What Types of Plant Galls Are There?

Plant galls are classified as either histoid or organoid galls. Some galls cannot be clearly assigned to either category and display characteristics of both histoid and organoid galls.

Organoid Galls

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

Histoid Galls

Histoid galls are changes in plant tissue that cannot be further differentiated into 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, which obtain 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 it begins to bend. The leaf tip begins to roll up, forming a protective layer around the gall inducer. Leaf rolls and leaf folds arise either through faster growth of the lower surface of the leaf or through inhibited growth of the upper surface. 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. However, 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, enclosing 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 the 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 to the outside. The growth response is confined to the internal tissues. Free galls, by contrast, extend through the tissue layers of the host organ.