Katy explains with the photograph: “This is a fluorescence image of a cross-section through an Acacia rigens phyllode (leaf). The central round portions are the nerves (veins) that run from the base of the cylindrical phyllode to its tip. They are attached to the surface of the phyllode by sclerophyllous caps.” For an idea of what Katy means by cylindrical phyllode, she provides another photograph of needle wattle: Acacia rigens — the long, tubular leaves are very evident.
Sclerophyllous caps refers, I believe, to the spokes of cells radiating outward from the centre. The walls of these cells are lignified (containing lignin) and are the cause of the phyllodes being stiff and tough. On the surface, it might appear that this is an adaptation to prevent water loss or herbivory. While this may be the case in some instances, it is mainly considered a strategy to promote growth in phosphorus-poor soils, since lignin production doesn’t require phosphorus (or requires very little – sorry, I’m not a biochemist). This strategy is so prevalent in some places that entire areas of Australia (and a few regions elsewhere) are considered sclerophyll forests.