2. Lichens, though not parasites, are produced on trees in consequence of damp air and flagging energy in growth. While the growth in girth is very slow, the bark is shed so gradually that lichens are able to fix themselves on this. And by mechanically closing the lenticels or air-holes of the bark through which the tree transpires and absorbs oxygen, the health of the tree is injured, and branches die off inside the crown. Unless checked this injury will often spread quickly and kill off a large number of trees. In the case of ornamental trees this can be remedied by scraping and pruning; but where bad cases occur to a large extent in woodlands, it is a sign that the situation is not well suited to the kind of crop grown on that particular spot.

3. Beard-mosses (Usnea barbata) and similar growths are also, like other lichens, due to dampness of the atmosphere, and are rather indications of want of vigorous growth and of unsuitable environment than actual symptoms of any diseased condition. Where they are plentiful, however, fungous diseases (Acidium, &c.) will generally be found attacking the timber-crops in unusual numbers, due partly to the lowered vitality of the trees and partly to the dampness of the atmosphere.

Extermination.-Lichens and Mosses can often be destroyed by brushing with a 10 per cent solution of sulphate of iron (1 lb. to a gallon of water). Caustic Alkali Wash (1 lb. caustic soda and 1 lb. pearl ashes, each dissolved in 5 gallons water, then mixed, and lb. of soft soap added) is stronger and more effective, but must be used with care. (See Board of Agriculture Leaflet No. 70, Winter- Washing of Fruit-Trees.)

4. Small Climbing Plants.-The woody-fibrous Honeysuckle or Woodbine (Lonicera) and Traveller's Joy or Old Man's Beard (Clematis) do damage by twining round the stems of saplings and young poles, constricting the stems and often finally strangling them; while the climbing Bindweed (Convolvulus) and Wild Hops (Humulus) act as a dead weight in bending. down slender stems in coppices and osiers-beds. They can only be exterminated by being dug out with all their roots.

C. Parasitic Plants.-With the exception of mistletoe and dodder, nearly all the parasitic plants in our woodlands are fungi, which cause diseases of various kinds, some of which extend in dangerous epidemic form,

Mistletoe (Viscum album) is commonest on Apple-trees (seldom on Peartrees) in orchards, but is also found on Poplars, Lime, Willows, Birch, Maple, Scots Pine, and Silver Fir; rarely, if ever, on Oak, and never on Beech, Alder, Larch, or Spruce. It often forms large bushes with greenish-yellow leaves persistent throughout the winter. Its extension and reproduction are probably mainly due to thrushes. They feed on its white berries, and leave portions of them, containing some of the seeds, on the bark of the tree when cleansing their beaks from the sticky flesh of the fruit.

On seeds germinating on a smooth thin-barked branch some rootlet finds its way, under favourable circumstances, in through the bark of the tree to the xylem, and forms the first penetrating rootlet (haustorium or sucker). During the following year this rootlet gets overgrown and enclosed in the new annual ring formed. But it retains direct connection with part of the plant flourishing outside of the branch infected; and as the mistletoe grows, this first

penetrating rootlet also enlarges and extends sidewards through the phloem, annually throwing out new haustoria or suckers. As these live a long time and gradually become overgrown with many annual rings, they often extend as deep as 4 inches into the wood. And as they rapidly decompose when they die, the infected part is riddled with holes. The part of the branch above the swelling caused by the mistletoe usually dies some time afterwards. If the damage is confined to branches it is of no great consequence. But in woods of Silver Fir and Scots Pine mistletoe sometimes appears in large quantities, and when it manages to get into the stem it makes the bole swollen and deformed, while the holes render its timber useless for technical purposes.

The extermination of mistletoe is easy enough in orchards, if desired. In woodlands, branches infected should be pruned off, and stems attacked should be thinned out to prevent the spread of this parasite by birds; but cutting off the visible outside part of the plant is of little use, as the roots within the tree soon send out fresh shoots to elaborate the sap drawn from the host.

