Disease in plants

CHAPTER XXII.

NATURAL WOUNDS.

Natural wounds are produced in a variety of ways during the life of the plant, and, generally speaking, are easily healed over by the normal process if the area destroyed is not too large, and the parts remaining uninjured are sufficiently provided with foliage, or with supplies of food-materials stored up in the roots, rhizomes, medullary rays, etc., to feed a vigorous callus.

The nature of such wounds and the mode of healing are explained by what we know of artificial wounds, and it only remains to point out that the principal danger of ordinary wounds is not so much the direct traumatic action, because the simpler organisation of the plant does not involve matters connected with shock, loss of blood, etc., as in animals; the danger consists, rather, in their affording access to other injurious agents, especially fungi, and the treatment of wounds frequently resolves itself into cutting or pruning in order to get clean surfaces which can heal readily.

Wounds on leaves imply loss of foliar surface—i.e. of chlorophyll action—and the remarks on page 193 apply.

Burrows may be taken as comprising all kinds of tunnel-like excavations in the various organs of plants, including those cases where insects burrow into hollow stems of grasses, etc., as indicated by the perforations they make in the outer tissues.

Bark-boring is done by many species of beetles, especially Scolytidae, which excavate characteristically formed branching passages tangentially in the inner bark of Conifers and other trees. Some of them also bore down to the surface of the sap wood (e.g. Tomicus bidentatus) or even burrow right into the latter (e.g. T. lineatum). It commonly happens that the external apertures show up clearly, owing to the brown dust and excrement, sometimes accompanied by turpentine, which exude from them. Many of these Bark beetles only attack trees which are already injured by fire, lightning, etc.; possibly they cannot bore through a cortex which swamps them with sap, as a vigorous one might do.

Wood-boring is also done by many of the bark-beetles as well as by Longicorns, e.g. Saperda in Poplars and Willows, the young shoots of which often show characteristic swellings with lateral holes indicating the points of exit. From the external apertures comminuted wood, like saw-dust, is frequently ejected in quantity and betrays the presence of the insects. Certain wood-wasps (Sirex) and the larvae of moths (Cossus) also make large perforations in the wood of Willows and other trees, often destroying it completely. In the case of these larger borers, whose tunnels may be as broad as the little finger, the foul smell as well as abundant "saw-dust" betray the evil.

Excavations in wood are by no means caused only by insects: several of the larger Hymenomycetes—Stereum, Thelephora, Polyporus, etc.—tunnel the timber in characteristic ways and often after a fashion very suggestive of insects. They usually obtain access through fractures.

Tunnels in leaves are invariably due to the activity of miners belonging to the smaller moths and beetles—e.g. Tinea, Orchestes, etc.—the larvae of which eat out the mesophyll but leave the covering epidermis or cuticle untouched, and since the insect bores forwards only, in an irregular track, and leaves its excrement in the winding passage, the effect is very characteristic.

Whitish leaf tunnels in Peas are excavated by Phytomyza.

Characteristic foxy-red tunnels are mined in the leaves of Apples by Lyonetia, Coleophora, etc.

Falling of fruit, of Apples, Plums, Apricots, etc., before they are ripe, is frequently due to insects, of which the various species of Grapholitha or Carpocapsa are conspicuous: the fallen fruits show a small hole leading by a labyrinth of passages to the "core" or "stone," and in which the grub and its excrement are visible. The cutting off of the vascular bundles and disturbance of the water supply only partly explain the premature fall.

Pith-flecks are minute brown specks or patches found in the wood-layers of many trees, and consist of dead parenchymatous thick-walled cells, reminding one of the structure of pith. They are explained as due to the borings of minute insects, Diptera or Beetles, the larvae of which pierce the cortex and phloem and bore their way into the cambium. The latter then occludes the tunnels by filling them up with cells, and continuing its wood-forming activity gradually buries them deeper and deeper in the wood. Such pith-flecks are common in Willow, Birch, Alder, Sorbus, etc. It is possible that they may be due to other causes also in other trees.

Erosions or irregular wounds on leaves are caused by large numbers of grubs and caterpillars and other insects, such as earwigs, as well as slugs, snails, and other animals; but it must by no means be assumed that all marginal leaf wounds, for instance, are caused by animals, since many fungi which rot the tissues, as explained below (p. 208), also cause such erosions, the putrescent parts falling out—e.g. the Potato disease.

