To find markers that distinguish the different Cactaceae species, by using near infrared Raman spectroscopy and scanning electron microscopy, we studied the occurrence, in the stem, of solid deposits in five Cactaceae species ( Coryphantha clavata, Ferocactus latispinus, Opuntia ficus-indica, O. robusta, and O. strepthacantha) collected from their natural habitats from a region of México. The deposits in the tissues usually occurred as spheroidal aggregates, druses, or prismatic crystals. From the Raman spectra, the crystals were identified either as calcium oxalate monohydrate (CaC₂O₄H₂O) or calcium oxalate dihydrate (CaC₂O₄·2H₂O) Opuntia species (subfamily Opuntioideae) showed the presence of CaC₂O₄·H₂O, and the deposition of CaC₂O₄·2H₂O was present in C. clavata and F. latispinus (subfamily Cactoideae, Cacteae tribe). As a punctual technique, Raman spectroscopy seems to be a useful tool to identify crystal composition. In addition to allowing the analysis of crystal morphology, this spectroscopic technique can be used to identify Cactaceae species and their chemotaxonomy.

The morphology and elemental composition of crystals in embryos, seedlings and adult plants of the globose cacti Mammillaria uncinata were studied. Samples of mature fruits and adult plants were collected. To obtain embryos and seedlings, seeds from mature fruits were germinated under laboratory conditions. Both embryos and seedlings as well as stem and root of the adult plants were processed by conventional microtechniques and tissue macerations to isolate the individual crystals. The crystal morphology was evaluated by light and scanning electron microscopy while its chemical composition was analyzed by energy dispersive spectroscopy, IR and Raman spectroscopy. The results demonstrated the occurrence of calcium oxalate crystals in the three stages of plant growth. Solitary and small crystals are present in embryos and seedlings in contrast crystals are aggregates or conglomerates in the adult plants. The compositional analysis showed that seedling crystals contain carbon (50.37%), oxygen (45.29%) and calcium (3.36%) while in adult plants the percentage has changed to carbon (12.54%), oxygen (53.06%) and calcium (34.38%). In the IR spectrum, the vibration bands around 1321 and 1621 cm-1 are attributed to the calcium oxalate in the dihydrate state (weddellite), the Raman peak at 1475 cm-1 shows also that crystals correspond to the pure state of calcium oxalate dihydrate state. The crystal size was also different for seedlings and adult plants, mean values varied from 12.11 to 13.38 μm for width and length, respectively in seedlings and from 65.10 to 73.90 μm, in adult plants. It is concluded that the elemental composition, size and morphology of crystals in M. uncinata depend on the growth stage, as it happens in other plant species.