Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/282

 PARASITISM [VEGETABLE which now occupies so important a place in medicine and the veterinary art (see p. 269 infra, and VETERINARY SCIENCE). (p. GE.) VEGETABLE PARASITISM. The name of parasites has been given to those plants which are nourished wholly or partially at the expense of other living organisms. The degree and nature of the benefit thus obtained varies greatly with different plants, and the effect produced upon the host ranges from an almost imperceptible one to complete destruction. At one extreme are certain forms which, while drawing the nourishment necessary for life from their hosts, yet do so in such fashion that both organisms continue to live in intimate association, and, it may be, rendering mutual help. From these by a series of gradations we come to parasites of such destructive influence as to cause wide spread death to certain animal and vegetable forms of life. This physiological group is closely related to another, the saprophytes, which obtain their nourishment from the dead remains of organisms. True parasites belong ex clusively to the dicotyledonous flowering plants and the fungi. A few algaj are partial parasites. The remarkable appearance presented by most parasitic flowering plants undoubtedly attracted notice in remote times. They are frequently mentioned by early writers, but there is no evidence sufficient to enable us to deter mine whether they were regarded as independent plants or merely as pathological excrescences unless in the one case of the mistletoe, which was recognized as the former by Pliny, who gives an account of its reproduction by seed. The effects of the attack of parasitic fungi were also observed in very early times, as there is abundant evidence to show, but the plants themselves which caused the damage were necessarily not detected as such from their minute size and obscure nature. We must come to the middle of the 18th century for the first attempt to establish a botanical group of flowering parasites. Pf eiffer, in his treatise on the Fungus melitensis (Cynomorium coccineum), divides all flowering parasites into three groups, according as they infested the whole plant or attacked but one place or were confined to the root ; but he includes many epiphytes, such as ivy, lichens, &c. After this remarkable classification a knowledge of native and exotic forms grew up, and nothing noteworthy occurred in the history of the subject until the end of last century and beginning of the present one, when there was a relapse to the old theory that parasites were no more than degener ate outgrowths from their hosts. For example, Meyen attempted to account on anatomical grounds for the exist ence of Lathrsea squamaria on its host, and more absurdly still, Trattinick, in a letter to Schlechtendal, gave a short list of plants to which parasites bear a very superficial resemblance, and gravely affirmed his belief that the latter are but specific degenerations of these. Thus he con tended that Balanophora is but an Arum, Cylinus a Cotyledon, Raftlesia a cabbage, &amp;lt;kc. De Candolle made the first genuine attempt in 1832 to establish a classifica tion of parasites on morphological and physiological grounds; Unger followed in 1840 with a purely morpho logical arrangement, and, though he advanced matters considerably, his treatise contains much speculation not borne out by facts. Martius s classification of about the sanvj time is on much the same lines as De Candolle s. The knowledge of parasitic fungi has advanced gradually with the improvement of the microscope, and the accumu lation of the life-histories of forms has grown up under the hands of numerous observers, among the earliest of whom Knight performed admirable service. With increas ing knowledge of native and exotic forms, and the advance made in the fields of vegetable anatomy and physiology, the whole group of vegetable parasites has become mere strictly defined, the last noteworthy service being tho establishment by De Bary (jforpk. u. Physiol. dtr Pilte, Flechten u. Myxomycelen) of the physiological group of saprophytes&quot; to receive those plants which differ from the parasites in obtaining their nourishment from the dead bodies of organisms and from soil rich in humus. 