When stressed or under attack, especially from fungal or bacterial canker diseases, trees - particularly fruit trees - can often release a clear gum through their bark – a process known as gummosis. This substance can run down the trunk and branches, especially in wet weather, accumulating wherever it gradually solidifies – often with small air bubbles trapped inside.
Similar gums, commonly known as gum Arabic, are produced by acacia trees and are widely used in the food industry, for example as emulsifiers and thickeners in sauces. They are mostly composed of various forms of sugars, but are rich in minerals and can contain almost 3 per cent protein, so have been used as chewing gum and as an emergency food.
The 18th-century botanist William Withering claimed that a besieged army of 100 men survived “for near two months without any other sustenance than a little of this gum taken into the mouth sometimes, and suffered gradually to dissolve”; and the traveller and naturalist Fredrik Hasselquist, his contemporary, described how Abyssinian traders travelling through deserts survived on an emergency diet of plant gum.
Main image: Canker induced Gummosis on an ornamental pendulous cherry. ⓒ Roger Griffith/wikipedia/creative commons
Phil Gates taught biology at Durham University and writes for The Guardian’s Country Diary column.