Skeleton leaves frequently result from the ravages of caterpillars, which leave the coarser ribs and veins untouched, but much finer skeletons with the minute veins almost intact may be found on plants infested with certain insects—e.g. Selandria on Cherries. Skeletonised patches on Cherry leaves, often pink or brown-pink, are eaten out by this grub.

Shot-holes are perforations in leaves presenting the appearance, from their more or less rounded shape, of gunshot wounds. They may be due to insects which bore through the young leaves while still folded in the bud—e.g. Willow Beetle—or which gnaw out the tissue—e.g. the Beech Miner. Similar but usually more torn and irregular holes are eaten out by many caterpillars—e.g. the Cabbage Moth.

Shot-holes on Peas may be the work of Thrips.

Leaf perforations are commonly caused by severe hail-storms, the hail-stones beating right through the thin mesophyll. Certain chemicals used for spraying have also been known to cause shot-holes by killing the tissue beneath the standing drops.

There is, however, a class of shot-holes in thin leaves which are due to the action of minute fungi, the mycelium of which so rots the tissues in a more or less circular area round the point of infection, that, in wet weather, the decomposing mass falls out and leaves a round hole—e.g. certain Chytridiaceae, Peronosporeae, Gloeosporium, Exoascus, etc. If dry weather supervenes these holes frequently dry at the edges, and the leaves appear as if eaten out.

Shot-holes in Cherry, Walnut, Tobacco, and Plum leaves are due to Phyllosticta, in Cherry leaves also to Clasterosporium, and in Potato leaves to Haltica.

Frost-cracks.—The trunks of trees exposed to the north-east, and occasionally with other aspects, are apt to show longitudinal ridges which realise on a larger scale the features of healed wounds scored with a knife. These wounds are due to the outer layers of wood losing water from their cell-walls as it congeals to ice in their lumina, more rapidly than do the warmer internal parts of the trunk; as this drying of the wood causes its shrinkage, especially in the tangential direction, the effect of a sudden frost and north-east wind is to rend the wood, which splits longitudinally with a loud report, as may often be heard in severe winters. Since the cortex and bark are ruptured at the same time the total effect resembles that of a deep knife-cut, and the same healing processes result on a larger scale when the wood swells and closes up the wound again in spring. But this recently-closed lesion is evidently a plane of weakness, and if a similarly severe winter follows the wound reopens and again heals, and so on, until after a succession of years a prominent Frost-ridge results, which may finally heal completely if milder winters ensue or the tree be eventually protected.

Strangulations.—We are now in a position to understand the so-called strangulations which result when woody climbers, telegraph wires, etc., kill or injure trees by tightly winding round them. If strong wire is twisted horizontally round a stem, the growth in thickness of the latter causes the trapping of the cortex and cambium, etc., between the wire and the wood, and a ringing process is set up in consequence of the death of the compressed tissues. A callus then forms above the wound, as in the case of true ringing by means of a cut, and eventually bulges over the upper side of the wire: in the course of years this overgrowth may completely cover in the wire, and, pressing on to the lower lip of the wound, may at length fuse with the cambium below. Hereafter the thickening rings of wood are continuous over the buried wire. The process is obstructed by all the impediments referred to in dealing with ringing, and of course the stem thickens more above than below the wire. If the sapwood is thin, and the bark is so thick as to put great obstacles in the way of the junction of the upper and lower cambiums, death may result—the tree is permanently ringed. (See p. 201.)

Spiral grooves are frequently met with where Wood-bine or other woody climbers have twined round a young stem or branch, the upper lip of the groove always protruding more than the lower. If a kink or a crossing of two plants or branches of the twiner results in a complete horizontal ring, the results are as in the above cases of ringing and strangulation. Naturally grooved walking sticks are often seen.

Buried letters, etc.—These processes of healing by occlusion enable us to understand how letters of the alphabet, cut into the wood of trees, come to be buried deep in the timber as successive annual rings cover them in more and more. Chains, nails, rope, etc., have frequently been found thus buried in wood.

Notes to Chapter XXII.

CHAPTER XXIII.

EXCRESCENCES.

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.

CHAPTER XXIV.

EXCRESCENCES (continued).

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.

CHAPTER XXV.

EXUDATIONS AND ROTTING.

I put together in one artificial class a varied group of diseases, the principal symptom of which is the escape of fluids from the tissues, under circumstances which betray an abnormal state of affairs, often obvious, but sometimes only to be inferred. In many of these cases bacteria abound in the putrefying mass, and some evidence exists for connecting these microbes causally with the disease in a few of the more thoroughly investigated cases, but in no case has this been sufficiently demonstrated; and considering the ease with which bacteria gain access via wounds caused by insects and fungi, as well as by other agents, the necessity for rigid proof must be insisted upon before we can accept such alleged examples of Bacteriosis.

