Functional Anatomy of the Calcium-Excreting System of Gleditsia triacanthos L.

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Abstract
Very young, growing leaflets of G. triacanthos L. (honey locust) contain a few druse-shaped calcium oxalate crystals (DC) in the phloem parenchyma of the midvein and in the mesophyll near veins. During leaf expansion and maturation, prismatic crystals (PC) form in abaxial bundle sheath cells (BSC) associated with sclerenchymatous fibers of the major veins, while terminal veins remain free of PC. BSC without PC are elongate, but most PC-containing BSC are isodiametric, suggesting that cell division precedes the formation of PC. Large numbers of very small, irregular crystals form in the walls of the epidermis cells, but not in the stomatal apparatus. In mature, fully expanded and differentiated leaflets, large PC develop in virtually all BSC of major and minor veins, and DC form in the mesophyll, particularly in the palisade parenchyma; as the cells of the latter differentiate into crystal idioblasts, they divide, increase in diameter, and undergo structural changes. In Gleditsia stems, PC are deposited in suberizing cells near fibers and sclereids, and DC form in parenchymatic cells of cortex, phloem and rays. The extended accumulation of calcium oxalate crystals (CX) in stem and leaf tissues along the transport system of the tree implies that, throughout the growing season, Gleditsia removes calcium from the transpiration stream, i.e., the entire tree acts as one large water-softening column. The formation of insoluble CX as sinks for excess Ca2+ is postulated to be the solution to the physiological problem of maintaining low cytoplasmic Ca2+ levels in the face of a continuous influx of Ca2+ with the transpiration stream.