Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/286

 P A K A S I T I S M [VEGETABLE. consequent loss or deficiency of the power of assimilation. For a comparison of this abnormal condition with the normal state a subject is found ready to hand in the nourishment of one organ by another, as exemplified in the growth of young seedlings, which in the case of seeds containing endosperm (cocoa-nut, date-palm, and many other monocotyledons) absorb by means of a definite organ the nourishment necessary for their development. Young plants nourished from the reserve-materials stored in bulbs and the like, and the young shoots of a tree from winter buds, afford a comparison which is even closer in an anatomical respect, since in this case there is present, as in the intimate association of parasitic haustoria with the host, a continuity of tissues which is not so strongly marked in the union of the absorbing organ of a seedling with the endosperm. Looking at the subject wholly from the point of view of the process of nutrition, there seems to be little essential difference between parasite and saprophyte, since we have not only experimental instances of the nutrition of parasites on artificially prepared solutions, but the natural union of both habits in the same individual (salmon-disease, ike. ; see also the experiments of Grawitz on the growth of saprophytic fungi in the blood of animals). These are exceptional instances, however, and it is manifest that other properties must be brought into play, since most parasites affect peculiar hosts, and many of them certain regions only of the plant. It is equally true that many saprophytes are able to grow only in peculiar substrata. That parasitism is often but partial is apparent from such instances as the mistletoe, Rhinanthus, Thesium, itc., which probably obtain from their hosts in the main only water and mineral substances in solution, to be prepared for plant food in their green leaves. It is most likely, however, that a small quantity of certain organic com pounds is a necessary accompaniment in all such instances. Here again there exist the means for comparison with green saprophytes. The taking up of ash constituents from the soil may occur in such parasites as Orobanche, which possesses rootlets, though undoubtedly the whole of the necessary carbon compounds are obtained from the host. This mode of life not only acts upon the host, but reacts upon the parasite itself, as is manifested by the aberrant and degraded structure of the parts (directly and indirectly) concerned in nutrition, and even of the reproductive system. This is strongly marked in the case of the embryo. It is apparent that large transpiratory surfaces are unnecessary, and would even be of detriment to a parasite ; and with this the formation of wood so intim ately connected with the process of transpiration keeps pace in degradation. In the mistletoe, for example, the bulk of w r ood is in relation to the small transpiratory surface, and in the cases of parasites without chlorophyll it dwindles to insignificance. No other abnormal mode of life so influences the structure of a plant as a parasitic or a saprophytic one, though we see an approach to it in the adaptations existing in insectivorous plants. The effect upon, the host ranges from local injury to destruction on the one hand, and, on the other, in the case of stimulus, from the local j (reduction of galls to the com plete hypertrophy and transformation of at least large regions of a plant. The exciting of definite reparative processes is an indirect effect. It must be noticed also that many parasites, especially fungi, cause in the host enormous destruction of food material far exceeding that necessary to their maintenance. In this way the parasite frequently commits suicide as it were, and the act is in striking contrast to the relations of symbiosis as exemplified in the lichen thallu.s. The change of or alternation between two different hosts is adapted to suit the requirements of the parasite. This is notably so in the case of the corn-mildew, which passes an intermediate stage on the barberry until a period when the wheat plant has sufficiently developed to become a suitable host. Most fungi are endophytic, and certain phanerogamic parasites, such as JRqfflesia, develop within the cortex of the host, while on the other hand the fungal part of a lichen encloses the algal. The existence and complete dependence upon its host of a parasite culminating in the production of seed after its kind is one of the most impressive relationships physio logy presents. 1 SYMBIOSIS. This, the consortism of organisms in such fashion that mutual services are rendered sufficient to make the alliance profitable and successful to the whole community of organisms, is a mode of life closely related to parasitism, in which, however, as has been seen, the profit is one-sided and the alliance ends with the exhaustion of the host or the detachment of the parasite. The term was first employed by De Bary (Die Erscheinuny dtr Symbiose, 1879), but the relations expressed by it were first brought into general notice by the epoch-marking discovery of the dual nature of the lichen thallus by Schwendener in 1868, and established after prolonged and searching controversy, more especially by the classical histological researches of Bornet, and the actual artificial lichen synthesis (by sowing fungus on alga) by Staid. Some theory of reciprocal accommodation was necessary to account for the duration of such relations between a fungal organism and an algal ; and, though it is not yet precisely known in what way these relations are maintained, specu lation has been active enough. It may safely be inferred that the fungal portion of the thallus is nourished by the exosmose of starch and the like in much the same fashion as the colourless cells of a plant are fed by those bearing chlorophyll ; and there can be little doubt that the algal cells benefit in return by the endosmose of the waste pro ducts of the fungal protoplasm. In the reproductive pro cess an adaptation exists in certain lichens for the supply of gonidia to the new lichen. Hymenial-gonidia (the offspring of the thal-lus-gonidia) are present in the apothecia, from which they are cast out along with the spores, and falling with them are subsequently enclosed by the germ tubes (see FUNGUS, vol. ix. p. 835). Jt may be noted here that, though the fungal portions of the thallus retain the marks of near relationship to ascom)-- cetous fungi, they arc yet considerably modified by this mode of life, and unfitted most probably in nearly every case for the distinctly parasitic or saprophytic life normal to fungi. The algal portion, on the other hand, is capable of independent existence after liberation from the fungal thallus. The complete symbiotic community represents an autonomous whole, living frequently in situations where neither alga nor fungus is known to support existence separately. 2 The presence of chlorophyll in animals (Hydra and Vortex) was discovered by Max Schultzc in 1851, and con firmed more recently (Hydra and Spongilla} by the spectroscopic evidence furnished by Lankester and by Sorby. That a chlorophyll-bearing animal is able to 1 I. Grawitz, &quot; Ueber Srhimmelvegetationen ini tieri.schcn Orprnnisrnus,&quot; in Virchoics Archie, Ixxxi., 1880; and &quot; KxprriiKontcllcs zur Infectionsfrage,&quot; in IlerliH. klinische Wochenschrift, No. 14, 1881 ; Brefcld, Butaniaclie Untertuchwtgen liber Sehtmmelptlze (1881), and Ueler llefenjntze (!88:J). A very graphic account of the physiology of parasitism is to be found in Sachs, Vorlesitnyen iibcr 1 Jlanzfn- J kysio!oyie, 1882. See also Pfeiffer, Pftanunphytiologie, 1881. 2 Scliwendener, Untersuch.itber den t lech. en-Tliullus, 18(i8; Stahl, Reitriiye zur Enliciel-elungsffeschirhte der Flecliten, 1S77 ; IJornct, &quot; Kechcrclu s sur 1. sgonidios des Lichens.&quot; Ann. Sci. Nat., 5th sen, 1873; DC Bary, Morphologic v. I himiolixjie der Pilze, F/echten, und Myxomycetcn (18CG), and Die Ertcheinung clt-r Symbioff (1879), which includes an account of the association of Azolla with AnabMia, and of the relations of Nostoc to cycad roots.