The potato first appeared in Europe around 1570, having been imported from South America by the Spaniards, and was introduced to Ireland around 1585. The plant is a member the Solanaceae family, which includes tomatoes, aubergines and peppers.
Although there are about 150 species in the wild, only Solanum tuberosum is grown outside the Andes, but within this species more than 600 different varieties are now known in Europe. The potato is an almost perfect food, rich in calories, minerals, vitamins and protein, and virtually free of fat.
The potato thrived in Ireland, and by the middle of the 19th century it had become virtually the sole means of nourishment of most of the population. If anything were to happen to cause a failure of the crop, the result would be disaster. And that, as we know, happened in 1845, bringing about the Great Irish Famine.
Although a great many social and political factors contributed to the strange and terrible events of 1845 and thereafter, the immediate cause was the prevalence of a combination of meteorological factors. The weather during those years was ideal for the spread of potato blight - a fungal disease which thrives only in very specific atmospheric conditions.
Potato blight spreads when high temperature and very high relative humidity coincide. For the spores to infect the plants, three conditions must be fulfilled over an extended period: the leaves must be wet, the relative humidity must exceed 90 per cent, and the temperature must be greater than 10 degrees Celsius.
Such weather prevailed over northern Europe in the early summer of 1845. The blight first appeared in Belgium in late June, spread rapidly, and by the second week in September it had reached Ireland.
On continental Europe and in Britain the following summers were dry and hot, and so the blight died out. But in Ireland the weather in 1846 was again abnormally wet and warm, and when blight wiped out most of the Irish crop that August, people who had consumed everything to get through the winter of 1845 had nothing left for viability.
By the end of 1846 the few salvageable potatoes, together with indigenous nettles, blueberries, edible nuts and cabbage, were all exhausted; people starved, and famine fever began to take its toll.
The population of the island, which had been rising steadily before the Famine, dropped by two million people, or more than a fifth, as a direct result of the disaster from the combined effects of death and emigration. It began a trend which continued relentlessly until relatively recently.