In 'Tolerance' graph, the x-axis is 'Range of Environmental Variable' and y-axis is 'Population.' A bell shaped curve has been depicted.

1. Mid portion of the bell is 'optimum range', marked in deep blue.
2. Left end of the bell is 'Lower limit of tolerance' and right end of the bell is 'Higher limit of tolerance.'
3. Portion to the left and right of bell curve is the region where species are absent from zones of intolerance.