Dodder (Cuscuta), far more harmful to field-crops than to nurseries or plantations, sometimes attacks osier - beds and spoils the rods where the haustoria or suckers pierce them. Shoots attacked should be cut and burned about the end of June. As the dodder-seed retains germinative power for two or three years, when once the osiers begin to be attacked the damage may continue for the next year or two, so that a careful watch has to be kept and the cutting out of rods continued till the pest is eradicated.

Fungous Diseases.-Many of the organic disturbances in plants, varying from petty interference in vegetative processes up to serious disease, resulting in the death of trees, are directly due to attacks of parasitic cryptogamous plants called Fungi.

Neither fungous nor any other diseases of trees can be transmitted through seed collected from infected trees, though the plants raised from such seed may perhaps be found weakly and predisposed to attacks from insects and fungi.

Fungi are lowly organised plants consisting only of cells, which may sometimes become hardened. They do not assimilate like green plants. They have no chlorophyll, and consequently cannot elaborate carbon for themselves, but are dependent for carbon on the supplies they can withdraw from other organic combinations. Such essential nutriment they must therefore obtain from either living or dead animal or vegetable organisms.

Life-history of Fungi.-All the fungi which attack trees and damage them to a greater or less extent belong to the higher or filamental order of this class of plants, namely, to those whose vegetative portion (mycelium) providing nourishment, and living on in the substratum, consists of long branching filaments (hypha) capable of growing at their points, and whose elongated cells are (except in the Phycomycetes) divided into cell - rows by transverse walls (septa). The mycelium develops from the spores or reproductive organs. The conditions required for the germination of the spores are merely water, oxygen, and a slight degree of warmth; given these the germinal hyphae can be produced anywhere, but darkness favours the growth of the fungus.

In many fungi the vegetative portion does not adhere merely to the typical simple form of mycelium, but assumes the more complex forms of compact bundles

of mycelial strands (rhizomorpha) with branching root-like processes. The tissue of these strands differs from that of higher plants in their extreme ramification, and in the separate hypha gradually uniting to form the rhizomorpha. In fungi which contain little water, the hypha become filled with nutrients, and then unite to form tuber-like masses, with very much thickened cell-walls. These are called sclerotia, and fungi which develop sclerotia have always a long vegetative existence; they may lie dormant, and may for a long time appear dry and dead, but once given favourable conditions they resume active vegetation again, and proceed to grow and produce spores.

The protoplasm of fungi usually contains a large proportion of water and a good deal of oil, but no starch and no organs for providing chlorophyll. The cell wall does not consist of cellulose (except in the Saprolegniacea and Peronosporacea), but to a greater or less extent of chitin (the substance of which the wing-cases and outer covering of insects consist), together with considerable quantities of substances containing little or no nitrogen (but not true cellulose), and of other carbohydrates used for constructing cell-walls.1

Usually from about a quarter to a half of the ash of fungi is found to consist of potash and phosphoric acid; but otherwise it contains much the same constituents as the ash of green plants, except lime, sufficient supplies of which the fungi can always obtain from its host.

The mycelium of parasitic fungi lives either epiphytically on the surface of the parts of plants attacked, or else endophytically in the interior of its host. Epiphytic fungi obtain their nourishment by means of special side-branches extending between the walls of the epidermal cells; whereas, on the other hand, endophytic fungi either extend their mycelium intercellularly (e.g., Uredinea or rusts), or again by means of special haustoria or suckers, when they withdraw the cell-contents by pure osmosis, or else intracellularly, when they pierce the living cells of their host. As the growing-points of the hyphae extend they generate and give off various crude ferments which enable them to bore through the cell-walls and imbibe their nourishment. By means of these ferments the cellulose and the pectine contained in the cell-walls is dissolved, the woody tissue is broken up and the cellulose consumed, and the various glucosides (such as coniferin, salicin, amygdalin, &c.) are transformed into carbohydrates serving for the nourishment of the fungi, while albuminoids, starch, and oils are all likewise dissolved.

Fungi which complete the whole of their life-cycle upon one host-plant are called autoxenous or autœcious, while meto.xenous or heterœcious fungi are those which spend one part of their life on a second kind of host; and in this latter case the second host is usually quite a different genus of plant from the first, while the fungus during this intermediate stage appears as if also belonging to quite a different genus (see p. 171 below).

