Notes to Chapter XXII
In addition to the notes to the last chapter, the reader may be referred to Fisher in Vol. IV. of Schlich's Manual of Forestry, Chap. VI., for an account of Hess' excellent work on Boring Beetles, etc.
The authority on Wood-fungi is Hartig, see especially his Zersetzungs-erscheinungen des Holzes, the principal results of which are condensed in his Diseases of Trees already referred to. As regards "Pith-flecks," the reader should consult Frank, Krankh. der Pflanzen, B. I., p. 212: the subject needs further investigation.
CHAPTER XXIII.
EXCRESCENCES
Herbaceous excrescences, or galls—Erineum—Intumescences—Corky warts, etc.—Pustules—Frost-blisters—Galls and Cecidia—Root nodules.
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.
Erineum.—The simplest excrescences on plants are certain hair-like developments of epidermal cells due to the irritation of species of Phytoptus, and similar insects which rise in clusters on the surfaces of leaves and by their colours, consistence, arrangement in patches, spots, etc., so simulate fungi that Persoon was deceived by them and gave them the genus name Erineum. They occur on most of our trees, e.g. Poplar, Lime, Oak, and are very common in the Tropics. Usually pale or even white at first, they turn brown as the hair-like outgrowths die and lose their sap, but since the latter may be bright coloured—yellow, red, purple,—the patches are sometimes very conspicuous objects on smooth leaves.
In many cases these hairs exactly resemble in shape and other characters the abnormal root-hairs found on roots exposed to the effects of poisonous reagents, or of unsuitable food-materials, or the rhizoids developed from wounded Algae, etc.
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.
Corky warts.—Several forms of disease are known in which the pathological condition is expressed by the formation of cork in unwonted places and quantities. The Scab or Scurf of Potatoes is a case in point. The tissue of the lenticels absorbs water and the outermost cells are cut off by cork and die: the cells below them burst the dead bark-like masses thus formed, and again cork is formed and cuts off the outer masses, and the rough cork warts—Scab or Scurf—are the result.
The causes predisposing to scab have been variously assigned to dampness, want of lime, action of bacteria and fungi—e.g. Sorosporium, Oospora, Spongospora,—the latter making their way into the ruptured tissue of the lenticels and irritating the cells to further growth.
It seems probable that several different kinds of scab exist in Potatoes, as well as in roots—e.g. Beets, and the whole subject needs further investigation. The scab-like rough scaly bark of Pear trees in dry districts may also be mentioned here.
Cork-wings are well known on the young branches of Elms, Maples, etc., some varieties of which have received specific names on this account.
Corky excrescences on leaves occur occasionally in the Gooseberry, Holly and other plants, for which no cause has been discovered.
Lenticels are also formed on some leaf-galls, and are remarkable as being structures not normal on leaves.
Pustules.—This term may be employed generally for all slight upheavals of the surfaces of herbaceous organs, which subsequently burst and give egress to the spores, etc., of the organism causing them, or merely fray away at the top if no organism is discoverable. They are often due to fungi—e.g. Synchytrium, Protomyces, Cystopus, and Ustilagineae,—and we may extend the use of the general term also to those cases where the stroma of the fungus itself bursts through the cortex of older parts and forms the principal part of the pustule—e.g. Monilia, forming white or grey pustules on Apples, Roestelia and other Æcidia, forming yellow or orange pustules on leaves, etc.; Cucurbitaria and Nectria (red) breaking through the cortex of trees, and Phoma and numerous other Ascomycetes which form black cushions. Pustules on the leaves of Lysimachia, Ajuga, etc., are due to the parasitic Alga Phyllobium.
Cylindrical stem swellings are caused by Calyptospora: they are due to the hypertrophy of the cortex of Bilberry stems permeated by the hyphae. Epichloë, which clothes the sheaths and halms of grasses with its stroma, at first snowy white and later ochre-yellow as the perithecia form, is another example.
The cylindrical layer of eggs of a moth such as Bombyx on a twig must not be confounded with these cases.
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 Crucifers, 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, Juncus, 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.
Notes to Chapter XXIII
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 Küster, "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 générale de Botanique, Apl., 1900, p. 157, where the histology is dealt with.
The nodules of the roots of Leguminoseae 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.
CHAPTER XXIV.
EXCRESCENCES (continued)
Cankers—Burrs—Sphaeroblasts, and other excrescences of woody tissues—Witches' Brooms.
