Fluorescence intensity distribution of holdfast at various ages labeled with 100 μg/ml fluorescein-WGA-lectin, (a) 7.5 ± 2.5 min, (b) 17.5 ± 2.5 min, (c) 27.5 ± 2.5 min, and (d) 37.5 ± 2.5 min. Note that the intensities fall into two groups, indicated as I and II. Approximately 10% of holdfasts are in group I, whose intensities remain very low. Inset in (c) is a combined phase and fluorescence image of 27.5 ± 2.5 min old cells, showing a few examples of the two groups of holdfasts with different fluorescence intensities. The fluorescence intensities of two holdfasts indicated by arrows are much weaker than the others. These two cells are identified as group I cells in co-existence with several group II cells.