Parasitic fungi may either attack only one particular kind of tree, or they may infest different kinds and portions of trees, and in different stages of growth and conditions of health. But seedlings, saplings, poles or trees predisposed to infection by unsuitable soil or situation, insufficient light (dominated stems, &c.) or weakly condition (after insect attacks, &c.), are of course far more liable to infection than healthy plants growing under normal conditions, and with favourable environment. Some fungi only attack live tissue during the non-vegetative winter period of rest of a plant, while others infest it during the vegetative period. Some fungi can only penetrate into the seed-leaves, so that plants outgrow danger on shedding their cotyledons; others can only attack foliage while the leaves are

1 Klein, Forstbotanik, in Lorey's Handbuch der Forstwissenschaft, 2nd ed., 1903, pp. 382-411. This excellent and most recent résumé of the fungous diseases of trees has mainly been utilised in bringing the following pages up to date.

still without the protection of a good tough cuticle. Mild damp weather increases danger of fungous infection; and in this respect our mild moist climate, with its long spring and autumn, and its comparatively mild winter, is most distinctly favourable to the dissemination and the growth of parasitic fungi, and the spread of the serious epidemic diseases they often cause in timber-crops. The more destructive of these sometimes in Continental woodlands destroy the whole of extensive natural regenerations, sowings, and replantations. The effects on polewoods in Britain are only too well known in the case of the Larch-canker; and even old woods can be seriously injured, as in the case of the Pine-canker. But the results of destructive fungous epidemics are always greatest when infection precedes, is simultaneous with, or follows attacks by insects.

The Fructification of Fungi takes place by means of well-defined sporeproducers (sporangia) found on special branches (sporophores) of the hypha, which produce vast numbers of germinative cells of different kinds, called spores generally, or sporidia when they are secondary spores formed on a promycelium formed by the germination of hibernating or resting-spores, and gonidia or conidia when formed at the points of hyphae usually growing erect. On being set free all these different kinds of spores can, under suitable climatic and atmospheric conditions, develop into a new generation of fungi.

Those spores which are produced sexually through the union of two cells or energids are termed ovispores (oospores or zygospores), and are "resting-spores" well provided with nutriment, by means of which they can germinate even after lying dormant for a very long time (up to three or four years). Those produced asexually are called conidia or gonidia, and (except the chlamydospores) perish in a few days unless they find, on being scattered far and wide in enormous numbers by wind, insects and other animals, water, &c., conditions favourable to their germination. These asexually-formed conidia are usually much more minute, and are produced in far greater number than the sexually formed oospores; but in both cases fructification is usually most common in autumn, about October.

The conidia are formed singly, or in rows at the end of simple sporophores (special hypha or mycelial filaments which usually grow erect). At first they are usually 1-celled, but in most cases they become many-celled through subdivision.

When the mycelium produces no conidia-bearers, but is wholly or partially divided into short pieces like conidia, the spores are called oidia; while they are termed chlamydospores when the membrane of such oidia becomes thickened and reserve-nutrients are stored up in it. A fungus can have more than one form of fructification, and then it is said to be pleomorphous.

Spores usually retain life for a longer period than the mycelium, and especially during time of drought, when many spores have great power of withstanding the effects of high temperature. And at the same time most spores can stand almost any degree of cold, whereas low temperature often kills the mycelium, especially if it be soft and oily (but not when it forms hard sclerotia).

