Gall

Gall (Cecidia)

Galls, also known as cecidia, are abnormal swelling growths that occur on the external tissues of plants. These growths are analogous to benign tumors or warts found in animals. The phenomenon of gall formation is a complex interaction between a plant and an inducing agent, which can be a diverse array of organisms.

Causes and Inducing Agents

Plant galls are induced by a wide spectrum of biological agents. These include microscopic entities such as viruses, fungi, and bacteria, as well as larger organisms like other plants, insects, and mites. The study dedicated to understanding these plant galls is known as cecidology.

Insect Galls

Insect galls are particularly distinctive and highly organized plant structures. They are formed by certain herbivorous insects, serving as their own microhabitats. These galls provide both a habitat and a food source for the insect responsible for their creation. The interior of an insect gall can contain edible, nutritious starch and other plant tissues. Some galls function as "physiologic sinks," concentrating resources from surrounding plant parts towards the gall itself. Additionally, galls can offer insects physical protection from predators. Insect galls are typically induced by chemicals injected by insect larvae, possibly combined with mechanical damage. After formation, the larvae develop within the gall until maturity. The formation of galls usually occurs during the growing season, when plant cell division is rapid.

Gall-inducing insects are often species-specific and sometimes tissue-specific on the plants they affect. This group includes gall wasps (such as cynipids), gall midges, gall flies, leaf-miner flies, aphids, scale insects, psyllids, thrips, gall moths, and weevils. The diversity of gall-inducing insects is vast, with estimates suggesting hundreds of thousands of species, not including their parasitoids.

Other Inducing Agents

Mites, particularly those in the family Eriophyidae, are significant inducers of gall formation, with thousands of described species attacking numerous plant types. Nematodes, microscopic worms, can cause galls on plant roots, commonly seen with Meloidogyne species (root-knot nematodes).

Fungi also play a role, with many rust fungi inducing gall formation. Examples include western gall rust on pine trees and cedar-apple rust. Some fungi, like Ustilago esculenta associated with wild rice (Zizania latifolia), produce edible galls that are a valued food source in certain regions of China.

Gall-causing bacteria such as Agrobacterium tumefaciens and Pseudomonas savastanoi are also known. Viral agents, like the rice gall dwarf virus, can cause gall formation along rice plant leaves and sheaths, leading to discoloration and reduced tillering.

Even other plants, such as the hemiparasitic mistletoe, can form woody structures sometimes referred to as galls on their hosts.

Historical Uses

Galls have been historically significant due to their rich content of resins and tannic acid. They have been widely employed in the manufacturing of permanent inks, notably iron gall ink. Their astringent properties made them useful in ointments, and they played a crucial role in dyeing processes and leather tanning. Medieval Arabic literature extensively documents the uses of galls, known as عفص (ˁafṣ), including their use as a mordant for black dyes, for making high-quality ink, and as a medicinal treatment for fever and intestinal ailments.