Tumescence.—It occasionally happens that herbaceous parts of plants pass into a condition of over-turgescence from excess of water in the tissues, an abnormal state which indicates pathological changes resulting from various causes, often not evident and therefore regarded as internal. Such disease was formerly termed Œdema or Dropsy. This disease is frequently due to the excessive watering of pot plants with large root systems and deficient foliage, in hot-houses with a saturated atmosphere: it is, therefore, primarily referable to diminished transpiration. It can sometimes be brought about by covering potato plants, for instance, with a bell-jar in moist, hot weather; and this, and the prevalence of the disease in hot-houses as compared with plants grown out of doors, point to the above explanation. Similar phenomena do occasionally occur out of doors in hot, moist situations or during wet seasons, however, and the watery shoots of rank vegetation are merely particular cases of the same class. Moreover, the well-known tendency to succulence of sea-side varieties of plants which have thin herbaceous leaves when growing inland, points to the action of the environment in these matters, excess of salts being no doubt one factor in such cases.

Rankness affords another example where superfluity of water is concerned, though it does not involve simply this, because the plant may also contain excessive quantities of nitrogenous and mineral matters taken up by the roots.

Rankness is, in fact, in many respects analogous to etiolation in so far as the tissues are soft and surcharged with water, but it differs fundamentally in the deep green of the chlorophyll: this may lead to abundant assimilation if free access of air and drier conditions can be gradually brought about. Any sudden drying, however, may be fatal to the tender tissues.

Rankness commonly depends on excess of food materials, especially nitrogenous manures, as may be seen in meadows and cornfields where the manure heaps have remained on the ground and saturated it to excess as compared with the rest of the soil; this may often be observed with weeds, etc., in the neighbourhood of farm-buildings. If the period of rank growth is accompanied and followed by days of suitably bright sunshine and dry air, the increase of vegetative structures usually results in increased flowering, heavy crops, or strong wood; but if the rankness continues too long, or is accompanied by wet and dull weather, the watery tissues are peculiarly susceptible to attacks of fungi and insects, and to damage by sudden frosts or chilly winds. Rankness affords, in fact, a typical illustration of predisposition to disease.

Damping off.—When seedlings are too closely crowded in beds kept too damp, or in moist weather, they are very apt to rot away, with all the symptoms—spreading from a centre, contagious infection, mycelia on and in the tissues, etc.—of a fungus attack. The commonest agent concerned is one of the species of Pythium, the propagation of which is favoured by the rank, over-turgid, and etiolated conditions of the plants. Species of Mucor, Botrytis, and other fungi, may also be met with.

Bursting of fleshy fruits, such as Tomatoes, Grapes, etc., is due to over-turgescence in rainy weather or excessively moist air. But the phenomenon is by no means confined to such organs. Hot-house plants when oedematous not infrequently put out watery blisters from the cortex or leaves, which rupture; and the stems of fleshy fasciated (e.g. Asparagus) or blanched and forced plants (e.g. Celery, Rhubarb) are particularly apt to crack here and there from the pressure of the turgescent tissues on the strained epidermis. Beets, Turnips, and other fleshy roots show the same phenomena in wet seasons. That these ruptures and exposures of watery tissues afford dangerous points of entry for parasites and moulds will be obvious—e.g. Edelfäule, a rotten condition of the grapes in the Moselle district.

Root-rot is a common disease in damp, sour clay soils after a continuance of wet weather—e.g. Wheat, especially if root-drawn and exposed to thaw water.

In the disease known as Beet-rot, the roots turn black at the tip, where the tissues shrivel and become grooved and wrinkled extensively. Inside the flesh also blackens and finally rots. In earlier stages, only the vascular bundles are brown and blocked with gum-like substances. In advanced stages there is much gummy material in the lumina, and even large cavities filled with this gum may be found.

The rot of Cherries, Pears, Apples, Plums, etc., in store may be due to several fungi, of which Botrytis, Monilia, Mucor, Penicillium, and Aspergillus are the chief. The fruit may be attacked while still on the tree, but very often fungi and bacteria gain access to the tissues, through bruises, cracks, etc., formed in the fruit lying in the storage baskets or on the shelves.

Rot in Onions, Hyacinth bulbs, etc., is frequently due to the access of Botrytis or Sclerotinia, followed by moulds, yeasts, and bacteria in the stores.

