GALLS. This term may be applied to any excrescences caused in plants or animals by the action of living animals, usually insects. In animals, galls may be produced under the skin of mammals or birds by acarids or by dipterous insects. The term, however, is mostly confined to vegetable excrescences of a particular type.
Accepting the definition of Lacaze-Authiers they comprise "all abnormal vegetable productions developed on plants by the action of animals, more particularly by insects, whatever may be their form, bulk or situation." History.—The hypertrophies of plant tissues known as galls have been known from early times. Theophrastus (372-286 B.c.) referred to the gall-nuts as an article of trade and to the superior quality of those from Syria. The nature of galls was, however unknown until recent times. It was naturally assumed that they arose spontaneously, i.e., directly from the plant, without the in tervention of any other animal or plant. John Evelyn, the diarist states in his Sylva (published 1664) : "Pliny affirms that the Galls break out all together in one Night, about the beginning of June, and arrive to their full growth in one Day." Evelyn was familiar with the oak galls of commerce imported for their tannin content but he did not realize that the oaks in England bore numerous galls of different kinds-8o–I oo kinds are known. The association of insects with most galls appears to have been discovered by Martin Lister (163o-1712) who was physician to Queen Anne. Malpighi (162o 1694) the physician and botanist seems to have been the first to deal systematically with galls, publishing a treatise entitled De Gallis which dealt with the galls of Italy and Sicily. In England, Dr. Derham, Canon of Windsor, seems to have been the first to take up the study of galls; he re fers to Malpighi's work in his Boyle Lectures given in 1711 and 1712.
Galls arise from the abnormal development of meris matic tissue of the plant as a result of an abnormal stimulus. The stimulus is caused by an ani mal parasite. The parasite is usually an insect which lays an egg in the plant tissue but some galls are due to the action of eelworms (nematodes). The stimulus causing the abnormal development of the plant cells may be due in some cases to a chemical substance injected by the insect but in many cases, as in those of the wasps producing oak galls, gall formation is due to the action of the larva developing from the eggs; in this the movements of the larva probably play a part. It must be borne in mind that the gall is the result of the interaction of two living organisms, the plant and the parasite. It is true that the gall is built up out of the tissues of the plant, but its special form and structure depends on the nature of the animal. The interaction is made clear from the fact that the same plant attacked by different insects will produce very different galls. In fact the differences between galls on the same plant may be the best means of distinguishing the insects, as in the case of gall-gnats of the willow, where ten different species attack Salix humilis, the insects being almost impossible to distinguish in the fully grown state except by the galls they produce.
Galls are very variable in form and size but very con stant for the special insect and special host plant concerned. Often the galls are entirely different from the structure on which they have developed or which they have replaced. Commonly, how ever, the tissues outside the larval chamber enclosing the insect, i.e., tissues which give the gall its distinctive form, indicate the special organ (leaf, etc.) from which the gall has developed. In the gall known as "Robin's pincushion" or "bedeguar" (produced by the action of the female gall-wasp, Rhodites rosae, on a leaf or bud in spring) the moss-like covering represents rudimentary leaves, consisting of fibro-vascular bundles with very little paren chyma between. Other galls represent in the case of the oak the arrested acorn. The size of the galls produced by insects varies enormously, some may be only a millimetre
inch) or two in length while those on the roots of old oak trees may reach the size of a man's fist. Many of the various galls are brightly coloured.
The most satisfactory classification is that by the insects which cause them and the plant on which they are produced. Different types can however be distinguished by their external appearance and internal structure. Most of them are unilocular or monothalomous (containing but one chamber or cell), while others are plurilocular (polythalamous), i.e., many-celled and contain a number of insects. The ordinary oak galls pro duced by the attack of a wasp (Cynips) on a leaf bud of oak are unilocular. Following mainly Kerner and Oliver and Swanton (see Bibliography) galls may be distinguished as the simple and compound. The simple galls are produced from a single plant organ such as a leaf. These may again be distinguished as (I) felt galls where the epidermal cells become hypertrophied and grow into hair-like projections, and (2) mantle galls where a chamber is developed round the insect. Mantle galls may further be dis tinguished as scroll galls (where the leaf or petiole becomes rolled and thickened), pocket galls (where a pocket or excavated cham ber is produced) and covering galls (where the insect is covered in by the over-arching of tissues but the tissues do not fuse where they meet, and finally shrivel and leave a slit for the escape of the insect). The simple galls also include (3) the solid or tuber cular galls of which the well known spherical marble gall on the oak is a good example, and the root galls. In addition there are compound galls in the growth of which several organs of the plant are concerned. These may be (I) bud-like galls which may arise from modified foliage buds or modified flower buds; (2) galls at the base of shoots, where the upper part of the shoot continues a normal growth beyond the gall; (3) rosette galls which may be developed either in the foliage or the floral region.
