What is being measured?
A cell length needs a calibrated scale. An eyepiece graticule has arbitrary divisions until it is calibrated against a stage micrometer. Recalibrate when the objective changes. A scale bar on an image represents a stated real length and can be used through a ratio of measured lengths.
Count cells consistently
For a root-tip count, identify cells in each stage using the same criteria. Count a defined set of fields and avoid choosing only fields with many dividing cells. Mitotic cells include prophase, metaphase, anaphase and telophase; interphase cells are part of the total but not the mitotic numerator.
Use the correct calculation
Magnification is image size divided by actual size in matching units. Mitotic index is cells in mitosis divided by total cells observed; multiply by 100 only when a percentage is requested. Show the numerator and denominator so the calculation can be checked.
What does the result support?
A higher mitotic index means a greater proportion of the observed cells was in mitosis. It does not, by itself, prove a faster cell cycle: a treatment might lengthen mitosis. Estimating the proportion of cycle time spent in a stage also needs an appropriate actively cycling population and representative sampling.
Evaluate the method
Poor staining or overlapping cells can make stage classification uncertain. More independent fields improve representation; repeatedly counting the same field does not provide new biological samples. Keep calibration separate from counting uncertainty.
Worked data example
A graticule calibration shows that 40 divisions correspond to 100 μm. A cell spans 12 divisions. In a separate root-tip count, 30 of 200 cells are mitotic.
One division = 100/40 = 2.5 μm. Cell length = 12 × 2.5 = 30 μm. Percentage mitotic index = 30/200 × 100 = 15%. These calculations describe different measurements.
Why recalibrate after changing the objective?
The same graticule divisions then correspond to a different real distance in the specimen.