Crystal masses formed on the top and within the cells of sectioned Cercis canadensis (Fabaceae) pulvinus tissue during preparation for SEM, were determined to form during the fixation process. The number of crystals is greater when sections are fixed in fresh FAA (formaldehyde, acetic acid, ethanol) than in 4% glutaraldehyde in a sodium phosphate buffer. Normal crystal formation, a single druse crystal within a vacuole, was observed in samples that were not fixed or fixed and then sectioned. Analysis of the crystals using energy-dispersive spectroscopy (EDS) showed peaks for calcium. Time series experiments in which sectioned pulvinus tissue was fixed in FAA and in 50% ethanol for 30 minutes up to 24 hours showed that crystal formation occurred within 30 minutes in both the FAA and in the 50% ethanol. Crystal development was followed microscopically. A pulvinus is a differentiated motor organ that is responsible for leaf movement. Differences in the turgor pressure between regions of cortical cells within the pulvinus causes it to bend and straighten.. Pressure changes are correlated with the movement of ions such as K+, Ca2+, Cl-, and malate - across cell membranes. The calcium in the crystals formed comes from the plant itself. but the other components could come from either the plant or the fixatives. Calcium chloride is soluble in alcohol, acetone and acids (all fixatives), calcium oxalate and calcium malate are soluble in all but acetic acid (FAA and 50% ethanol fixatives) and calcium phosphate (buffer in glutaraldehyde fixative) is insoluble in alcohol. Other components of the calcium crystals will be analyzed in the future. Meanwhile, considering the number of calcium crystals formed, calcium is one of the major ions involved in the movement of the pulvinus of C. canadensis.

Key words: calcium crystals, Cercis canadensis, EDS, fixation, pulvinus