Developmental aspects of crystal idioblasts were studied in roots of Vanilla planifolia L. and Yucca torreyi L. In Vanilla, root crystal idioblast initials are first recognized by intense fluorescence of cytoplasmic RNA with acridine orange; nuclear and nucleolar enlargement gradually ensue. Feulgen microspectrophotometry showed that all idioblast nuclei are endopolyploid. Frequency distribution of individual DNA content measurements and nuclear structures suggest endomitosis to the 8C level and heterochromatin underreplication in higher endopolyploid nuclei. Many ultrastructural features of Vanilla idioblast development are common to differentiation of similar cells observed in other plants. Excised primary roots of Yucca torreyi were cultured in various media, and development of coritical raphide crystal idioblasts observed with CTEM and HVEM. Idioblasts were consistently produced, required 24 hours to fully differentiate from initials in isolated root cultures, and developed normally relative to those cells in intact roots. These isolated root cultures are shown to be a well-suited system for physiological investigations of oxalate metabolism and control of idioblast cytodifferentiation. Cytochemical localization of three peroxisomal enzymes, glycolate oxidase, urate oxidase, and catalase was performed on tissues from Psychotria punctata leaves and Yucca torreyi roots which contain developing idioblasts. Reaction product deposition attributable to glycolate oxidase activity was never observed in any developing idioblast in either plant; pathways involving glycolate or glyoxylate intermediates are probably not involved in oxalate synthesis for crystal production. A survey of callus cultures from nineteen higher plant species showed that structural differentiation of idioblasts in culture is media dependent, species specific, and influenced by types of growth regulators. Also, crystal shape is determined by the cell during its differentiation and under general genetic control. Media modifications which favor cytodifferentiation of idioblasts include use if IAA as culture auxin, deletion of ammonium nitrate, and high calcium supplement. This study concludes that crystal idioblasts probably have multiple functions which vary with tissue and species specificity; however, it is not likely that these cells ever serve as depositories for oxalate as a metabolic waste product.