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these proximal causes, or agents, is ever solely responsible; and it is very easy to err in attributing a diseased condition to any of them, unless the relative importance of primary and subordinate agencies is discoverable. For instance, a Fungus epidemic is impossible unless the climatic conditions are such as to favour the dispersal and germination of the spores; and when plants are killed off owing to the supersaturation of the soil with water, it is by no means obvious whether the excess of water and dissolved materials, or the exclusion of oxygen from the root-hairs, or the lowering of the temperature, or the accumulation of foul products of decomposition should be put into the foreground. In every case there are chains of causation concerned, and the same factors will be differently grouped in different cases.

Bearing in mind these precautions, we may classify the proximal causal agents of disease as—

I.—External agencies.

A. Non-living

a. Material

1. Physical—

Soil.

Water.

Atmosphere.

2. Chemical—

Soil.

Water.

Atmosphere.

b. Non-material.

1. Temperature.

2. Illumination.

3. Other agencies.

B. Living.

a. Animals.

1. Vertebrata.

2. Invertebrata.

b. Plants.

1. Phanerogams.

2. Cryptogams.

II.—Internal agencies.

While such a classification may serve its purpose as a sort of index, it must be confessed that the limits of its usefulness are soon reached. In the first place, the so-called “internal causes” of disease is probably a mere phrase covering our ignorance of the factors at work, and although a certain convenience attaches to the distinction between those cases where tender breeds of plants apparently exhibit internal predisposition to suffer more readily than others from parasites, low temperatures, excessive growth, &c.—as is the case with some grafted plants, cultivated hybrids, &c.—the mystery involved in the phrase “internal causes” only exists until we find what action of the living or nonliving environment of the essential mechanism of the plant has upset its equilibrium.

I.—Passing to the recognized external agencies, the physical condition of the soil is a fruitful source of disease. If too closely packed, the soil particles present mechanical obstacles to growth; if too retentive of moisture, the root-hairs suffer, as already hinted; if too open or over-drained, the plant succumbs to drought. All those properties of soil known as texture, porosity, depth, inclination to the horizon, &c., are concerned here. Many maladies of plants are traceable to the chemical composition of soils—e.g. deficiency of nutritive salts, especially nitrates and phosphates, the presence of poisonous salts of iron, copper, &c., or (in the soil about the roots of trees in towns) of coal-gas and so forth. But it is worthy of special attention that the mere chemical composition of agricultural and garden soils is, as a rule, the least important feature about them, popular opinion to the contrary notwithstanding. Ordinary soils will almost always provide the necessary chemical ingredients if of proper physical texture, depth, &c. (see Fungi and Bacteriology).

As regards water, its deficiency or excess is a relative matter, and although many of the minor maladies of pot-plants in windows and greenhouses controlled by amateurs depend on its misuse, water alone is probably never a primary cause of disease. Its over-supply is, however, a frequent cause of predisposition to the attacks of parasitic Fungi—e.g. the damping off of seedlings—and in saturated soils not only are the roots and root-hairs killed by asphyxiation, but the whole course of soil fermentation is altered, and it takes time to “sweeten” such by draining, because not only must the noxious bodies be gradually washed out and the lost salts restored, but the balance of suitable bacterial and fungal life must be restored.

The atmosphere is a cause of disease in the neighbourhood of chemical works, large towns, volcanoes, &c., in so far as it carries acid gases and poisons to the leaves and roots; but it is usual to associate with it the action of excessive humidity which brings about those tender watery and more or less etiolated conditions which favour parasitic Fungi, and diminish transpiration and therefore nutrition. It is customary to speak of the disastrous effects of cold winds, snow, hail and frost, lightning, &c., under the heading of atmospheric influences, which only shows once more how impossible it is to separate causes individually.

