid growth," "depressed growth" and "final swell." Lott (1933) indicated a similar pattern in peach (P persica Batsch) drupes. Tukey (1934) reported a three-stage sequential development in se cerasus L. (sour cherry). Growth of at least one variety of plum (P domestica L.) has only slight cyclic growth (Lilleland, 1934).
Simkiss and Wilbur (1989) defined biomineralization as the conversion by organisms of ions in solution into solid minerals. In 1982, Arnott outlined three systems of biomineralization in plants, with products including calcium oxalate, carbonates and silicon.
Cystoliths and calcified or silicified cells have been widely reported. Solereder (1908) noted the presence of silica and carbonates in the wood and leaves of many genera within the Urticales (his Urticaceae). Satake (1931) and Pireyre (1961) found cystoliths in leaves of Celtis. Werner (1931) reported the presence of "Nebencystolithen" (secondary cystoliths) in leaves of Celtis occidentalis L. Metcalfe and Chalk (1950) described hairs, as well as epidermal cell walls, of Celtis leaves as calcified or silicified, and noted the presence of cystoliths in the epidermis and silicification in the cortex of the young stems of Celts and Ulmus. In an anatomical study of the Ulmaceae, Schweitzer (1971), found cystoliths composed of both calcium carbonate and silica and that trichomes were often silicified. Setoguchi et al. (1986) discovered calcium, silicon and magnesium in cystoliths in the leaves of Celtis sinensis. Okazaki et al. (1991) described similar results on the same species. Yanovsky et al. (1932) reported the presence of silica and carbonates in fruits of extant Celtis, but there are no reports of the chronology or levels of deposition in the fruit walls. …