As already stated insects are the chief agents while eelworms play a part, fungi not being properly included. Of the insects the classes concerned may be Hymenoptera (wasps and sawflies), Coleoptera (beetles), Lepidoptera (moths), Dip tera (flies), as well as Hemiptera (aphides), and Acari (mites). The eelworms (Nematoda) belong to such genera as Heterodera and Tylenchus. Among the Hymenoptera (q.v.), the group Cynipideae (gall-wasps) are best known as gall producers. The members of this group (e.g., Cynips, Andricus, Biorrhiza, Rho dites), have received more attention as gall producers than any other insects. This is partly due to the fact that they are re sponsible for many of the galls so commonly found on the oak but also to the fact that many of the insects show alternation of generation, i.e., the young produced do not resemble their parents but their grandparents, and with these two different generations different galls may be associated. In the case, for example, of Biorrhiza pallida the female gall-wasp pricks the oak bud and lays its eggs therein with the result that the familiar gall known as "oak-apple" is produced. From this gall the wasps emerge in July, the males winged and the females wingless or with rudimentary wings only. The female, who is smaller than her parents, crawls down the oak trunk, reaches the ground, and then pierces the roots of the oak producing the other type of gall, the root gall. In other cases the same organ (leaf or bud) may be attacked in each case but different galls are produced. The gall-wasps are small insects with straight antennae and a compressed and usually very short abdomen with the second or second and third segments greatly developed, the rest concealing the partially coiled ovipositor. The transformation from the egg through the larval state to the imago (the perfect insect) takes place in the gall, the imago boring its way out of the gall usually in the autumn. Among the galls pro duced by Cynips and its allies are the "oak-apple" or "oak sponge" produced by Andricus terminalis, the currant or berry galls of Spatheguster baccarum and the "oak spangles" of Neurotenes len ticularis. The marble galls or "Devonshire woody galls" of oak buds which often destroy the leading shoots of young oak trees are produced by Cynips Kollari. They were first introduced into Devonshire about 1847 and after a time spread widely. The large purplish Mecca or Bassorah galls produced by Cynips insana have been regarded by some as the Dead Sea fruit, mad apple or apples of Sodom alluded to by Josephus and others, but more probably the fruit referred to is that of a species of Asclepias.
Oak galls or gall nuts (produced by cynips) were once a valu able article of commerce (for ink making, etc.)—they may contain as much as 7o% of tannin The "blue," "black" or "green" galls still contain the insect ; the inferior "white" galls are lighter col oured and are gathered after the insect has escaped.
The saw-flies (also members of the Hymenoptera) so named on account of the saw-like nature of the egg depositing apparatus (the ovipositor) produce galls on willow, rose and other plants; on the willow, bud galls, stem galls and leaf galls are all pro duced. The red, oval or kidney shaped swellings on the blade of the willow leaf are perhaps the best known saw-fly galls in Great Britain. Of beetles (Coleoptera) in spite of their large numbers (150,000 or more species) only a very few are responsible for plant galls. Gorse and vetches may be attacked, and also the toadflax (Linaria vulgaris) . Ceuthorhynchus sulcicollis attacks the underground portions of various plants (turnips, swedes, cabbages, etc.) of the family Cruciferae. The swellings produced are sometimes confused with the malformations due to the dis ease, "finger and toe," caused by a fungus Plasmodiophora bras sicae.
