_Tumescence._--It occasionally happens that herbaceous parts of plants pa.s.s 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 _Oedema_ 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 cla.s.s. 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 quant.i.ties of nitrogenous and mineral matters taken up by the roots.

Rankness is, in fact, in many respects a.n.a.logous 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 a.s.similation 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 ill.u.s.tration 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._ _Edelfaule_, 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 ma.s.ses 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 a.n.a.logy 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 circ.u.mstances.

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 gra.s.ses 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 quant.i.ties 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 ca.n.a.ls and thus open the way to an outflow through cracks in the bark. _Agaricus melleus_ not only invades the resin pa.s.sages, but stimulates the tree to produce abnormal quant.i.ties 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 quant.i.ties 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 ma.s.ses 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 gla.s.sy 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 conidioph.o.r.es.

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 ma.s.s 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 princ.i.p.al 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 pa.s.ses 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 a.s.sociated 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 ma.s.s has a crumbly consistency.

When the flesh remains white, but a.s.sumes a powdery consistency and dry-rot, with the cork destroyed here and there, Frank refers the damage to _Ph.e.l.lomyces_. 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 gla.s.s-like, starting at the haulm end, and this may be owing to the invasion of _Rhizoctonia_.

NOTES TO CHAPTER XXV.

The rotting of bulbs, roots, etc., has been much discussed during the last few years in the pages of the _Gardeners'

Chronicle_, _Zeitschrift fur Pflanzenkh._, and elsewhere. The princ.i.p.al references to Bacteriosis--the rot in which bacteria are stated to be the primary agent causing these and similar diseases--may be found in Ma.s.see, _Diseases of Plants_, pp.

338-342, and more fully in Russell, _Bacteria in their Relation to Vegetable Tissue_, Baltimore, 1892; and in Migula, _Kritische Uebersicht derjenigen Pflanzen-krankheiten, welche Angeblich durch Bakterien verursacht werden_, Semarang, 1892.

The most convincing accounts, however, are since that date; see Smith, "Pseudomonas Campestris," _Cent. f. Bakt._, B.

III., 1897, p. 284, and Arthur and Bolley, _Bacteriosis of Carnations_, Perdue University Agr. Expt. Station, 1896, Vol.

VII., p. 17. Woods has lately shown that this disease is due to Aphides only, the bacteria having nothing to do with the disease primarily, _Stigmonose_, _Bull. 19_, U.S. Dept. Agr., 1900; but it is necessary to bear in mind that actual penetration of the cell-walls from without must be proved, as De Bary proved it for germ-tubes of fungi, before the evidence that Bacteria are truly parasitic in living plants can be called decisive. This is a difficult matter, but until it is settled we do not know whether these organisms are really parasitic in the sense that _Phytophthora_ is, or merely gain access by other means--I have traced them through dead fungus-hyphae--to the vessels, dead cell-walls, etc. The proof of infection _via_ water pores and vessels is given for one species by Harding, "Die Schwarze Faulnis der Kohls," etc., _Cent. f. Bakt._, Abh. II., B. VI., 1900, p. 305, with literature.

Concerning the "Damping off" of seedlings, see Marshall Ward, "Observations on the Genus Pythium," _Quart. Journ. Microsc.

Soc._, Vol. XXIII., 1883, p. 485, and Atkinson, _Bull. 94 of Cornell University Agric. Expt. Station_, 1895, p. 233.

On Bacteriosis in Turnips, see Potter, _Proc. R. S._ 1901, Vol. LXVII., p. 442.

CHAPTER XXVI.

NECROTIC DISEASES.