Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/283

VEGETABLE.] PARASITIS M 265 pushes up its stem, which meeting with the stem of the host plant j develops a papilla-like body at the point of contact. From the papilla there proceeds the true haustorium, which penetrates the tissues of the host as far as the vascular system, where it expands slightly and terminates in a broad surface. The haustorium is furnished with a central vascular bundle originating in the vascular system of the dodder stem. When this haustorium has been developed the root of the dodder dies off and all connexion with the soil ceases, while the stem above the hauatorium continues to wind round its host, producing fresh haustoria at short intervals, and gradually enveloping and destroying the plant. The influences exerted are of two kinds: (1) a truly parasitic influence, since the dodder, possessing neither connexion with the soil nor chlorophyll, obtains all its nourishment from its host by the action of its haustoria ; and (2) a mechanical influence, in depriving its host of air, light, &c. , and preventing the development of branches, leaves, &c. (see fig. 2). The commonest species are Cuscuta Epithymum, Murr., distributed throughout Europe, growing on Thymus Ser- pyllum, Calluna vulyaris, Genista, &c. ; var. trifolii on clover, to which crop it is enormously destructive ; G. curopsea, L., occurring throughout Europe on hops, vines, &c., and C. Epilinum, "VVeihe, commonly found throughout Europe growing on flax. 1 There are at least five genera of Serophulariacess which are partially parasitic, viz., lUiinanthus, Mclampyrum, Pedicularis, Euphrasia, and Striga. They all contain chlorophyll, and possess true roots on which small haustoria are developed. Euphrasia, occurring in both north and south temperate regions, is partially parasitic on roots of grasses. Pedicularis is common in alpine and arctic regions of the northern hemisphere, Mdampyrum and FIG. V. Cuscuta glomerata, Choisy. A, Parasite entwin ng host; B, section through union between parasite and host; c, stem of host; <i, stem cf Cuscula; h, haustoria. After Dodel-Port. Jlhinanthus in the north temperate zone, and Striga is a native of Asia, Africa, and Australia. The last possesses perhaps more dis tinctly parasitic habits than the others though the cultivation experiments of Decaisne, Cornu, and others tend to show that parasitism is necessary in the cases of Melampyrum, Rhinanthus, and Euphrasia. 2 The Orobanchcse (Broomrapes) possess erect, simple or little- branched sterns bearing numerous scale-like leaves, and are variously coloured, but destitute of chlorophyll. They are parasitic on the roots of many different herbs and shrubs by means of their haustoria, which penetrate to the vascular system oif the host. They attach themselves thus immediately after germination. There are about one hundred and fifty so-called species of Orobanche, of which the following are perhaps best kuown: Orobanche rubens, Wallr., parasitic on and very destructive to lucerne; 0. minor, Sutr., on red clover; 0. major, L. , which attains a height of 2 feet on roots of furze and other leguminous plants; and 0. J-tapitm, Thuill. Phelipsea ramosa, Mey., attacks particularly hemp and tobacco. Lath r sea, which according to Solms Laubach belongs to Scrophulariacese, is parasitic on the roots of trees such as hazels. 3 The Cytinaccse are a very remarkable order of truly parasitic plants which are wholly destitute of chlorophyll, and of a very degraded structure. Cytinus possesses a scaly stem bearing sessile flowers, while Rafflesia and Brugmansia consists one may say of a single flower, measuring in the case of Rajflesia as much as 3 feet across. These flowers appear first in the form of knobs emerging from the host plant, and before expanding resemble an unopened cabbage. They remain expanded only for a few days, when putrefaction begins and a smell as of putrescent flesh is emitted, serving thus to attract insects which probably aid in effecting fertilization, since the stamens are in different flowers. There are about twenty-four species in the order, and these are mostly tropical. Cytinus Hi/pocifitis, L., which is parasitic on the roots of Cissus, occurs in southern Europe. Rafflesia and Brugmansia are limited to the Malay Islands, and Sapria has a wider distribution throughout the same region. Rafflesia is parasitic on both roots and stems, the latter generally prostrate. Pilostylcs, a native of America and Africa, and Apodanthes, confined to America, are parasitic on branches. Hydnora, found in tropical and south Africa, grows on succulent plants, chiefly Euphorbiaceee ; and closely allied to it is J rosopanche, an American genus. 