Sour-rot in Grapes, and other fleshy fruits which need much sun to ripen them, is probably a usual result of continued cold, wet weather at the cropping season, setting in when the fruits are beginning to swell.

Flux.—It is a common event to see fluids of various kinds issuing from wounds in trees, or congealing in more or less solid masses about them; and owing to the prevailing tendency to compare plant diseases with those of animals, we find such expressions as Gangrene, Ulcer, and so forth, applied to these "open sores." In so far as such outflowings frequently indicate diseased states of injured tissues which are incapable of healing up, the analogy is perhaps a true one; but it must be remembered that very different structures and processes in detail are concerned. Moreover, liquid excretions more or less indicative of diseased states are by no means confined to wounds or definitely injured tissues, in which case such terms are wholly misapplied.

Honey-dew.—The leaves, or other organs, of many plants are sticky in hot weather, owing to the excretion of a sweet liquid containing sugar, the consistency and colour of which vary according to circumstances. This honey-dew must not be confounded with the normal viscidity of certain insectivorous plants—e.g. Sundew—or with the sticky secretion on the internodes of species of Lychnis, etc., where it plays the part of a protection against minute creeping things.

Honey-dew is often met with on Lime trees, Roses, Hops, etc. In many of these cases the honey-dew is excreted by Aphides, which suck the juices of the leaves and pour out the saccharine liquid from their bodies. The sweet fluid is in its turn sought after by ants, and also serves as nutritive material for various epiphytic fungi—e.g. sooty mould, Capnodium, Fumago, and Antennaria—which give the leaves and honey-dew a brown or black colour. Certain Coccideae also excrete honey-dew, especially in the tropics.

At least one case is known where honey-dew is formed as the result of the parasitic action of a fungus, namely Claviceps purpurea in its conidial stage on the stigmas of cereals, and this may be compared with the sweet odorous fluid excreted by the spermogonia of certain Aecidia. In both cases the sweet fluid attracts insects which disperse the spores.

Honey-dew may also be formed without the agency of fungi or insects, when hot and dry days are followed by cool nights, with a saturated atmosphere, e.g. Caesalpinia, Calliandra and other trees in the tropics, which are called rain trees owing to the numerous drops of fluid which drip from the leaves under the abnormally turgescent conditions referred to.

Cuckoo-spit.—The leaves of Willows, Meadow grasses and herbs, etc., are often seen with froth on them, in which is a green insect, Aphrophora, which sucks the juices from the tissues and excretes the frothy watery cuckoo-spit from its body.

Slime-flux.—The trunks of trees may sometimes be observed to pour out a slimy fluid from cracks in the bark, or from old wounds, or branch scars. In some cases, e.g. in Oaks, the slime has a beery odour and white colour, and abounds in yeasts and other fungi to the fermentative activity of which the odour and frothiness are due. In other cases the slime is red e.g.—Hornbeam; or brown—e.g. Apple and Elm; or black—e.g. Beech, the colour in such cases being due to the mixture of yeasts, bacteria, and fungi with which these slimes abound. The phenomenon appears to be due to the exudation of large quantities of sap under pressure—root pressure—and is primarily a normal phenomenon comparable to the bleeding of cut trees in spring: the fungi, etc., are doubtless saprophytes, but their activity is concerned with the putrefactive processes going on in the diseased wood, and which may lead to rotting of the timber.

The origin of the wounds in the bark and cortex, and which extend into the wood and other tissues as the putrefactive and fermentative processes increase, appears to be in some cases at least due to lightning.

Resin-flux or Resinosis.—The stems of Pines and other conifers are apt to exude resin from any cut or wound made by insects, or by the gnawing of other animals; but in many cases the flow is due to fungi, e.g. Peridermium, the hyphae of which invade the medullary rays and resin canals and thus open the way to an outflow through cracks in the bark. Agaricus melleus not only invades the resin passages, but stimulates the tree to produce abnormal quantities of resin, which flows down to the collar and roots, and exudes in great abundance at the surface of the soil. Various other plants also exude resin from wounds, and in some cases the flux seems to be increased by degeneration of the tissues, e.g. Copaifera.

Gummosis.—Cherries, Apricots, Acacias, and many other trees are apt to produce abnormal quantities of gum, which flows from any wound or exudes through cracks in the bark. Degeneration of the wood-cells, and especially of the cell-walls of a soft wood formed by abnormal activity of the cambium, points to its origin being due, in some cases at any rate, to a conversion of the cellulose, and fungi are sometimes found in the masses of gum; but beyond the fact that gummosis is a pathological phenomenon we know very little of the disease.

