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Raphide revisitation reveals relationships
The synthesis and accumulation of oxalic acid and/or calcium oxalate occurs in many species distributed throughout the plant kingdom. In monocots, calcium oxalate is most often laid down and usually as bundles of needle-shaped crystals known as raphides. Scott Zona (Fairchild Tropical Gardens, Miami, Florida, USA; pp. 415-421) states that raphides are ubiquitous in palms, and quotes from earlier literature to show that they are deposited in roots and in all the aerial parts of the plant. However, this is not the whole story because it had been previously noted that raphides also occur in the embryos of some palms. This has led the author to carry out a survey amongst 127 taxa of palms to determine the distribution of embryo-located raphides. This work, combined with a more limited data set from an earlier author, gave a survey covering 148 taxa. Embryos were dissected out from fresh or preserved seeds, stained in toluidine blue and examined under the microscope. The author comments that if raphides were very abundant they could be detected under normal light, but their presence became much more obvious under polarized light. Raphides were classified as absent, scarce, present or abundant and the distribution of species in these categories was compared with the current taxonomy of the palms. The occurrence of embryonic raphides was clearly not random within the palm family; in particular, they were very scarce in two subfamilies but very abundant within two tribes of a third sub-family. The author suggests that this relatively simple molecular feature may be a further aid in studying the evolution and taxonomy of the palms. Finally, the question of the function of embryo storage of calcium oxalate is discussed. The author considers it unlikely to be an anti-feedant in embryos. It is more likely, he suggests, to be a storage compound.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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