Many so-called weeds are, however, partially parasitic, with their roots on the roots of others - e.g. Rhinanthus, Thesium, etc., and much damage is done to meadow grasses and herbage by the exhaustive tax which these semi-parasites impose.
This is carried still further in the case of such root-parasites as Orobanche, where the host-plant is burdened with the whole support of the pest, because the latter, having no chlorophyll, is entirely dependent on the former for all its food.
Even ordinary climbing plants may injure others by shading them, either by scrambling over their branches - e.g. Bramble, or twisting their tendrils round the twigs - e.g. Bryony, or twining round them - e.g. Woodbine, Convolvulus, etc. The principal direct injury is in these cases owing to the loss of light suffered by the shaded foliage, but the weed-action is often increased by the competition of their roots - e.g. briars; and in the case of woody climbers the gradually increased pressure of the woody-coils round the thickening stems compresses the cambium and cortex of the support and induces strictures and abnormalities which may be fatal in course of time.
Epiphytes, or plants which support themselves wholly on the trunks, branches, or leaves of other plants, also injure the latter more especially by shading their foliage - e.g. tropical Figs, Orchids, Aroids, etc.; and similar damage is done by our own Ivy, the main roots of which are in the soil, but the numerous adventitious roots of which cling to the bark.
When the climber or epiphyte is also parasitic, as in the case of the Dodder, Loranthus, Mistletoe, etc., the direct loss of substance stolen from the host by the parasite comes in to supplement any effect of shading that the latter may bring about if it is a leafy plant.
Of Cryptogams, apart from a few epiphytic ferns, and the intense weed-action of certain Equisetums, the rhizomes and roots of which are as troublesome as those of twitch and other phanerogamic weeds, it is especially the fungi which act as agents of disease, and which, as we now know, are par excellence the causes of epidemics.
The action of fungi may be local or general; and restricted, slow and insidious, or virulent and rapidly destructive.
Examples of local action are furnished by Schinzia, which forms gall-like swellings on the roots of rushes; Gymnosporangium, which induces excrescences on the stems of junipers, and numerous leaf-fungi (.Puccinia, JEcidium, Septoria, etc.), which cause yellow, brown, or black spots on leaves, as well as by Ustilago, which attacks the anthers or the ovary of various plants, and so forth. In such cases the injury done by a few centres of infection is very slight, but prolonged action may bring into play secondary effects such as the gradual destruction of the cambium round a branch, when, of course, the effect of ringing results; or if the fungus becomes epidemic and myriads of leaf-spots are formed, the destruction of foliar tissue, gradual taxing of the assimilatory cells, etc., may end in rapid defoliation, and renewed attacks soon exhaust the plants and lead to sterility and death, as often occurs with Uredineae - e.g. the coffee leaf-disease.
It is highly probable that such fungi are particularly exacting owing to their exhausting demands for compounds of potassium, phosphoric acid, and other bodies.
Examples of virulent and rampant general action are afforded by finger and toe in turnips, etc., where the roots are invaded by Plasmodiophora, which induces hypertrophy and rotting of the roots; and by the damping off of seedlings, where the fungus Pythium rapidly invades all parts of the seedlings and reduces them to a water-logged, putrefying mass; or the potato-disease, which is due to the rapid spread of Phytoplithora in the leaves and throughout the plant, which it blackens and rots in a few days.
Many fungi not in themselves very virulent or aggressive do enormous harm owing to the secondary effects they induce. Some of the tree-killing hymenomycetes, such as Agaricus melleus, for instance, penetrate the wood of a pine at the collar, and the result of the large flow of resin which results is to so block up the water passages that the tree dies off above with all the symptoms of drought. Similarly, the Peziza causing the larch disease, having obtained access to the stem about a foot or so above the ground, will gradually kill the cambium further and further round the stem, and so girdle the tree as effectually as if we had cut out the new wood all round. In all such cases - and the same applies to the leaf-diseases referred to above - the fungus may be compared to an army which is not strong enough to invade the whole territory, but which, by striking at the lines of communication, cuts off the supplies of water, food, etc., and so brings the struggle to an end. Indeed we might compare the cases of fungi which attack the root and collar, and so strike at and cut off the water supply, to a compact army which at once cuts off the enemy from his narrow base; whereas the innumerable units which bring about an epidemic attack on the leaves, and so surround the enemy and cut off his food supplies all round, is rather like a much larger army which cannot get in beyond the natural barriers of the tissues, and so puts a cordon all round the territory and seizes the multitudes of food-stuffs at the frontiers. The end result is similar in both cases, but the methods of warfare differ.
Many fungi, however, though they make their presence noticeable by conspicuous signs, cannot be said to do much damage to the individual plant attacked. The extraordinary malformations induced by parasites like Exoascus, which live in the ends of twigs of trees and stimulate the buds to put out dense tufts of shoots, again densely branched-Witches' brooms - are a case in point. Also the curious distortions of nettle stems swollen and curved by Aecidium, of maize stems and leaves attacked by Ustilago, and of the inflorescences of Capsella by Cystopus, etc., are not individually very destructive; it is the cumulative effects of numerous attacks, or of large epidemics, which tell in the end.
Some very curious effects are due to fungi such as Aecidium elatinum, which, living in the cortex of firs, stimulate buds to put out shoots with erect habit, and with leaves which are radially disposed, annually cast, and differently shaped from the normal-characters quite foreign to the species of fir in its natural condition.
Equally strange are the shoots of Euphorbia infested with the aecidia of Uromyces, those of bilberries affected with Calyptospora, etc. In all these cases we must assume a condition of toleration, so to speak, on the part of the host, which adapts itself to the altered circumstances by marked adaptations in its tissue developments, mode of growth and so forth.
This toleration is perhaps most marked in the case of those cereals which, though infected by the minute mycelium of Ustilago while still a seedling, nevertheless go on growing as apparently healthy green plants indistinguishable from the rest, although the fine hyphae of the parasite are in the tissues and keeping pace with the growth of the shoots just behind the growing points. As the grains of the cereal begin to form and swell, however, the hyphae suddenly assume the part of a dominant aggressor, consume the endosperm of the enlarging seed, and replace the contents of the grain with the well-known black spores known as Smut.
The reader will find a summary of such fungi as are here concerned in Massee, A Text-Book of Plant Diseases, 1899, or Prilleux, Maladies des Plantes Agricoles.
For further details the student should consult the works of Frank and Sorauer referred to in the notes to Chapter IX., and Tubeuf, The Diseases of Plants, Engl. ed. 1897, pp. 104 - 539.
For experiments on the effects of grass on orchard trees, see Report of the Woburn Experimental Fruit Farm, 1900, p. 160.
For the further study of weeds, the interesting bulletins of the Kansas State Agricultural College, 1895 - 1898, will show the reader what may be done in the matter of classifying them according to their biological peculiarities.
In regard to insects, the reader will find the following list embraces the subject: Somerville, Farm and Garden Insects,
1897; Theobald, Insect Life, 1896; Ormerod, Manual of Injurious Insects, 1890, and Handbook of Insects Injurious to Orchards, etc., 1898.
The admirable series of publications of the U.S. Department of Agriculture under the editorship of Riley and Howard, and entitled Insect Life, 1888 - 1895, also abounds in information.
Further, Taschenberg's Praktische lnsektenkunde, 1879 - 1880, and Judeich and Nietsche, Lehrbuch der Mitteleurop. Forst. Insektenkunde, 1889.
For an elementary introduction to the study of fungus diseases, see Marshall Ward, Diseases of Plants, Soc. for Promoting Christian Knowledge, London.