With regard to such gumming, it is significant how frequently pruned trees—Cherries, Oranges, Lemons, Plums, etc.—suffer.

Manna flux.—Certain trees, such as the Manna Ash, species of Tamarisk, etc., yield manna from wounds, and in some cases the latter are due to insects, e.g. Cicada.

The Potato-disease is best known by the pale whitish fringe, giving an almost mealy appearance to the margins of the brown to black patches in damp weather. In dry weather the brown patches shrivel and dry, and as they are apt to be at the edges and tips of the leaflets, these curl up. The young disease spots are yellowish, and the leaves of badly affected plants are apt to be sickly yellow throughout.

This Potato-disease due to Phytophthora must be distinguished from the curling and puckering, with wilting and browning of the leaves and yellow glassy look of the stems, due to the invasion of the vessels by a fungus which lurks in the tubers, and gains access thence to the shoots.

In the disease traceable to Phytophthora the stock remains green and the leaves plump and plane, and only the brown patches slough out in wet or shrivel in dry weather, and are bordered by the pale whitish zone of conidiophores.

In the leaf-curl the yellow and flaccid appearance of all the leaves of a stalk, or even of the plant, is the striking symptom, and the stem soon droops and blackens just above the soil, a white mould appearing also at the black spots. Subsequently black spots appear higher up, and bacteria gain an entrance. The stolons rot, and eventually the roots and the leaves wither. The tubers appear sound, but are small; they are apt to rot in the store, the vascular zones turning brown.

This leaf-curl has been ascribed to Pleospora, Polydesmus, Verticillium, and other parasites, as well as to excessive manuring and other agencies, but it still needs explanation.

Rot of Potato tubers in the soil, or in store, may be brought about by very different agents.

If Phytophthora has obtained access, the fungus hyphae spread between the cells, starting from the haulm, and cause the flesh to turn yellowish and then brown in patches. On the exterior are discoloured patches, depressed, with the flesh beneath brown and soft. The mycelium spreads mostly in the outer layers, which though they turn deep brown remain firm.

Wet rot of potatoes may be due to various fungi, and, in excess of water, to putrefactive bacteria (e.g. Clostridium), which destroy the cell-walls. The flesh becomes soft, then soup-like, and finally putrefies to a liquid mass with a vile smell of butyric acid, etc., in which the starch grains may be seen floating.

Tubers are often found with the cork burst and peeling in shreds, the flesh more or less converted into a putrid and stinking pulp, with a spotted brown boundary of partly destroyed but firmer tissue between the dark utterly rotten and the white and still firm healthy flesh. The principal agent in the destruction of the tissues is Clostridium, an anaerobic bacillus which consumes the cell-walls but leaves the starch intact. Hence a thoroughly decomposed tuber consists of a cork bag full of starch and foetid liquid. In the dried condition the flesh shows a brown marbling; this passes into a soft soupy starchy part, and here and there may be violet grey cavities lined with Spicaria, Hypomyces, etc., the white stromata of the latter often appearing externally. The excavations are filled with loose starch grains, and bounded by cork and cambium formed in the peripheral cells. The cell-walls eventually undergo slimy decomposition.

Spicaria, Fusisporium, various moulds, and bacteria may all be associated with wet-rot.

Dry-rot of Potatoes is also due to various fungi and bacteria, but the destructive action goes on slowly, owing to there being no more moisture than the tissues afford. The flesh becomes excavated here and there, owing to the slow destruction of the cell-walls by Clostridium: the destroyed tissues are brown, and the uninjured starch grains powder them all over. Finally the whole shrunken mass has a crumbly consistency.

When the flesh remains white, but assumes a powdery consistency and dry-rot, with the cork destroyed here and there, Frank refers the damage to Phellomyces. Where the dry-rot is due to Fusarium the chalk-white stromata may often be detected breaking through the periderm; but it must be remembered that the soil-contaminated, broken skin of a potato-tuber is a favourable lurking spot for many fungi, and Periola, Acrostalagmus, and others have been detected therein.

Brown spots, depressed into the flesh, sometimes result from the ravages of Tylenchus, the minute worms being found in the diseased tissues.

In some cases the flesh turns watery and soft, grey, almost glass-like, starting at the haulm end, and this may be owing to the invasion of Rhizoctonia.

Notes to Chapter XXV.