“a sort of botanical glory-hole”
― John Wyndham, The Day of the Triffids, 1963
That animals eat, and make homes from, plants, is something we learn at a very young age. But fewer of us seem to be aware of the very much more complex manipulations of living plants that some organisms conduct. Sometimes the plant tissue itself is distorted by another organism. A growth forms. We call these growths, galls.
The nature and shape of these galls can vary wildly: from hard nutty growths, to soft fleshy lumps; from tiny specks on leaves, to giant weighty tumours on trees. The ’causer’, to give the parasitic organism its correct term, is often a very small invertebrate such as a mite, midge, wasp (if you are thinking of large stinging black an yellow things, you are in the wrong ballpark altogether), or aphid; but can also be fungi, bacteria, and even other plants (mistletoe for example). The ‘hosts’ are often specially chosen and parasitised by specialist causers, meaning that if you see a nail gall on the leaf of a Common Lime, it has been caused by a different species to the similar gall on the closely related Small-leaved Lime.
When invertebrates are involved, galls are formed to provide shelter, sustenance, and/or protection to the causer and/or its offspring. Sometimes the lifecycle of a gall causer can be so complex that different stages of metamorphosis can require different, sometimes multiple, host plants (I have a pear tree in my garden infected by a fungus that will require juniper to conduct the next stage of its vital process).
Galls fascinate me. Unlike birds and butterflies, they stay still when you want to study or photograph them. They straddle the worlds of botany and zoology (the study of plant galls is called Cecidology, in case you were wondering), and form a welcome distraction to the lack of interesting migrant birds on the patch during the quiet summer months.
Armed with no equipment, other than a camera phone, and very limited background knowledge, I have managed to photograph and identify at least thirty types of plant gall on my local patch. I am confident that I shall find many more.