Calcium Oxalate Many plants detoxify soluble oxalic acid as insoluble calcium salts. Calcium oxalate crystallizes in characteristic forms; these shapes can be very important for diagnostic purposes (Figures 7.12 and 7.13). The exact crystallographic forms of these crystals are determined by measuring their angles. This is difficult and many technical and nontechnical terms are used to describe them. Calcium oxalate crystals are birefractive (birefringent), which makes them visible in polarized light (Figure 7.13a-c). Druses are clusters of calcium oxalate crystals that are shaped like diamonds (Figure 7.13a); raphides are composed of slender, needle-like crystals (Figure 7.13b). Prismatic crystals are rhomboidal in shape
(Figure 7.13c). The following various types of crystals can be found:
Prismatic crystals are rhomboidal or prismatic in shape, with plane faces (e.g., Allium sativum; Figure 7.13d). Groups of fibers or fibers along vascular bundles are often accompanied by a large number of solitary prism crystals, forming a calcium oxalate prism sheath. Cluster crystals (druses) are spheroidal aggregates of calcium oxalate having numerous faces and sharp points (e.g., Rheum spp., Senna alexandrina, Ginkgo biloba; Figure 7.13e).
Acicular crystals are thin and elongated needle-like crystals that taper at both ends. Various texts may distinguish raphides and needles. Most commonly, the term raphide refers to relatively long needles that are typically found in large numbers, aligned parallel to one another, and aggregated into bundles (e.g., Aletris farinosa, Cephaelis ipecacuanha, Chamaelirium luteum) (Figure 7.13f). They occur in idioblasts (cells that differ markedly from surrounding cells), often embedded in mucilage. Bundles of raphides may be broken apart during tissue preparation, making it appear as though the needles naturally occur individually. Raphides are not always aligned parallel
to one another in plant tissues (e.g., milkweeds, Epilobium spp.). They occur rarely, are characteristic of certain plant families (e.g., Lauraceae, Liliaceae, Rubiaceae), and can therefore be helpful when identifying unknown plant material. Relatively short, needle-like crystals that can be relatively wide at the center are sometimes referred to simply as needles. These are often found in great numbers disordered in one corner of a cell (e.g., in Gentiana lutea root and in covering trichomes in members of the mint family; Lamiaceae).
Styloids are long and prismatic in shape with flat faces. They may be found singly (e.g., Quillaja saponaria; Figure 7.13g) or in pairs (e.g., Inula helenium, Iris germanica, I. pallida; Figure 7.13h) and are rare, occurring primarily in several monocot families (e.g., Agavaceae, Iridaceae, Liliaceae).
Crystal sand consists of very small (2-5 ^m) crystals that usually occur in masses in cells (e.g., Atropa belladonna, Cinchona succirubra, Senna alexandrina). The structure of the crystals may be pyramid-like, prismatic, tetrahedric (microsphe-noidal), or irregular and is difficult to determine using a light microscope. Cells containing crystal sand appear dark in bright field illumination but can be distinguished using polarized light.
Calcium Carbonate A number of plant families contain irregular concretions of calcium carbonate formed on narrow ingrowths of the cell wall known as cysto-liths. Cystoliths occur in cells called lithocysts that often become greatly enlarged to accommodate the growth of the crystal. They usually occur in epidermal cells and are confined to the Acanthaceae, Curcurbitaceae, Moraceae, and Urticaceae. In some other families, calcium carbonate can be found on leaf surfaces and encrusted on the walls of vessels and trichomes; in these cases, the concretions are not called cystoliths (Figure 7.14). In contrast to calcium oxalate crystals, calcium carbonate is not birefractive and therefore is not visible in polarized light. It can be distinguished from calcium oxalate by mounting it in dilute acetic acid; this will cause it to dissolve with effervescence, whereas calcium oxalate will remain insoluble.
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