What Types of Plant Galls Are There?

Plant galls are divided into histoid and organoid galls. Some galls cannot be assigned clearly to either category and display characteristics of both histoid and organoid galls.

Organoid Galls

Organoid galls are modifications in the development of individual plant organs. They can be further divided into anomalies of form, anomalies in leaf arrangement, branching anomalies, and the formation of new organs.

Histoid Galls

Histoid galls are changes in plant tissue that cannot be further classified according to individual organs. They include erineum galls, leaf rolls, leaf folds, pouch galls, covering galls and pith galls.

Erineum Galls

In erineum galls, a fine layer of hairs, known as erineum, develops on the plant epidermis. It is inhabited by gall-inducing organisms, most commonly eriophyid mites, and provides them with protection from adverse environmental conditions and predators.

Leaf Rolls and Leaf Folds

Plant galls formed by leaf rolling or leaf folding alter the natural growth of the leaf at a point where the leaf begins to bend. The leaf tip starts to roll up and forms a protective layer around the gall inducer. Leaf rolls and leaf folds arise either through faster growth of the lower side of the leaf or through inhibited growth of the upper side. This produces both a characteristic bend and an oval-shaped fold on the upper surface that encloses the gall inducer like protective armour. The resulting layer protects the gall inducer from adverse environmental conditions and predators, although it is relatively easy to open because the end of the roll usually does not fuse with the upper surface of the leaf but merely rests against it.

Pouch Galls

Pouch galls develop when the activity of the gall inducer causes the cells in all layers of the leaf tissue to elongate, resulting in an outward bulge. This growth creates a large cavity inhabited by the gall inducer. A small opening often remains on the underside of the leaf and is frequently covered by a fine layer of erineum, allowing air to circulate.

Covering Galls

Covering galls are produced when the gall inducer stimulates strong, ridge-like growth around itself on the leaf surface. The growth eventually closes over the inducer and encloses it. A small exit hole often remains, allowing the gall inducer to leave the gall once its development is complete.

Pith Galls

Pith galls are gall forms in which growth originates in the inner tissues of the plant. They can be divided into closed galls and free galls. In closed galls, the plant tissue is not breached towards the outside. The growth response is confined to the internal tissue. Free galls, by contrast, penetrate the tissue layers of the host organ.