Page:EB1911 - Volume 21.djvu/789

Rh wounds due to plants which entwine, pierce or otherwise materially injure trees, &c., on a large scale.

4. Excrescences.—Outgrowths, more or less abnormal in character, are frequent signs of diseased organs. They are due to hypertrophy of young tissues, which may undergo profound alterations subsequently, and occur on all parts of the plants. The injury which initiates them may be very slight in the first place—a mere abrasion, puncture or Fungus infection—but the minute wound or other disturbance, instead of healing over normally, is frequently maintained as a perennial source of irritation, and the regenerative tissues grow on month after month or year after year, resulting in extraordinary outgrowths often of large size and remarkable shape. Excrescences may be divided into those occurring on herbaceous tissues, of which Galls are well-known examples, and those found on the wood stem, branches, &c., and themselves eventually woody, of which Burrs of various kinds afford common illustrations. Among the simplest examples of the former are the hairs which follow the irritation of the cells by mites. These hairs often occur in tufts, and are so coloured and arranged that they were long taken for Fungi and placed in the “genus” Erineum.

Cecidia or galls arise by the hypertrophy of the subepidermal cells of a leaf, cortex, &c., which has been pierced by the ovipositor of an insect, and in which the egg is deposited. The irritation set up by the hatching egg and its resulting larva appears to be the stimulus to development, and not a poison or enzyme injected by the insect. The extraordinary forms, colours and textures of the true galls have always formed some of the most interesting of biological questions, for not only is there definite co-operation between a given species of insect and of plant, as shown by the facts that the same insect may induce galls of different kinds on different plants or organs, while different insects induce different galls on the same plant—e.g. the numerous galls on the oak—but the gall itself furnishes well adapted protection and abundant stores of nutriment to its particular larva, and often appears to be borne without injury to the plant. This latter fact is no doubt due to the production of an excess of plastic materials over and above what the tree requires for its immediate needs. Galls in the wide sense—technically Cecidia—are not always due to insects. The nodules on the roots of leguminous plants are induced by the presence of a minute organism now known to do no injury to the plant. Those on turnips and other Cruciferae are due to the infection of Plasmodiophora, a dangerously parasitic Myxomycete. Nodules due to “eel-worms” (Nematodes) are produced on numerous classes of plants, and frequently result in great losses—e.g. tomatoes, cucumbers, &c.; and the only too well known Phylloxera, which cost France and other vine-growing countries many millions sterling, is another case in point. Fungus-galls on leaves and stems are exemplified by the “pocket-plums” caused by the Exoasceae, the black blistering swellings of Ustilago Maydis, the yellow swellings on nettles due to Aecidium, &c.

In many cases the swellings on leaves are minute, and may be termed pustules—e.g. those due to Synchytrium, Protomyces, Cystopus, many Ustilagineae, &c. These cases are not easily distinguished superficially from the pustular outgrowth of actual mycelia and spores (stromata) of such Fungi as Nectria, Puccinia, &c. The cylindrical stem-swellings due to Calyptospora, Epichloe, &c., may also be mentioned here, and the tyro may easily confound with these the layers and cushions of eggs laid on similar organs by moths. There is a class of gall-like or pustular outgrowths for which no external cause has as yet been determined, and which are therefore often ascribed to internal causes of disease. Such are the cork-warts on elms, maples, &c., and the class of outgrowths known as Intumescences. Recent researches point to definite external conditions of moisture, affecting the processes of respiration and transpiration, &c., as being responsible for some of those. The “scab” of potatoes is another case in point. Frost blisters are pustular swellings due to the up-growth of callus-tissue into cavities caused by the uprising of the superficial cortex under the action of intense cold.

Turning now to outgrowths of a woody nature, the well-known burrs or “knaurs,” so common on elms and other trees are cases in point. They are due to some injury—e.g. bruising by a cartwheel, insects—having started a callus on which adventitious buds arise, or to the destruction of buds at an early stage. Then, stores of food-material being accumulated at the injured place, other buds arise at the base of or around the injured one. If matters are propitious to the development of these buds, then a tuft of twigs is formed and no burr; but if the incipient twigs are also destroyed at an early stage, new buds are again formed, and in larger numbers than before, and the continued repetition of these processes leads to a sort of conglomerate woody mass of fused bud-bases, not dead, but unable to grow out, and thus each contributing a crowded portion of woody material as it slowly grows. There are many varieties of burrs, though all woody outgrowths of old trees are not to be confounded with them, e.g. the “knees” of Taxodium, &c. Many typical burrs might be described as witches'-brooms, with all the twigs arrested to extremely short outgrowths. Witches'-brooms are the tufted bunches of twigs found on silver firs, birches and other trees, and often present resemblances to birds' nests or clumps of mistletoe if only seen from a distance.

