Excrescences, or out-growths of more or less abnormal character from the general surface of diseased organs, are very common symptoms, and widely recognised. They are due to hypertrophy of the tissues while the cells are young and capable of growth, and may be induced by a variety of causes, among which the stimulus of insect-punctures and of the presence of insect eggs are best known; but that of fungi, though less widely recognised, plays an equally important part, and, as we shall see, galls and other excrescences may be due to widely different agents.
Galls or Cecidia are protuberances of the most varied shapes, colours, and sizes found on herbaceous parts attacked by insects, fungi, etc. In the simplest cases the insects only pierce and suck the young cellular tissue - e.g. Phytoptus, Aphides, etc. - but in others the stimulus to hypertrophy starts by the puncture of the embryonic tissue of a leaf, root, etc., by the ovipositor of the female insect, which then lays an egg - e.g. Cynips, Cecidomyia, etc. - the presence of which appears to intensify the irritating action, or such only occurs when the young larva escapes.
Our knowledge of the primary cause of gall-formation amounts to very little. Generally speaking, only embryonic or very young cellular tissue reacts, and galls on adult leaves and branches have usually been initiated long before. The same gall-insect may induce totally different galls on different plants, or even on different parts of the same plant, and different insects call forth different galls on any one plant. These facts point clearly to the co-operation of both plant and insect in the gall-formation, and the best hypothesis yet to hand is to the effect that a gall is a hypertrophy of cells, the normal nutrition, growth, and division of which have been disturbed owing to the action of some poison or other irritant derived from the insect, or fungus, or other organism. Attempts have been made to reproduce galls by injecting the juices of similar galls into the tissue, but as yet without success, and this may point to the co-operation of mechanical irritation during the hypertrophy in normal gall-formation.
Galls, in the broad sense, are not always preceded by a wound, however. Insects on the outside of young tissues may cause such irritations that the parts in contact with the animal are arrested in their growth, while those further away grow more rapidly - e.g. where Mites, etc., cause puckers and leaf-rolling. In true galls the hypertrophy may consist merely in the enlargement of cells already present, and no new cell-divisions and, still less, changes in the nature of the tissues result - e.g. some pocket galls on Viburnum, Pyrus, etc., and the hairy outgrowths of the epidermis known as Erineum. In other cases there is not only hypertrophy of existing cells, but new cell-divisions are instituted: these cell-divisions may be confined to the direction perpendicular to the epidermis, and the tissues grow - only in the direction of the surface, producing puckerings - e.g. the Aphis galls on Ribes, Phytoptus galls of Salvia, leaf galls on Tilia, Acer, Alnus, etc., and the curious galls on Plums due to Cecidomyia Pruni, and which must not be confounded with the "pocket plums" and similar galls due to Exoasci.
In a third series of cases, cell-divisions occur parallel to the surface of the leaf, and galls are formed which grow in thickness, and develop the most extraordinary and complicated new tissues - proteid-cells surrounding the egg or larva deposited inside, followed by a protective layer of sclerenchyma encasing this food layer, and around this again softer tissues which may assume the structures and functions of respiratory tissues, water-storing tissues, starch reservoirs, assimilatory, or protective tissues of various kinds, and over all may be a well-marked epidermis, with stomata, or cork with lenticels.
The chief seat of these hypertrophies and - what is more remarkable - development of new tissue elements not found elsewhere in the leaves, or even in the species, is the mesophyll, and various speculations and hypothesis have been founded on these curious phenomena.
Intumescences are similar trichomatous outgrowths not associated with insects or fungi, and due to some disturbance of the balance between transpiratory and assimilatory functions of their leaves, as indicated by the less localised occurrence and by their non-appearance when the plant is under favourable cultural conditions. Structures not unlike these have been artificially induced by exposure to particular lights, and also by painting spots with dilute corrosive sublimate, indicating that poisons may impel the epidermis cells to grow out abnormally.
Frost-blisters are pustule-like uprisings of the cortex, where the living tissues below have formed a callus-like cushion into the cavity beneath the dead outer parts of the cortex which were killed by the frost; they occur on the stems of young Apples, Pears, etc.
