This article appeared in Tree Life 205

Galls, abnormal growths on plants, have fascinated botanists for many years and even Charles Darwin discussed them as a possible contradiction to his theory of evolution by natural selection. Whilst certain general aspects are known about them, much of the finer detail about how they come about and the relationship between gall-former and plant host are still the subject of some speculation. The treatise by Mani (1964) remains the basis of much of the knowledge of galls today.

What is a gall?
Galls are cells, tissues or organs which have developed abnormally by cell enlargement (hypertrophy) and cell proliferation (hyperplasia) and are caused by the action of biologic organisms such as bacteria, fungi, nematodes, mites and insects.

However, the definition of a gall is not always quite so clear-cut particularly since there are many gradations between what is normal and what is grossly abnormal. For example, some scientists may classify all abnormalities of a leaf caused by a fungus as a gall whereas the stricter definition would require there to be evidence of excess growth i.e. swelling with the plant playing an active role rather than a passive one. Excessive growths can also be caused by mechanical injury and non-biological chemicals but these are not generally considered as galls. Galls may affect any part of a plant including the vegetative parts, reproductive organs and root systems as well as any fruits. Usually they are a direct consequence of the attack by the gall¬-former but in some cases the gall may form at a site distant from the parasite. In such cases the tissue of the gall usually shows normal histology under the microscope (organoid galls) e.g. flower abnormalities in Scabiosa columbaria due to attack of the roots. Organoid galls are usually caused by fungi, mites and aphids. The cells of histioid galls on the other hand differ from those of the organ on which they develop and no normal organ or function can be discerned. This is comparable to the abnormal cell growth on human cancers and, similarly, the changes can be less marked (benign) or extreme (malignant).

Why and how are galls formed?
There is still some debate about whether galls represent a simple parasitic relationship between the gall-former and the plant with the former deriving all the benefit i.e. shelter, nourishment, dispersal to the detriment of the host i.e. loss of nutrients, altered growth patterns, premature decay. There are examples of where the plant itself benefits such as the nitrogen-fixing nodule galls on Leguminosae but in most of these cases the benefit is rather indirect. A logical argument is also that the gall is a simple result of a protective mechanism against the parasite in an attempt to limit the attack and prevent death. It can further be argued that within this interplay there has been mutual adaptation between host and parasite over the years reaching the present state of specialisation. Under these discussions, galls can be due to either parasites or symbiotic organisms.

The discussion on why galls form requires an understanding of how they form but this area is far from being clearly understood. In some cases it may be that the gall-former produces plant hormone analogues which stimulate growth or particular growth patterns, or the parasite is able to mobilise plant-produced hormones. It may also be true that the altered growth may be due to production of cytokines by the plant in response to the attack. In considering these issues one needs to bear in mind the situation of galls produced by insects (Diptera and Hymenoptera) which are usually of a particular growth on a particular part of a particular plant.

Examples of galls.
Galls can be caused by bacteria, fungi, nematodes, mites as well as insects. Viral attack does not seem to commonly give rise to galls.

Galls caused by bacteria.
Agrobacterium tumefaciens causes uncontrolled proliferation of cells at the top of root systems i.e. crown gall, in most hardwoods although there are many species which are not recorded as being hosts. The bacterium enters the tissues through a wound, stimulates increased DNA replication and cell growth and is then sloughed off with external cells back into the soil. However, the gall continues to grow and may even spread to other parts of the plant.

Galls caused by nematodes.
Nematodes almost invariably cause root galls although the attack of the roots can result in organoid galls elsewhere on the plant. The root-knot nematode (Meloidogyne spp.) affects over 1500 species of plants including about 100 species of trees both conifers and hardwoods. The female feeding on the root causes giant cells to form in the root: Cell walls dissolve, nuclear division is increased and the cells enlarge. The female nematode is an endoparasite and lives within the gall feeding on the few giant cells formed. Eggs are extruded together in a sac-like structure on the surface of the gall and the young hatch and may penetrate the root in the same general area.
Some nematodes transmit plant viruses and are important in the transmission of plant disease.

Galls caused by fungi.
Many rust diseases caused by fungi can give rise to gall formation on conifers in particular but also on other trees. Examples are the fusiform rust on southern US pines (caused by Cronartium fusiforme), pine-oak rust (Cronartium quercuum), pine-pine rust (Endocronartium harkenssii) and cedar-apple rust (Gymnosporangium spp.)

Galls caused by mites.
Mites (Acarina) are small eight-legged relatives of spiders which are able to produce much cellular hypertrophy due to their ability to attack individual cells. Some of the galls on inflorescences can develop into witches-brooms i.e. multiple branching. This effect can however also be caused by other sap suckers and fungi.

Galls caused by insects.
The galls caused by insects are numerous. Often the insect lays its eggs within the plant stimulating a gall which then provides nutrition to the grub until it is sufficiently grown. Some of the galls actually have intricately designed hatches or a dehiscent character for dispersal of the inhabitant(s). The galls of Hymenoptera are of the most specialised. The chalcid gall on Eucalyptus sieberiana is noted for its agglomeration of oval, sessile swellings which contain elongated larval chambers.

Zimbabwean Tree Galls
The galls on Zimbabwe’s trees and other plants are not well documented. Some of the more numerous or obvious include the small hairy disc-like scales on the underside of leaves of Parinari curatellifolia which can apparently develop into grey, pointed chambers. Combretum molle galls are large spherical, rugiform (hairy or rug-like) creations usually around leaf axils while those of Terminalia sericea are ovoid, woody and affect the growing shoot of branches.

Galls are a fascinating and mysterious aspect of the interaction of plants with their environment. Further study of the galls and gall-formers of our local vegetation will help to gain a fuller understanding of the interrelationships which exist in our native woodlands and the effect of man on them.

Mani MS (1964). The ecology of plant galls. Junk: The Hague
Manion PD (1981) Tree disease concepts. Prentice-hall: Enmglewood Cliffs.
Macnaughton SJ (1983) Physiological and ecological implications of hervivory. In: Lange OL, Nobel PS, Osmond CB, Ziegler H (eds.)
Physiological Plant Ecology Ill: responses to the chemical and biological environment. Springer¬-Verlag: Berlin.

-Douglas Ball.