4 The genus Cassytha (Laurincee), of which there are about fifteen species occurring in the tropics, but mostly in Australia, strongly resembles Cuscuta. The plants are exceedingly alike in appearance and in parasitic habit, for which reason the name of "dodder laurels" has been given to the Cassythse. They are wholly without chlorophyll, and their thin, twining, cylindrical stems, bearing scaly leaves, envelop their hosts, to which they are attached by means of papilla-like haustoria. The seeds germinate in the soil, and the roots subsequently die off as in Cuscuta. 5 The Loranthaceae. are parasitic on the stems and branches of trees, but, since they bear mostly thick and leathery leaves containing chlorophyll, their parasitism cannot be considered so complete as in those cases where chlorophyll is absent. The order is for the most part a tropical one, but it is represented in Europe by Loranthus europeeus, L. , and Viscum album, L. , the common mistletoe. Lor anthus is a large tropical genus containing upwards of three hun dred species. Arceuthobium occurs in southern Europe. The mode of parasitism of f^iscum album, L., the mistletoe, may be taken as illustrative of the order. Its seeds adhere to the young shoots of trees by means of the viscid pulp of the fruit (used in the pre paration of bird-lime). On germination it shoots out rootlets which traverse the cortex of the host mostly in the direction of the axis, sending down numerous haustoria into the wood, where the cells of the parasite become partly lignificd, and thus attain an intimate connexion with the wood-cells of the host. A layer of meristem is formed in the haustorium where it passes through the cambium region of the host stt-ni, thus enabling the parasite to keep pace with the growth in thickness, and gradually to become more deeply fixed. The function of the growing point, which soon passes over into permanent tissue, is thus transferred to this region of the haustorium. Ultimately this layer of meristem is also transformed into permanent tissue, and the activity of the parasite in this direc tion ceases. The haustoria arc commonly situated close together in considerable numbers, and an excessive demand upon the host is thus brought about, causing local death and a hurtful influence throughout the plant, exhibited in its defective development. Where a tree has been attacked by mistletoe a corroded and dis torted appearance is presented, owing to the drying up of the tissues and the reparative processes that ensue. When the mistle toe has thus exhausted one region of supply it frequently sends out adventitious shoots, which, attacking the host in fresh places, give rise to new growths of the parasite. The mistletoe grows on a large number of different trees, such as the apple, lime, elm, maple, willow, thorn, poplar, and even on conifers. Though exceedingly plentiful on the apple, it rarely attacks the pear tree, and the Lcmbardy poplar seems to be exempt, while other poplars suffer considerably. A r ery rarely does it attack the oak, and Dr Bull, who made exhaustive inquiry (Jour/i. Bot., vol. ii.) into the matter, succeeded in discovering only seven authentic cases in Dissertat., 1865, and loc. cit., also in Ahhandl. <J. yaturforsch. Ges. zu Halle, xiii., Koch, " Untersuch. liber <1. Entwlckelung d Satmns d. Orobanchen," in Ja/trb. f. u-issensch Sot., xi.; Caspavy, " L eber Sum en. Keimung, etc., der Orobanchen," F,ora, 185-1; Lory, " Suv la respiration et la structure des Oro- banch.," Ann. d. Sci. Nat., ser. iii., 1847. 1 R. Brown, " An Account of a New Genus of Plants, named Rajflesia, Trans. Linn. Soc., xiii. (published also in Afiscel/aneous Works); Id.. "On the Female Flower and Fruit of Rafflesia Anwldi, and on Jfydnora africana," Ibid., xix.; Solms Laubach, loc. cit., and " Ueber das Haustorium der Loranthaceen und den Thallus der Rafflesiaceen und Balanophoieen," Abltandl. d. A aturfonch. Ges. zu Halle, xiii.; Id., "Ueber den Ban der Samen in der Fam. tier Kuffiesiaceen und Hydno:-ecn," Bot. Ztg., 1S74 ; Beccari, "Os^ervaz s. alcune Rajflesiacex," A uovo f/iorn. bot. Ital., 1875; Ttysmann, "Xouvelles recherchcssurla culture de Rajflesia Arnoldi," Batavia. 1836 ; De Bary, 1 Prosopanche Burmeisteri, cine neue Ilydnorce aus Sud-America," Abhandl. d. Naturf. Ges. zu Halle, x.; Schimper, Die V egeta- tionsoi gane von Prosopanche Burmeisleri," Ibid., xv. ; Baillon, "Sur le developpe- ment du Cytinus," Bull.de la Soc. Linn, de Paris, 1874; Archangeli, Etude sur le Cytinus Ilypocistis," Atti del Congresso internaz. botan., Florence, 1874. 5 Poulsen, " UebLT d. morphol. VVerth d. Ilaustoriums v. Casujtha," Flora, 1877. XVIII - - 34