Cankers are irregular excrescences due to the perennial struggle between tissues attempting to heal up a wound, and some organism or other agent which keeps the lesion open. A canker always originates in a wound affecting the cambium, and usually in a small wound such as an insect puncture or frost nip; if undisturbed the dead parts would heal over by cork and callus, but if recurring frost-cracks break open the coverings, or if insects or fungi penetrate the callus and invade the cambium, irregularities of growth due to the occluding tissue on the one hand, and continued growth of the still unimpaired cambium on the opposite side of the injured shoot on the other, result in the canker. Frost cankers occur on fruit-trees, Vines, Beeches, etc.
Cankers due to insects are found on Apples, the cortex of which is punctured by the woolly Aphis (Schizoneura) while the twigs are young, and the wound is kept open by the insects nestling in crevices in the occlusion tissues. Species of Coccus, Lachnus, and Chermes also produce cankers on forest trees.
Cankers due to fungi usually originate in a wound primarily due to an insect puncture or bite, or to frost, the invading fungus hyphae making their way into the wounded tissues and gradually extending more and more into the cambium and the occluding callus. Among the best known of these wound fungi which cause cankers are Dasyscypha Willkommii the peziza of Larch disease, Nectria ditissima and N. cucurbitula on Beech and Conifers; less common are Scleroderris on Willows, Aglaospora on Oaks and some others.
Peridermium Pini and Aecidium elatinum also cause cankers under certain conditions, as also does Gymnosporangium, but in these cases the fungi are more truly parasitic.
In some cases—e.g. Ash, Pine, Olives—bacteria are concerned as associated organisms in the cankering of trees.
Burrs or Knauers are irregular excrescences, principally woody, with gnarled and warted surfaces. They are frequently due to some previous injury, such as the crushing or grazing of cortical tissues by cart-wheels. The excitation of the tissues thus wounded results in the development of shoots from adventitious or dormant buds at the base of old tree trunks, or in the starting of the same process where a branch has been broken off. The new bud begins to develop a shoot, but soon dies at its tip owing to paucity of food-supplies to the weak shoot, while new buds at its base repeat the process next year with the same result, and each of these again in turn, and so on. The consequence is an extremely complex nest of buds, all capable of growing in thickness and putting on wood to some extent, but not of growing out in length. In course of time this mass may attain dimensions measurable by feet, forming huge rounded and extremely hard-knotted burrs, the cross-section of which shows the vascular tissues running irregularly in all directions, and, owing to the very slow growth, extremely dense and hard. The dark spots in such sections—e.g. Bird's-eye Maple—are the cut bud-axes all fused together, as it were. On old Elms such burrs are common at heights on the stem which preclude the assumption of any coarse mechanical injury, and similar structures occur on the boles of other forest trees suddenly exposed to light by the felling of their companions, which suggests that these epicormic shoots result from some disturbance due to the action of light.
Witches' Brooms are irregular tufts of twigs often found among the branches of trees such as Birches, Hornbeam, etc., where they look like crows' nests, and similar structures are to be found on Silver Firs and other conifers. In the former case they are due to Exoascus, in the latter to Aecidium, fungi which are perennially parasitic in the shoots, and stimulate the twiggy development of a number of buds which would normally have remained in abeyance, or not have been formed at all, and only do so now in a fashion different from that of normal branches.
Rosette-like formations, depending on similar disturbing causes on the part of insects, occur in conifers—e.g. Gastropacha Pini.
Dense tufts of twiggy shoots may be developed on many trees by pruning in such a way as to stimulate the shooting out of basal buds which would otherwise remain dormant, e.g. Elm, Ash, and thus it occurs that injuries such as frost, insect bites, etc., may induce the production of such tufts in a tree crown. The dense nests of stool-shoots thrown up from felled tree-stumps are of essentially the same nature—partly adventitious and partly dormant buds being enabled to grow out because they can now be supplied with materials previously carried beyond them while the trunk was still there. Suckers, if repeatedly cut down, may also behave similarly.
Wood-nodules or Sphaeroblasts are curious marble-like masses of wood which protrude with a covering of bark from old trunks of Beeches, etc., and can be readily dug out with a knife. The nodule has arisen by the slow growth of the cambium of a dormant bud, the base of which separated at an early date from the wood beneath; the cambium then closed in over the base and laid on thickening rings all round the axis of the bud except at the extreme apex. When the separation occurred the cambium of the wood beneath covered over the previous point of junction, and thus the woody bud was pushed out with the bark, and now protrudes covered with a thin layer of the latter. Similar nodules are occasionally found on Apple trees.
Notes to Chapter XXIV
For further information on Cankers the student should read Marshall Ward, Timber and some of its Diseases, Chapter X. Further, the discussion as to the causes of canker in Frank, Krankheiten der Pflanzen, B. I., p. 207, and B. III., pp. 167 and 172, and various papers in Zeitschrift für Pflanzen-krankheiten.