Fig. 9.2-4. Clearing of banana leaf (Musa). The three vertical strips of tissue are leaf vascular bundles, with the pink color being xylem and the brown being latex. The leaf was cleared by placing large pieces of it in warm potassium hydroxide and a stain solution. Latex tubes typically appear empty in histological slides because the latex drains out of the cut ends of the long tube-like cells. But these are articulated laticifers – a structure made up of many cells. The articulations are visible between the individual cells because the brown color is due to stained latex, and the latex is excluded from the area between cells by the presence of the cells’ end walls (which did not stain). There is a narrow bit of brown color between some cells – apparently the end walls have a small hole or two that lets latex move from cell to cell.
The laticifers run parallel to the vascular bundles, but just below them; when the microscope is focused on the laticifers, the xylem is so far above the plane of focus that its red-stained helical secondary walls merely look like the pink strip we see here. If we focused up to see the xylem, the laticifers would be indistinct.