This cross section of an oleander (Nerium oleander) leaf reveals two beautiful mineral crystals inside. Leaf cells are stained red. Adapted to dry conditions, this leaf possess three epidermal layers to prevent water loss, below which there are tightly packed palisade mesophyll cells that are specialized for harvesting light and loosely packed spongy mesophyll cells allowing efficient gas exchange. These calcium oxalate crystals are deposited by specialized cells (the “crystal idioblasts”) in the leaf through highly controlled intracellular processes. Calcium oxalate crystalizes in an astonishing variety of forms in plants, and their distribution and morphology can be diagnostic features of specific species/families. Generally, crystal formation is considered as a response to regulating surplus calcium in the plant. There is also a growing body of evidence to support that crystals sequester poisons (e.g. heavy metals) and are involved in defense against herbivores — in spite of the stunning appearance, it may have very unpleasant flavors. Like “diamonds are forever”, these crystals may remain intact even if the plant is degraded or burned.
Contributed by Min Ya, a second year graduate student in the Organismal and Evolutionary Biology program at Harvard University, and our Featured Artist for April 2017. To meet Min Ya and see more of her art, click here.

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