attacks of the fungi, or if some antecedent predisposing condition be necessary. There are a number of well-known fungi that produce diseases of the skin in man and other vertebrates. Achorion Schoenleinii produces favus in man, rabbits, cats, fowls and other birds and mammals. Trichophyton tonsurans (Malmsten), is the fungus of tinea or ringworm in man, oxen, horses, dogs and rabbits. Saccharomyces albicans (Reess), produces thrush of the mouth in young herbivora and birds. Actinomyces bovis (Harz) is associated with swellings on the jaw-bone of cattle and kangaroos, but has been found in pigs and human beings.
The fungi parasitic on plants are much better known and are responsible for a large number of diseases. They display every gradation from occasional to complete parasitism. Amongst the Pyrenomycetes, the group Erysipheae contain a large number of common parasites; the main body of the fungus is usually epiphytic as in various mildews (q.v.). Ergot (q.v.) is the most familiar example of the group. The Discomycetes are chiefly saprophytic, being common on dead fruits, roots and so forth, but many of them kill living plants: Exoascus on plums, peaches and cherries. Sclerotinia is most common on dead juicy fruits, but will destroy turnips in store, and has been known to attack living Phaseolus and Petunia. The Hymenocytes are naturally saprophytes, but when they gain access through wounds are the most destructive parasites of living timber. The Ustilagineae are endoparasites in Phanerogams, and are especially notorious for their attacks on grain-crops and grasses. The species of Ustilego set up hypertrophy in the tissues of their hosts, and the enlarged spaces thus formed become filled with the spores of the parasite. The Uredineae are also endoparasites of the higher plants and produce the diseases known as rusts which specially affect cultivated plants. The Peronosporeae are all parasites of plants and are the most destructive enemies of agriculture and horticulture. Phytophthora infestans (de Bary), the potato-disease fungus, is a typical example.
Algae.—The chlorophyll-containing green and yellow cells found in Hydroids and Planarians referred to in connexion with symbiosis and the small green algae that infest the hairs of sloths are on the border-line of parasitism. A species of Nostoc occurs in the intercellular spaces of other plants; Chlorochytrium is found in the tissues of Lemna, and Phyllosiphon arisari (Kühn) infests the parenchyma of Arum arisarum.
The flowering plants have a considerable number of representatives which have become epiphytes and which exhibit various degrees of parasitic degeneration. The Monotropeae allied to the heaths, are degenerate, with no chlorophyll and with scale-like leaves but the evidence as to their parasitism is more than doubtful; they are possibly only saprophytic. The allied Lennoaceae, a small group also devoid of chlorophyll and with scale-leaves, are true root-parasites. The genus Cuscuta of the Convolvulaceae consists of the true parasites known as dodders. They are destitute of chlorophyll and attach themselves to other plants by twining stems on which occur haustoria that penetrate the tissues of the host and absorb nutritive material. Cuscuta europaea, the great dodder, is a parasite of nettles and hops; Cuscuta epilinum is the flax dodder; Cuscuta epithymum attacks a number of low-growing plants; and Cuscutum trifolii is very destructive to clover. Several genera of Scrophulariaceae are partially parasitic; they contain chlorophyll but have degenerate roots with haustoria. Euphrasia, the eyebright, attacks the roots of grasses; Pedicularis, the lousewort, Rhinanthus, the rattle, Melampyrum, the cow-wheat and Bartsia are all partly parasitic on the roots of other plants. The Orobanchaceae or broomworts, are all destitute of chlorophyll and have scale-leaves; they are parasitic on the roots of other plants, species attacking various Leguminosae, ivy, hemp and hazel. The Cytinaceae are true parasites devoid of chlorophyll and leaves, with deformed bodies and conspicuous flowers or inflorescences. Most of them are tropical, and the group is widely scattered throughout the world. The Santalales are all parasitic; some members like Thesium linophyllum (the bastard toad-flax), a root parasite, and Viscum album (the mistletoe), parasitic on branches, have chlorophyll, but rather degenerate leaves; others like the tropical Balanophoraceae are devoid of chlorophyll and foliage leaves and have deformed bodies. Of the Lauraceae, a few genera such as Cassytha (the tropical “dodder-laurels,”) are true parasites, without chlorophyll and with twining stems.