Galls in the narrower sense are tissue outgrowths usually involving deeper cell-layers. They are so varied and numerous that classification is difficult. For symptomatic purposes we may divide them as follows:
Leaf-galls. - A well-marked type is that of the pocket-galls or bladders in which the whole thickness of the leaf is as it were pushed up like a glove-finger at one spot, so that if the upper surface of the leaf forms the outside of the gall the lower surface is its lining. Such galls are common on Limes (Phytoptus), Glechoma (Cecidomyia), Elms (Tetraneura), etc. Similar localised extension of the leaf surface, compelling it to rise up like a pocket, are caused by fungi - e.g. Taphrina on Poplars, Exoascus on Birches, etc., Exobasidium on Bilberries, Rhododendrons, etc.
Another type is that of the Gall-apple, so well known on Oaks, where the spherical swelling is solid - except for the inner cavity containing the eggs - Neurotus, Cynips, Hormomyia, etc. These are comparable in general characters to the nodules on roots.
Fungus galls with similar external features when young are found on Maize (Ustilago Maydis), and betray their nature by the black powdery spores as they mature.
Bud galls on Willows are due to Cecidomyia, which causes several internodes to swell out into a greenish barrel-shaped mass, from which leaves may spring.
Small irregular excrescences on Willow stems are referred to Phytoptus, and another species of the same insect induces similar swellings on Pines which are not surcharged with resin.
American Blight, or Woolly Aphis, on Apples especially, causes the tumour-like swellings covered with sticky white fluff, which is a waxy excretion of the insect. Galls on Pilea, in Java, are due to an Alga-Phytophysa.
Root-nodules or nodosities are frequently caused by insects - e.g. Centhorhynchus, a beetle which attacks Crucificers, Cynips and allied "gallflies" of Oaks, and the notorious Phylloxera. But similar root-galls are produced by Nematode worms, Heterodora, on Beets, Tomatoes, Cucumbers and numerous other plants, and by the Slime fungus Plasmodiophora, and it is not always easy to distinguish such cases from the fungus-galls (Mycocecidia) on the roots of Alders, funcus, and Leguminoseae where the symbiosis of bacteria or fungi with the roots are of benefit to the plant. Urocystis Leimbachii forms similar nodules at the collar of young plants of Adonis.
Heterodora javanica passes into the cortex of sugar-cane roots through fissures, and makes its way to the place where a young rootlet is about to emerge; here it sticks its beak into the growing-point and remains fixed.
Molliard has shown that in the roots of Melons, Coleus, etc., Heterodora causes the cells in immediate contact with its head, and which would normally become vessels of the xylem, to swell up into huge giant-cells, with their walls curiously folded, and containing large supplies of proteids and numerous nuclei, reminding us of the food-layer of insect galls and of the tapetal layer of pollen-sacs. While the stimulus exerted by the Nematode thus induces hypertrophy and storage with food-substances of these cells, those of the next layers undergo reticulate thickenings of their walls. Again instances of the evolution of new tissue elements by the action of the foreign organism.
So far as galls on leaves are concerned the amount and kind of damage done are in proportion to the area of chlorophyll action put out of play for the benefit of the plant, and the remarks already made on p. 193 apply here also. Where buds are destroyed the effects may of course extend further, but it rarely happens that leaf-galls are so abundant as to maim a tree permanently. Nevertheless we must remember that cases like Phylloxera are notorious.
Far more dangerous, however, are the root-galls due to such insects, because here the damage is not so local: the water-supplies are cut off, and injurious consequences result from the absorption of the products of decomposition in the soil.
In addition to the literature on galls quoted in the Notes to Chapter XIV., the reader should consult Dale "On certain Outgrowths (Intumescences) on the green parts of Hibiscus" Proc. Cambr. Phil. Soc., Vol. X., 1899, p. 192, and Brit. Ass. Rep., Bradford, 1900.
The detailed study of the anatomy and histology of Galls has been recently undertaken by Kiister, "Beitrage zur Kenntniss der Gallenanatomie," Flora, B. 87, 1900, p. 117, where the principal references will be found.
On the root-galls due to Nematodes see Atkinson in Science Contributions from the Agric. Expt. Station, Alabama, Vol. I., p. 1, 1889; Percival, "An Eel-worm disease of Hops" in Natural Science, Vol. VI., 1895, P. 187; and Molliard in Revue generale de Botanique, Apl., 1900, p. 157, where the histology is dealt with.
The nodules of the roots of Leguminosae are not part of the subject of this work: the literature is collected in Science Progress, 1895, Vol. III., p. 252, and Dawson, Phil. Trans., 1900.