Some plants have extremely sensitive biological thermometers  For example, the leaves of rhododendrons curl when the temperature of the air around them is below 0 degree C (Celsius)  Similarly, mature crocus blossoms open in temperatures above 2 degree C
So someone who simultaneously observed rhododendrons with uncurled leaves, crocuses with mature but unopened blossoms, and a thermometer showing 1 degree C could determine that the thermometer' s reading was accurate to within plus or minus 1 degree C
Neither rhododendrons nor crocuses bloom for more than a few weeks each year, and the blossoms of rhododendrons growing in any area do not appear until at least several weeks after crocuses growing in that area have ceased to bloom.