Although known from plants for a long time, the function of mineral inclusions – included amongst the so-called ergastic substances of old – remains speculative. Frequently composed of calcium oxalate and rather toxic, it has been speculated that druses may deter would-be herbivores, and/or act as store of calcium – often needed in large amounts as a secondary messenger in many signalling pathways within plants.
Well, another role is suggested by Harry (‘Jack’) Horner in leaves of certain succulent Peperomia species. Horner hypothesises that vacuole-located druse crystals in the palisade mesophyll cells – which are situated below recently discovered ‘skylight-like’ regions in the overlaying hypodermal cell walls – may help to focus light on to the chloroplasts that surround the crystals (see figure above). This arrangement may be an adaptation that permits more efficient photosynthesis in the low-light environment where these plants are found.
You’ll need to read the paper – in June’s Annals of Botany, and which is free access – to appreciate the full story, but this elegant structure–function study dramatically demonstrates how much still awaits our discovery and understanding. Plus, this paper was one of an extremely select few – ‘one in a thousand manuscripts’ – accepted for publication without any changes! So, like his namesake of old, our modern-day Jack Horner has produced a real plum: Read and enjoy.