They are branches in whicha perennial Fungus (Aecidium, Exoascus, &c.) has obtained a hold. This Fungus stimulates the main twig to shoot out more twigs than usual; the mycelium then enters each incipient twig and stimulates it to a repetition of the process, and so in the course of years large broom-like tufts result, often markedly different from the normal.

But undoubtedly the most important of the woody excrescences on trees are cankers. A canker is the result of repeated frustrated attempts on the part of the callus to heal up a wound. If a clean cut remains clean, the cambium and cortical tissues soon form callus over it, and in this callus—regenerative tissue—new wood, &c., soon forms, and if the wound was a small one, no trace is visible after a few years. But the occluding callus is a mass of delicate succulent cells, and offers a dainty morsel to certain insects—e.g. Aphides—and may be easily penetrated by certain Fungi such as Peziza, Nectria; and when thus attacked, the repeated conflicts between the cambium and callus, on the one hand, trying to heal over the wound, and the insect or Fungus, on the other, destroying the new tissues as they are formed, results in irregular growths; the still uninjured cambium area goes on thickening the branch, the dead parts, of course, remain unthickened, and the portion in which the Fungus is at work may for the time being grow more rapidly. Such cankers often commence in mere insect punctures, frosted buds, cracks in the cortex, &c., into which a germinating spore sends its hypha. The seriousness of the damage done is illustrated by the ravages of the larch disease, apple canker, &c.

5. Exudations and Rotting.—The outward symptoms of many diseases consist in excessive discharges of moisture, often accompanied by bursting of over-turgid cells, and eventually by putrefactive changes. Conditions of hyper-turgescence are common in herbaceous plants in wet seasons, or when overcrowded and in situations too moist for them. This unhealthy state is frequently combined with etiolation: what is termed rankness is a particular case, and if the factors concerned are removed by drainage, weeding out, free transpiration, &c., no permanent harm may result. With seedlings and tender plants, however, matters are frequently complicated by the onslaughts of Fungi—e.g. Pythium, Peronospora, Completoria, Volutella, Botrytis, &c. That such overturgescence should lead to the bursting of fleshy fruits, such as gooseberries, tomatoes and grapes, is not surprising, nor can we wonder that fermentation and mould Fungi rapidly spread in such fruits; and the same is true for bulbs and herbaceous organs generally. The rotting of rhizomes, roots, &c., also comes into this category; but while it is extremely difficult in given cases to explain the course of events in detail, certain Fungi and bacteria have been so definitely associated with these roots—e.g. beet-rot, turnip disease, wet-rot of potatoes—that we have to consider each case separately. It is, of course, impossible to do this here, but I will briefly discuss one or two groups of cases.

Honey-dew.—The sticky condition of leaves of trees—e.g. lime—in hot weather is owing to exudation's of sugar. In many cases the punctures of Aphides and Coccideae are shown to be responsible for such exudations, and at least one instance is known where a Fungus—Claviceps—causes it. But it also appears that honey-dew may be excreted by ordinary processes of over-turgescence pressing the liquid through water-pores, as in the tropical Caesalpinia, Calliandra, &c. That these exudations on leaves should afterwards serve as pabulum for Fungi—e.g. Fumago, Antennaria—is not surprising, and the leaves of limes are often black with them.

Flux.—A common event in the exudation of turbid, frothing liquids from wounds in the bark of trees, and the odours of putrefaction and even alcoholic fermentation in these are sufficiently explained by the coexistence of albuminous and saccharine matters with fungi, yeasts and bacteria in such fluxes. It is clear that in these cases the obvious symptom—the flux—is not the primary one. Some wound in the succulent tissues has become infected by the organisms referred to, and their continued action prevents healing. At certain seasons the wound “bleeds,” and the organisms—some of which, by the bye, are remarkable and interesting forms—multiply in the nutritious sap and ferment it. The phenomenon is, in fact, very like that of the fermentation of palm wine and pulque, where the juices are obtained from artificial cuts.

Comparable with these cases is that of Cuckoo-spit, due to the juices sucked out by Aphrophthora, on herbaceous plants of all kinds. Outflows of resin—Resinosis—also come under this general heading; but although some resin-fluxes are traced to the destructive action of Agaricus melleus in Conifers, others, as well as certain forms of Gummosis, are still in need of explanation.

Bacteriosis.—Many of the plant diseases involving rot have been ascribed to the action of bacteria, and in some cases—e.g. cabbage-rot, bulb-rot of hyacinths, &c., carnation disease—there is evidence that bacteria are causally connected with the disease. It is not sufficient to find bacteria in the rotting tissues, however, nor even to be successful in infecting the plant through an artificial wound, unless very special and critical precautions are taken, and in many of the alleged cases of bacteriosis the saprophytic bacteria in the tissues are to be regarded as merely secondary agents.

6. Necrosis—A number of diseases the obvious symptoms of which are the local drying up and death of tissues, in many cases