Unlike wheat and barley, broomcorn millet is a C4 plant and as such particularly well adapted to hot climates. When temperatures rise, plants usually close their stomatal pores in order to retain water. This, however, makes it difficult for them to absorb the carbon dioxide required for photosynthesis.

To cope with this problem, C4 plants have developed a mechanism that allows them to use even the tiniest quantities of carbon dioxide. They do so by means of a particular leaf anatomy, in which the vascular bundles (which supply leaves with water and nutrients) are surrounded by bundle sheath cells. This means that biomass is produced in two steps and in two isolated compartments.

Carbon dioxide is initially fixed in the mesophyll cells, producing large C4 molecules which are then diffused to the bundle sheath cells. In the bundle sheath cells, the carbon is extracted from the C4 molecules and fed into the Calvin cycle, which produces the carbohydrates that allow the plant to grow.

Although the diffusion of molecules requires additional energy, it allows C4 plants to be productive even in the hottest part of the day, when their stomatal pores are closed to avoid water loss.

This makes millet eminently suited to provide people with their daily bread – or, more accurately, their daily gruel – even in times of extreme drought. It did so in the Bronze Age, and its ability to withstand dry conditions has made it ripe for rediscovery as harvests are threatened by climate change.