Effect of Parasitism on Parasites.—The phenomena of parasitism occur so generally in the animal and vegetable kingdoms and are repeated in degrees so varying that no categorical statements can be laid down as to the effects produced on the organisms concerned. All living creatures have a certain degree of correspondence with the conditions of their environment, and parasitism is only a special case of such adaptation. The widest generalization that can be made regarding it is that parasitism tends towards a rigid adaptation to a relatively limited and stable environment, whilst free life tends towards a looser correspondence with a more varying environment. The summum bonum of a parasite is to reach and maintain existence in the limited conditions afforded by its host; the goal of the free-living organism is a varying or experimental fitness for varying surrounding conditions. And, if the metaphor be continued, the danger of parasitism for the parasite, is that if it become too nicely adjusted to the special conditions of its host, and fail to attain these, it will inevitably perish. The degeneration of parasites is merely a more precise adaptation; in the favourable environment the degenerate, or specialized parasite is best equipped for successful existence, but the smallest change of environment is fatal. Such a generalization as has been formulated covers nearly all the peculiarities of parasitism. Organs of prehension are notably developed; parasitic plants have twining stems, boring roots and special clinging organs; parasitic animals display hooks, suckers and boring apparatus. The normal organs of locomotion tend to disappear, whether these be wings or walking legs. Organs of sense, the chief purpose of which is to make animals react quickly to changes in the environment, become degenerate in proportion as the changes which the parasite may have to encounter are diminished. The changes correlated with nutrition equally conform with the generalization. The chlorophyll of the plant becomes unnecessary and tends to disappear; the stem has no longer to thrust a spreading crown of leaves into the tenuous air or groping rootlets into the soil, but absorbs already prepared nourishment from the tissues of its host through compact conduits. And so the parasitic higher plant tends to lose its division into stem and leaves and roots, and to acquire a compact and amorphous body. The animal has no longer to seek its food, and the lithe segmentation of a body adapted for locomotion becomes replaced by a squat or insinuating form. Jaws give place to sucking and piercing tubes, the alimentary canal becomes simplified, or may disappear altogether, the parasite living in the juices of its host, and absorbing them through the skin. So, also, parasites obtaining protection from the tissues of their host lose their intrinsic protective mechanisms.
The reproduction of parasites offers many peculiarities, all of which are readily correlated with our generalization. A creature rigidly adapted to a special environment fails if it does not reach that environment, and hence species most successful in reproduction are able to afford the largest number of misses to secure a few hits and so to maintain existence. High reproductive capacity is still more urgent when the parasites tend to bring to an end their own environment by killing their hosts. Reproduction in parasites, so far from being degenerate, displays an exuberance of activity, and an extraordinary efficiency. In parasitic flowering plants the flowers tend to be highly conspicuous, the seeds to be numerous, and specially adapted to ready diffusion. Amongst the fungi, the reproductive processes are most prolific, spores are produced by myriads, and very many special adaptations exist for the protection of the latter during their transference from host to host. It is notorious that the spores of bacteria and the higher fungi resist changes of temperature, desiccation, and the action of physical and chemical agents, to an astonishing extent. Vegetative reproduction is extremely active under favourable conditions, and resting reproductive bodies of varying morphological character are produced in great abundance. Amongst fungi, a phenomenon known as heteroecism is developed as a special adaptation to parasitic conditions, and recalls the similar adaptations in many animal parasites. At one stage of its existence, the fungus is adapted to one host, at another stage to another host. Puccinia graminis, the fungoid rust affecting many grasses, is a typical instance. It inhabits wheat, rye and other grasses, developing a mycelium in the tissues of young plants. During the summer, the mycelium gives rise to large numbers of simple processes which break through the tissues of the host and bud off orange-coloured uredogonidia. These small bodies are scattered by the wind, and reach other plants on which they germinate, enter the new host through the stomata and give rise to new mycelia. Towards autumn, when the tissues of the host are becoming hard and dry, darker-coloured teleutogonidia are produced, and these remain quiescent during the winter. In spring they germinate, produce