1 PHANEROGAM i A. The parasitic flowering plants are ex- j clusively dicotyledons confined to natural orders falling under the two divisions of Camopetalx and Monochlamydes. Among the Gamopetalse there are the (Hfonotropexl) Lennoacex, Citscutex (Convolvulacex), certain genera of Scrophiilariacex (such as Rhinanthus, Melampyrum, Eu- phrasia, and Pedicularis), and the Orobanchex. Among the Monochlamydetx there are the Cylinacex, Cassytlta (Laurinese), Loranthaceee, Santalacese, and Bcdanophoracese. The vegetative bodies of these exhibit various degrees of degradation, and this process may go so far that, excepting the parts concerned in reproduction, not only the external appearance but the whole structure of the tissues character istic of a vascular plant may be lost to the parasite. The roots in particular undergo considerable change of form and structure in adaptation to their peculiar function, and tho typical root of a vascular plant may lose all its character istics, retaining only its physiological properties. A ! degraded root or part of a root so adapted is termed a haustorium, and the mistletoe, dodder, Thesium, Balano phora, and J?((ftlesia exhibit such in various degrees of removal from the true type. 2 The arrangement of the orders as follows is that adopted in systematic botany. Their physiological relations will be afterwards indicated. The Monotropeae, which are allied to the heaths, possess no chlorophyll and only small scale-like leaves. Monotropa, which may be taken as a type of the group, undoubtedly subsists as a saprophyte on organic matter derived from the soil. There has been some controversy as to the parasitism of these plants. Perhaps the strongest evidence in its favour was offered by Drude, who stated that he found a parasitic connexion between Monotropa and the roots of Abies excelsa. Monotropa was then generally regarded as both parasite and saprophyte. Wore recently, however, Kamienski has denied the accuracy of Drude s interpretation of the case, and, affirming that Monotropa possesses no haustoria, upholds the view that it is no true parasite. Upon the evidence it may be taken that no case has yet been satisfactorily made out for the parasitism of this group. The suborder consists of ten or twelve species included in nine genera occurring in north temperate regions. Monotropa Ifypopitys, L., is distributed through Europe; var. glahra, Roth, mostly among deciduous trees ; and var. kirstita, Roth, commonly among conifers. 3 The Lennoacex are a very small order confined to Mexico and California. They are succulent herbs with simple or slightly branched stems bearing small scale-like leaves, and resemble in general habit the Monotropcai, to which they are allied. They possess no chlorophyll, and are probably always parasitic. 4 The CiiscutacciK (Dodders) are a suborder of Convolvulaccx, and are distinguished by their fibrous, climbing stems bearing very small scale-like leaves. They are entirely without chlorophyll, and are true parasites. The gioup consists of annual plants reproduced each year from their seed, which commonly ripens about the same time as that of the host plants. The seeds of host and parasite are frequently found mixed, and it consequently happens that they are sown together. When the seed of the dodder germinates it 1 Pfeiffer, Fungus melitensis, Linnams s Anianitat. Acad., Dissert. lx-., vol. iv., 1755 ; De Candolle, I hysiologie ve gcta/e, ill., JJes parasites phanerogames, 1832; Ungcr, &quot; Bfitr. zur Kenntniss dcr parasitischen Pflanzc ii,&quot; Ann. d. Wiener Jtfits., ii., 184&amp;lt;i; Muitius, &quot;Ueber &amp;lt;lio Vegetation der unccliten und ecliteu PiirasittMi zuniiclist in Brasilien,&quot; Gel. Am. d. K&amp;lt;jl. lair. Acad. d. Wissensch., xiv. 2 The following recent works deal more or less completely with parasitic floweiing plants usa group : Solms Laubacli, &quot; Ueber den Bun und die, Kntwiek- elung der Ernahrungsoigane pnrasitischer Phanerogamen,&quot; I ringsfieiiii s Jalirb. f. irissensch. Hot., vi.; Cliatin, Anatomic comparee des vegetaiix I lantes para sites, Paris, 18G2; Brandt, A onnulla de parasit. quibtisdam phanerogam, obs. Linntea, 1849; Pitra, &quot;Ueber die Anheftungsweisc einigcr phanerogamischer Parasiten an Hire N&hrpflanzen,&quot; Hot. Ztg., 18(jl. 3 Solms Laubach, loc. cit. ; Drude, iJie Biologie rnn Monotropa Ilypopitys, (iuttingen, 1873; Kamicnski, &quot;Die Vegetatlonsorgane von Monotropa Hypo- pityt,&quot; Hot. Ztg., 1881. 4 Solms Laubacli, &quot; Die Familie der Lennoaceen,&quot; Abhandl. d. Naturf. Gesell- at Ifa le, xi.