Due to its ubiquitous role as a signalling element in plant cells, the cytosolic concentration and cell-type distribution of calcium (Ca) must be tightly regulated in all cells. It is currently thought that most dicots accumulate Ca in mesophyll cells and phosphorus (P) in epidermal cells; this separation of Ca from P is necessary to avoid the deleterious precipitation of calcium phosphates. However, in several Australian Proteaceae species from severely P-impoverished soils, we discovered that P is actually accumulated in photosynthetically active mesophyll cells which is likely a strategy to increase P-use efficiency, by preferentially storing P where it is most needed. However, this shift in P-accumulation may result in the formation of deleterious precipitates with Ca. This project investigates if all Proteaceae accumulate P in mesophyll cells and if this results in a shift in Ca-accumulation away from P. The distribution patterns and concentrations of Ca and P were determined in a range of Proteaceae from both Australia and South America. Leaves of Proteaceae from south-western Australia, Chile and Brazil were dissected and rapid-frozen in liquid nitrogen before being cryoplaned and analysed in the fully frozen-hydrated state. Quantitative X-ray mapping and elemental analysis were performed on transverse leaf sections using a Zeiss Supra 55 FESEM, with an Oxford X-Max80 SDD X-ray detector. The accumulation of P in mesophyll cells was only found in Australian Proteaceae from severely P-impoverished soils, indicating that it is not a family-wide trait and is likely a strategy to increase P-use efficiency. Patterns of Ca-accumulation were not consistent with current understandings, instead showing significant variation, even between closely related species. Such variation in the patterns of Ca-accumulation within a single family and genus is unprecedented and requires further investigation into what factors influence Ca-regulation in leaf cells.