Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/284

266 PARASITISM VEGETABLE. England. Loranthus curopsens, L., occurs OH the oak in southern Europe. 1 The SantalacaK are mostly if not all partially parasitic shrubs or herbs their foliage containing chlorophyll. Santalum (S. album yields sandal wood), distributed throughout the East Indies, Malay Islands, and Australia, and Thcsium, a native of Europe, are parasites on the roots of plants, especially monocotyledons. Their liaustoria are more or less globular in shape, and emit from the surface in contact with the host a process which penetrates the tissues. Osijris also attacks the roots of trees. Henslowia and Myzodendron are partially parasitic on the branches of trees. The latter, a native of south temperate climates, attaches itself to its host by means of the feathered processes on its seeds. These retain them in contact with the branches on which they fall until germination (thus per forming the same function as the viscid pulp of the mistletoe), when the liaustoria penetrate the bark and become, as it were, grafted into the living tissues. 2 The BalanophoracesB are flowering plants of degraded structure, destitute of chlorophyll, and generally coloured led, yellow, or brown. In appearance they somewhat resemble Cyiinaccx, though there is no real ailinity in the case. The steins are succulent, some what knob-shaped or cylindrical, varying in height from a few inches to a foot, in which latter case they are sometimes branched, and bear imbricated scales in place of leaves. They are true parasites on the roots of woody Dicotyledons, rarely on Monocotyledons. The liaustoria vegetate in the tissues, frequently setting up exten sive hypertrophy. They occur chiefly in mountainous tropical regions some in Australia and the Cape. The order contains thirty-five species in fourteen genera, of which Balanvphora, Ci/no- morium, and Laiigsdoiffia are the best known. Cynomorium coccineiim the Fiutyus i/icHtensis of old writers is found in Malta, the Levant, North Africa, and the Canary Islands. 3 ALG.E. Several microscopic algae may very well be partial parasites, though it is"probable that in most cases they are little more than epiphytes in their relation to the plants in which they occur. They all possess chlorophyll and are able to assimilate ; but from their situation in the tissues of other plants a degree of parasitism may be inferred A species of Nostoc. occurs in the intercellular spaces of the roots, leaves, and thalli of other plants ; and C klorochytrium is found in the tissues of Lemna, Cerato- phyllum, and in another alga Schizonema. More distinctly parasitic is -the case of Phyllosiphon Arisari, Kiihn, which inhabits the parenchymatous tissue of Arum Ari- sarum.* LICHENES. Mycoidea parasitica, Cunn., was described and figured by Cunningham as a parasitic green alga. It, or a closely allied form, has been recently examined by Ward, who says, " It seems clear that the injury is not due to a direct parasitic action of the thallus ; even in the extreme case of Citrus I do not imagine the active development to depend so much on absorption of food from the living leaf as on the sheltered situation enjoyed by the ensconced thallus. 5 FUNGI. The absence of chlorophyll from all fungi, and the necessity thus thrown upon them of taking up the carbon compounds assimilated by other organisms, deter mines their mode of life, which is therefore either parasitic or saprophytic. The parasitic organ of the fungal thallus is the mycelium, upon which haustoria are sometimes developed in the form of lateral protuberances of various shapes and sizes. In the same species of parasitic fungus receptacles frequently occur of different kinds, succeeding each other more or less regularly in cycles, and sometimes in their course preying upon hosts of remote affinities among themselves. This course of life is of practical importance when effort is made to limit the ravages of such a parasite (see MILDEW, vol. xvi. p. 293). Many indiscriminately attack plants nearly allied to each other; numerous species are peculiar to one host; while others are confined to a single region such as the ovary, the stem, or the leaf of one or more species of the higher plants. The spores, invariably of microscopical dimen sions, represent the infectious agent, as the seeds of flowering parasites commonly do. They are conveyed by the atmosphere, by contact of one plant with another, by insects and other animals, &c., and germinate by the emission of a germ-tube, the production of zoospores sometimes intervening. Access to the host is obtained by the penetration of the epidermal tissue or by way of the open stomata. The main body of the fungus is either endophytic or epiphytic the spore-producing portion in nearly all cases opening externally. The amount of damage effected by the attack varies from slight local injury to the destruction of the host; in some cases cell- contents only are destroyed, while in others whole tissues perish. The effect produced is often in the direction of abnormal stimulus, and the hypertrophy of whole regions or the production of galls ensues. The parasite commonly prepares the way for the saprophyte, which steps in to break up the dead and decaying remains. In certain rare instances the union of parasitic and saprophytic modes of life in the same species has been observed (see below). The fungi which are concerned in the constitution of lichens maintain with the algal components throughout life relations of consortism which will be dealt with below, under "Symbiosis." 6 For the life-histories of the following groups the student is referred to the article FUNGUS (vol. ix. p. 827), and to the literature therein cited. Snprolegniess. The fungi of this suborder are many of them saprophytes, as their name implies, but some are of distinctly para sitic habits. Certain species of Pythium are parasitic on fresh water alga?, on the prothallia of vascular cryptogams, and in the tissues of the higher plants. Several species of Saprolegnia are parasitic on similar hosts, but one in particular, S. fcrax, Gruith, is well known for the part it, plays in the disease of fishes in fresh water commonly called the salmon disease. That this fungus possesses both parasitic and saprophytic modes of life is established, and the fact is one of remarkable importance, since it stands almost by itself in this respect among the higher fungi. 7 The Peronosporese are all parasites on vascular plants of many different orders. The mycelium inhabits the tissues of the host, and, in many of the species, while passing through the intercellu lar passages, sends globular or irregularly brandling filamentous haustoria (see fig. 3) into the adjoining cells. On the other hand the mycelial filaments of certain species, such as Phytophthora infestans, De Bary, the potato disease, possess no true haustoria, but they pene trate the cells, breaking down the cell-walls in their course. In the regions where the oospores of Pcronosporcte are formed hyper trophy of the tissues of the host sometimes occurs, and, the normal functions being checked, the parts in question die on . The Feronosporese are enormously destructive to the higher plants, and may be reckoned among the most dangerous enemies of agriculture and horticulture. Besides the potato disease, Cystnpus candidv.s Ttie following works have ^peeial reference to fungal parasites : Frank, Die Krankheiten der Pfianzen, 1880; Soraucr, Handbuth tier Pflanzenkrankheiten, 1874: O. Comes, Le Crittogame parassite del/e piante agrarie, Naples, 1882. Of historical interest arc Unger, Die Exantheme tier I flanzen (i883), and Beitrage zur reryleichenden Palfwlogie(l840); Meyen, Pflamen-l atholoyie, 1841. 7 Pringsheim, "Die Saprolefinieen," in inJahrb.f. vissensch. JM., i., ii., and ix. ; De Bary, " Einige neue Saprolegnieen," Hid., ii.; Lindstedt, Synopsis der Saprolefjnieen, Berlin. 1872; Cornu, " Monographic des Saprolegnldes," in Ann. Sci. Nat., ser. v., vol. xvi.; Hesse, Pythium tie Banmnum, Ac,. Halle, 1874; Sadebeck, " Untersuch. liber Pythium Kquiseti," in Cohn s lieitr. zur Bio!, d. Pjtanzcn, i.; T. H. Huxley and G. Murray, "On Salmon Disease," in Inspector of Fisheries Reports for 1881, 1882, 1883 ; Marshall Ward. "Observations on the Genus Pythium, in Quart. Journ. Aticroscop. Sri., vol. xxiii., hew ser.