What does 'ear' mean here? Why do 'the coroners prepare for active duty'?
Whenever the yellow scorch creeps down over the tender inside leaves about the ear, then the coroners prepare for active duty; for the oil of the country is burned out and it does not take long for the flame to eat up the wick.
On the Divide by Willa Cather
The meaning of "ears" is as pictured in the other answer.
The sense of the passage is that when a hot wind starts that destroys the corn crop (as evidenced by the "yellow scorch" reaching even the tender leaves around the ear of corn, then the farmers know that they won't have a full crop that year. Some of them are living so close to the end of their economic resources, that they will get depressed and commit suicide. That is why the coroners get ready.
There is a metaphor comparing the wealth of the countryside, which is its corn crop, to the oil in a lantern. After the oil has all burned up, only the wick is left to burn.
Corn has ears. This is an ear of corn.
When the outer leaves of the corn become yellow, it is autumn. Coroners are judges who investigate causes of death. In the British legal system, every death is investigated. If there is doubt over the cause of death, of if the death might have been preventable (and particularly if it may have been a crime) then a special judge will hold an investigation, like a trial, to discover why a person died, and if there is anything that we could learn from it.
"Yellow scorch on the leaves of ears of corn" is being used as a poetic way of saying "Autumn". And the coroners get ready for duty because (it is implied) that there are more unexplained deaths in autumn and winter, apparently because the country is (metaphorically) burnt out, in the way that when a candle burns up all the wax, the wick burns away and the candle goes out.
