You might do a simple assay for chlorophyll as a proxy for cell
number. Extract the cells in acetone or methanol, spin down debris,
and measure spectrophotometrically. In combination with Warwick's
suggestion, you could construct a calibration curve of cell number
(hemocytometer) vs. chlorophyll - but that is probably more than you
need, although it might be informative for the students to see the
relationship.
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