The incredible galls
Round and red like cherries they were, the oak-apples, a kind of plant gall, which I loved to collect as a child. But they had a biting-bitter flavour, which taught me that despite their enticing looks as far as their value as a food item was concerned, they were pretty useless. A very important lesson, indeed, for not everything that’s pretty on the outside has a valuable inside. Later I also learned that not all plant galls have to be smooth, round or red. There are some that are warty, chimney-shaped, pimple-like, white and even purple in colour and they may not even be confined to the blade of a leaf. As a young, first year student (I started my university life as someone who considered math as a major, but then decided on chemistry only to change a year later to fisheries and ultimately end up in Neurobiology) I described a gall that was formed by the actions of an aphid known as the spiral gall aphid (Pemphigus spirothecae). This “plant louse” causes the stem of a poplar tree’s leaf to undergo a helical twist combined with a bulbous swelling into which the aphid settles and starts its family. The species has recently been shown to be a “social insect”, exhibiting division of labour between individuals and altruistic behaviour traits similar to those of ants and honey bees.
For centuries some galls had practical roles (as a source of ink) or found uses in homeopathy and medical compounds, but none to the best of my knowledge served as a regular component of the human diet. The principal agents in plant gall formation are the larvae of certain beetles species, plant sap-sucking aphids, flies and solitary wasps. A few small moths, some mites and nematode worms are also on record to cause gall growths. Each gall former usually has an “agreement” with one particular species of plant. Through an as yet still poorly understood mechanism the growing embryo or larva in the plant tissue manipulates the metabolism and growth machinery of the plant, so that certain cells undergo rapid proliferation (as in cancers), creating a thick coat of multi-cellular, nutritive tissue around the insect: a tissue that is out of place and out of character for the plant; hence another similarity to a cancerous growth. The plant gall, as it continues to grow and acquire colour, becomes conspicuous to other insects and some, which originally have had nothing to do with the formation of the gall, may “decide to become tenants” so that a mature gall can become the home to a dozen or so different species of insects.
It surprises me immensely that cancer researchers and molecular geneticists or even nutrition specialists have not homed in on this wondrous interplay between an animal and a plant. If we knew how the plant makes the gall, couldn’t we program a plant to produce an edible product, a kind of seedless “fruit” that did not originate from a flower? Before our eyes, the gall-inducer manipulates the plant to “do” things it would not normally do, and the plant appears defenceless. But by complying, the plant actually confines the “pest” to one localized spot rather than allowing it to crawl unrestrictedly all over it and although the plant is “persuaded” to provide some nourishment to the gall residents, it can, in the end, rid itself of the uninvited and unwanted guests: by shedding its leaves.