Many plant species accumulate calcium oxalate crystals in specialised cells called crystal idioblasts. In one species of crystal-forming plants (Pistia stratiotes L.; forming raphide crystals), it has been shown that ascorbic acid is the primary precursor of oxalic acid. The question remains if this is true of other calcium oxalate crystal-forming plants. One way of answering the above question is by examining ascorbic acid as the oxalic acid precursor in diverse species with a variety of crystal types. In this study we tested ascorbic acid as the primary precursor of oxalic acid in four different species, each forming one of the four, thus far, unexamined crystal types: water hyacinth, styloid (and raphide); tomato, crystal sand; winged-bean, prismatic; water lily, astrosclereids with surface prismatic crystals. Pulse–chase feeding of 1-[¹⁴C]-ascorbic acid followed by resin embedding, microautoradiography and light microscopy were employed to examine incorporation of label into calcium oxalate crystals. For the species and crystal types studied, ascorbic acid is the primary precursor of oxalic acid and further, oxalic acid is added to crystals in patterns that correlate with the age and type of crystal involved.