A vascular bundle in a transverse section of a grass leaf, stained with the fluorochromes Calcofluor M2R (blue fluorescence = cellulose) and auramine O (yellow fluorescence = lignified cell walls). The red fluorescence is chlorophyll autofluorescing red in the blue excitation beam of the microscope.
The two big 'eyes' in this 'smiley face' (which is typical of a monocot vascular bundle) are metaxylem elements that transport water through the leaf. The bright blue fluorescence in the 'mouth' of the 'smiley face' is phloem, composed of larger sieve tubes and smaller rectangular (in cross section) companion cells, which together transport sugars, made by photosynthesis, out of the leaf. The bright yellow cells forming the neck of the 'smiley face' are lignified, providing a measure of rigidity in the leaf, and the band of cells along the bottom of the section are epidermal cells covered by a cuticle.