Turning to the non-material external agents, probably no factors are more responsible for ill health in plants than temperature and light. Every plant is constrained to carry out its functions of germination, growth, nutrition, reproduction, &c., between certain limits of temperature, and somewhere between the extremes of these limits each function finds an optimum temperature at which the working of the living machinery is at its best, and, other things being equal, any great departure from this may induce pathological conditions; and many disasters are due to the failure to provide such suitable temperatures—e.g. in greenhouses where plants requiring very different optimum temperatures and illumination are kept together. Equally disastrous are those climatic or seasonal changes which involve temperatures in themselves not excessive but in wrong sequence; how many more useful plants could be grown in the open in the United Kingdom if the deceptively mild springs were not so often followed by frosts in May and June! The indirect effects of temperature are also important. Trees, of which the young buds are “nipped” by frost, would frequently not suffer material injury, were it not that the small frost-cracks serve as points of entry for Fungi; and numerous cases are known where even high temperatures can be endured on rich, deep, retentive soils by plants which at once succumb to drought on shallow or non-retentive soils.

All chlorophyll plants require light, but in very different degrees, as exemplified even in the United Kingdom by the shade-bearing beech and yew contrasted with the light-demanding larch and birch; and as with temperature so with light, every plant and even every organ has its optimum of illumination. The “drawn” or etiolated condition of over-shaded plants is a case in point, though here again the soft, watery plant often really succumbs to other disease agents—e.g. parasitic Fungi—supervening on its non-resistant condition.

Animals and plants as agents of disease or injury form part of the larger subject of the struggle for existence between living organisms, as is recognized even by those who do not so readily apprehend that diseased conditions in general are always signs of defeat in the struggle for existence between the suffering organism and its environment, living and non-living.

The Vertebrata come within the scope of our subject, chiefly as destructive agents which cause wounds or devour young shoots and foliage, &c. Rabbits and other burrowing animals injure roots, squirrels and birds snip off buds, horned cattle strip off bark, and so forth. It is among the Invertebrata that epidemics of destruction are referred to, though we should bear in mind that it is only the difference in numerical proportion that prevents our speaking of an epidemic of elephants or of rabbits, though we use the term when speaking of blight insects; there is little consistency in the matter, as it is usual to speak of an invasion or scourge of locusts, caterpillars, &c. Insect injuries are very varied in degree and in kind. Locusts devour all before them; caterpillars defoliate the plant, and necessitate the premature utilization of its reserves; other insects (e.g. Grapholitha) eat the buds or the roots (e.g. wire-worms), and so maim the plant that its foliage suffers from want of water and assimilation is diminished, or actual withering follows. Many aphides, &c., puncture the leaves, suck out the sap, and induce various local deformations, arrest of growth, pustular swellings, &c., and if numerous all the evils of defoliation may follow. Others (e.g. miners) tunnel into the leaf parenchyma, and so put the assimilating areas out of action in another way. It should be remembered that a single complete defoliation of a herbaceous annual may so incapacitate the assimilation that no stores are available for seeds, tubers, &c., for another year, or at most so little that feeble plants only come up. In the case of a tree matters run somewhat differently; most large trees in full foliage have far more assimilatory surface than is immediately necessary, and if the injury is confined to a single year it may be a small event in the life of the tree, but if repeated the cambium, bud-stores and fruiting may all suffer. Many larvae of beetles, moths, &c., bore into bark, and injure the cambium, or even the wood and pith; in addition to direct injury, the interference with the transpiration current and the access of other parasites through the wounds are also to be feared in proportion to the numbers of insects at work. Various local hypertrophies, including galls, result from the increased growth of young tissues irritated by the punctures of insects, or by the presence of eggs or larvae left behind. They may occur on all parts, buds, leaves, stems or roots, as shown by the numerous species of Cynips on oak, Phylloxera on vines, &c. The local damage is small, but the general injury to assimilation, absorption and other functions, may be important if the numbers increase. In addition to insects, various kinds of worms, molluscs, &c., are sometimes of importance as pests. The so-called eel-worms (Nematodes) may do immense damage on roots and in the grains of cereals, and every one knows how predatory slugs and snails are. (See Economic Entomology.)

Plants as agents of damage and disease may be divided into those larger forms which as weeds, epiphytes and so forth, do injury by dominating and shading more delicate species, or by gradually exhausting the soil, &c., and true parasites which actually live on and in the tissues of the plants. It must be remembered that phanerogams also include parasitic species—e.g. Cuscuta, Loranthus, Viscum, Thesium, Rhinanthus, &c.—with various capacities for injury. These enemies are as a rule so conspicuous that