Most fungi are fortunately saprophytic, and only a comparatively small number are parasitic.1 Saprophytic fungi only live on dead and decomposing substances, and are therefore merely an accompaniment or consequence of disease, and not its cause. Parasitic fungi, however, attack healthy living plants, and are a direct cause of disease, often ending in death; but many parasitic fungi continue

The only known cases in which fungi are of use to trees are those in which the mycorhiza or thickly-matted mycelium of root-fungi underground give symbiotic aid in accumulating stores of nitrogen from the soil. They are especially active in the rootnodules of all leguminous plants, and also of the Alder. Similar fungi are also found on the roots of Beech, Oak, Willows, and conifers, &c., assisting in nutrition, and useful in aerating the soil. The mycodomatia inhabiting the bacteriads utilise the free nitrogen of the air, and do not form a mycelium.

to grow saprophytically when once they have killed the plant attacked. Few of the slime-fungi (Myxomycetes) are parasitic, most are saprophytic; and most of the Ascomycetes and Hyphomycetes are also saprophytic, whereas all the Uredineæ or rusts, Erysiphea, Peronosporaceæ, and Eroasceœ are parasitic. In wound-parasites (e.g., Polyporus), the fungus is usually first saprophytic, before becoming parasitic.

"When any spores settle on plants and find the conditions (damp and warmth) favourable to their development, germination follows-i.e., they throw out delicate, thin-walled, mostly colourless, tube-like processes (mycelial filaments or hyphæ), sometimes containing a golden-yellow oily fluid. These tubes, though often undivided, are as a rule divided into separate cells (septated), their contents being protoplasm and cell-sap. With the exception of the mildew-fungi, which merely vegetate on the exterior, the infection of leaves, fruits, bark, and wood always commences from the outside by the hyphæ pushing their way either through the stomata or the epidermal cells of young leaves or bark, or effecting an entrance into the interior of the plant through the unprotected portions of the roots, or at any wound-surfaces like those of broken branches.

'These hyphae either insinuate themselves between the cellular walls of the parenchym or the prosenchym, and vegetate in the intercellular spaces or the resin-ducts, whilst they send single short off-shoots or suckers (haustoria) into the interior of the cells, or else they bore through the cell-walls, and thus force their way from one cell to another. By extending in length, and at the same time throwing out side-processes-less frequently by dividing-they at length form a filamentous network or mycelium, which represents the complete vegetative body of the organism. Masses of mycelium form sclerotia, containing stores of food-supplies (chiefly proteids and oil) which, under favourable conditions, produce either new mycelium or sporophores of the fungus.

Sporangia or spore-producers spring from the mycelium and produce the reproductive organs, the spores. One and the same fungus often produces sporophores of the most varying shapes, and the form of these is much more characteristic (e.g., mushrooms, toad-stools, &c.) than the hidden mycelium, which is much alike in all fungi.

"When the sporophores are only single threads from the mycelium they are called spore-hypha, but in other cases the sporophores and sporangia are of the most varying forms, and have different names. Carpospores are a form of resting-spores which produce a second generation differing from the original form, whilst gonidia or conidia only reproduce the form on which they occur. Conidia therefore multiply one form of fungus very rapidly within the annual period of vegetation, whereas carpospores perpetuate the species from one year to the other."-(Studies in Forestry, 1903, p. 277.)

Fungous diseases may either spread from root to root underground by mycelial infection (e.g., Agaricus melleus, Trametes radiciperda, and Rosellinia quercina in nurseries and plantations), or by means of spores and gonidia carried by wind, animals, or other agencies (e.g., Phytophthora omnivora on seedlings, attacks of Trametes radiciperda at the base of the stem, and smut caused by Ustilago, bunt due to Tilletia, or rust on wheat, oats, &c., due to Puccinia).

The manner in which the host-plant reacts against the fungous infection differs according to the fungus and the host. Only in the case of leaf-fungi, of minor importance generally, does the point of attack remain localised and circumscribed within narrow limits. In other cases the action either causes destruction or some sort of transformation, or else both combined. The life of leaves and other organs is shortened; the form and anatomical structure of the parts attacked are changed, and the host becomes more or less diseased. Infection with fungous spores or hypha interferes first of all with normal vegetation (e.g., transpiration and decomposition of the carbonic acid in foliage infected). This gradually leads to cellwalls being destroyed and the cellular substance and contents being consumed; then chemical disturbance and physiological alteration result, generally accompanied by hypertrophy or morbid enlargement to a greater or less extent, and finally ending in the death of the part attacked. These hypertrophic changes. vary greatly in character, according to the part of the tree infected. Fungus-galls