The distinct shapes and birefringence of calcium oxalate
crystals, especially raphides and styloids, make them readily
observable, particularly in young, actively growing tissues,
although smaller, rounded druses are more easily missed.
Crystals normally form intracellularly, but extracellular
crystals have also been reported, e.g. on the outside surfaces
of palisade parenchyma in Tsuga leaves (Gambles and
Dengler, 1974). However, since these crystals have a covering
of ®brous material (Horner and Franceschi, 1978) they are
probably initiated intracellularly. Similarly, in Welwitschia
bainesiithe crystal-encrusted spiculate cells are covered by a
sheet-like layer (Scur®eld et al., 1973). Some tissues may
contain other crystal types in addition to calcium oxalate
crystals; in particular silicate crystals are characteristic of
the monocot superorder Commelinanae, many of which
have calcium oxalate crystals (see below).
The value of calcium oxalate crystals to normal plant
growth and development is largely unknown and probably
variable (Frey, 1929; Arnott, 1976; Franceschi and Horner