Galls caused by moths (Lepidoptera) are also infrequent and not of great economic importance. Rhyacionia resinella produces a gall upon Pinus silvestus, destroying the terminal bud and thus retarding the growth of the tree. Among the flies (Diptera) those which cause galls belong mostly to the gall midges (Cecidiomyidae) . Spiraea ulmaria (meadowsweet) and S. filipendula are attacked by Perrisia ulmariae, the galls appearing as small, glabrous, light green pustules; over 20o have been counted on one leaf. The ger mander speedwell (Veronica Chamaedrys) and the nettle (Urtica) are "galled" by species of Perrissia, as are also willows. On wil lows a number of galls are also produced by species of Rhabdo phaga. The frit-fly (Oscinis frit) causes the new shoots of cereals and pasture grasses to become swollen and distorted. The ash, maple, hornbeam, oak, grape-vine, alder, gooseberry, blackberry, pine, juniper, thistle and fennel are also galled by insects of this class. Among the order of Hemiptera, the Aphidae (green fly), the Psyllidae (plant lice) and the Coccidae (scale insects and mealy bugs) all include forms which are gall producers. The majority of these belong to the first class Aphidae the members of which are generally known as "blight" or "green-fly." Of the galls produced by aphids the best known are probably those found on the spruce fir (Abies excelsa), resembling immature cones in size and shape. They are sometimes called "pineapple" galls and are due to species of Chermes. The galls develop near one end of a twig and are large and plurilocular containing in some cases as many as twenty-five cavities; a single gall may contain 2,000 insects. The mites belonging to the order Acari of the class Arachnida (which includes the spiders and scorpions) are respon sible for a number of galls. The gall-formers have only two pairs of legs, no eyes and belong to the group Vermiformia ; they are very minute and are easily developed. The mite-galls are nearly always characterized by a felt of hairs which develop in associa tion with the gall by the abnormal growth of the epidermal cells of the leaf attacked. These galls are mostly caused by species of the genus Eriophyes; they are found on the sycamore, pear, plum, ash, alder, vine, mulberry, etc. Those on the sycamore leaf due to Eriophyes macrorrhynchus are very common in Great Britain in June and July, and several hundred may be found on a single leaf. They are small, elongated outgrowths, about 2 mm. broad by 3 mm. long, tapering somewhat at the free end. They are bright red in colour with a tuft of hairs at the base and also bearing hairs in the hollow cavity and at the aperture of the gall; they are borne on the upper surface of the leaf. The gall-mite, Eriophyes ribis causes "big-bud" in black currants and is a serious pest, while Eriophyes pyri is the pear leaf blister mite.
Among the order Nematoda the genera Heterodera, Tylenchus and Aphelenchus (belonging to the family Anguillulidae, to the members of which the term eelworms is applied) cause galls. Either the aerial or underground parts may be attacked. A num ber of the infections are of economic importance.
The insect galls often contain, besides the larva of the insect responsible for the gall, so called inquilines or lodgers. They feed on the substance of the gall and so deprive the normal "householder" of part of the food supplies and frequently kill oft the rightful owner. There are also definite parasites (generally small Hymenoptera belonging to the family Chalcididae) which kill and devour not only the primary occupant of the gall but also the "guest flies." Derham, who has been already referred to, recognized in the early years of the 18th century that galls may contain interlopers. His quaint language may be quoted : "I ap prehend we see many vermicules towards the outsides of many oak-apples, which I guess were not what the primitive insects laid up in the germ from which the oak-apple had its rise, but from some supervenient additional insects laid in after the apple was grown, and whilst it was tender and soft." The presence of dif ferent classes of insects within the same gall renders investigation very difficult.
Fungi.—A number of fungi cause marked malformations of the portions of the host plant attacked. For example, Synchy trium endobioticurn causes wart-like outgrowths of the potato, Plasmodiophora brassicae swellings on turnip and cabbage roots. In these cases, however, the tissue of the outgrowth is usually penetrated by the fungus so they are hardly comparable to the insects galls, where the abnormal tissue is purely vegetable in nature and grows round the insect responsible for the malforma tion. A few cases which are sometimes confused with true galls may be mentioned. Cultivated plums, the sloe and the wild cherry may show a condition in which the fruit is swollen and deformed; this is known as "bladder plum" and is due to the attack of a fungus, Exoascus pruni, belonging to the class Ascomycetes (see FUNGI). The so-called witches' brooms (q.v.), are found on various trees and are sometimes mistaken for galls ; they are con spicuous as groups of short, generally unhealthy twigs with re duced leaves, the branches of the "broom" growing upward. On birch these brooms may be caused by the fungus Exoascus tur gidus though they may also be caused by a mite of the genus Eriophyes. Those on cherry are due to Exoascus cerasi, and those on silver fir are due to the attack of a rust fungus Melanosporella elating. The large succulent often reddish swellings found on the leaves of the cowberry (Vaccinium Vitis-idaea) and the cranberry (Oxycoccus quadri-petalus) and the still larger ones on species of Rhodendron are due to fungi belonging to the genus Exobasidium (see FUNGI).
see A. Kerner von Marilaur, Natural History of Plants, Vol. II. (trans., F. W. Oliver, London, 1894) ; N. Adler and C. Straton, Alternating Generations: A Biological Study of Oak Galls and Gall Flies (London, 1894) ; E. T. Connold, British Vegetable Galls (London, 19o1) ; British Oak Galls (London, 19o8) ; Plant Galls of Great Britain (London, 19o9) ; E. Kiister, Die Gallen des Pflanzen (Leipzig, 1911) ; E. W. Swanton, British Plant Galls (London, 1912). (V. H. B.)
