1 175 3 XI. Ow the Parasitbm of the Nistletoe (Viscum album). By JOHNHARLEY, M.D., X.R.C.P., kc. Communicated by J. D. I~OOKER,M.D.,3 R.S.,P.L.S. (Plates XXVIII., XXIX., XXX.) Read March 5th, 1863. SOME three years ago I turned my attention to small Vegetable Parasites, hoping that they would throw some light on the cause of cancer and analogous diseases in the human subject, or that, at least, they would direct my investigations into the nature of these obscure growths. But on referring to the literature of our great vegetable parasite, the Mistletoe, I was surprised to find that, although the structure and development of the wood, the ovules, and the pollen of this plant, and the anatomy and germination of its seed, have been very fully and carefully investigated”, our knowledge of the anatomical and physiological relations of the parasite ta the plants upon which it grows was still imperfect. The observations of our own authors in particular are most fragmentary and superficial; md the English student, if he wanted definite information respecting the nature of the parasitism of the Mistletoe, would seek in vain for that information in our own language ; and, what is still more remarkable, the subject has never been illustrated by our own botanists. Our fellow-labourers in Germany have, however, advanced our knowledge of the subject very considerably, but yet their observations are incomplete and sonietimes contradictory ; and as I have found them in some essential particulars at variance with my own observations, I have thought it desirable that a subject so important to vegetable physiology as the nature of the parasitism of the Mistletoe should be more fully considered. The present paper professes to be an investigation into the anatomical relations of the Mistletoe to the plants upon which it grows, and a deduction therefrom of the general physiological relations existing between them. The Mistletoe attaches itself to the nourishing plants by roots, some of which are horizontal and confined to the bark, the others are contained within the wod. * Decaisne, M. : Mimoire sur le De”ve1oppement du Pollen, de l’Ovule, et sur la Structure des Tiges du Gui (Acad. Roy. Bruxelles, 1839, vol. xiii.). DBveloppement du Pollen dans le Gui (Acade’mie des Sciences, vol. 6.p. N1). De la Structure ligneuse du Gui (Comptes Rendus, 1839, p. 294). Kieser : M6m. sur I’Organisation des Plantes, 1814, p. 305, tab. 22. Bischoff: Lehrbuch, vol. ii. p. 62. Link, H. F.: Icones aelectae Anat. Bot. 1842, fascic. iv. tab. 8, all the seven figs. Icones Amt BoL faseic. ii. tab. x. 7, 8. Richard : in Jussieu’s Mdmoire (Ann. Mus. vol. xii.). Griffith, W. : On the Development of the Ovules of .hunthug and Piscum (Trans. Linn. SOC. d.XvG-). Loudon, J. C. : On the Germination of Yiscurn album (Arboretum et Fruticetum, vd. k p. tQz.1)- 2 A2 176 DR. EIARLEY ON THE PARASITISM OF THE MISTLETOE. Henslow%, Griffith-f., UngerS, Schachtg, and Pitrail all agree, so far as their indi- vidual statements extend, in the following particulars :--The young plant first sends into the bark of the noul*ishingplant a single root, sucker, or senker, which, pressing inwards, comes into perpendicular relation to the wood of the nourishing plant, in the cambial layer ofwhich the point rests, and there ceases to grow. In its passage towards the wood it gives off several horizontal or side roots, which run along the branch in the bark or upon the surface of the wood. These side roots give origin to perpendicular suckers (semker), which come into contact, like the origiiial root, with the surface of the wood. ‘6 The wood and bark of the mother plant, in their periodical increase, form layers around the suckers, which grow in exactly the same manner in the cambial stratum” (Pitra, p. 61), and thus the hardened suckers come to be imbedded in the body of the wood. I will now proceed to detail the result of iny own observation, introducing as occasion requires such particular statements of these several authors as are not mentioned here. First, as to the general characters and structure, and the arrangement and direction of that part of the Mistletoe which lies within the nourishing plant. The base of the Mistletoe gradually diminishes in size from the surface of the support- ing Branch inwards, that being the thickest part of the entire plant which corresponds in position to the outer surface of the last-formed layer of the wood. From this situation the base of the parasite, in its simplest condition, tapers as it passes towards the centre of the branch-gradually in the case of a young plant, so as to form a long tapering root (Pl. XXVIII. figs. 1, 5 c, &c.), suddenly in an old plant, forming a short, conical, woody plug, which, however, invariably ends in a slender cellular process (Pl. XXVIII. figs. 1b, 2 by 3 a; P1. XXIX. figs. 8, 10, & 11). But more commonly the base of the Mistletoe terminates in three or four, and some- times in five or six, such tapering roots. When the base of the parasite does not exceed at its thickest part &ths of an inch in diameter, itself and all its ramifications are composed of a delicate yellowish-green soft cellular tissue, which, shortly after making sections of a green branch charged with Mistletoe, shrinks below the level of the wood to the same extent as its younger layers of bark. When moistened, however, the young roots immediately swell up and project considerably above the surface of the wood. The young roots, and the equally soft cellular terminations of the older ones, are chiefly composed of delicate tubular cells, the &$h of an inch long and the &th of an inch wide, joined end to end, and arranged parallel to each other and to the long axis of the root (Pl. XXX. fig. 1’7 6). In cross sections of the root they have the appearance represented in figs. 14, 15. This parenchyma is pervaded by a few (the number depend- ing upon the age and size of the root) straggling plates of young prosenchyma, each composed of one or two layers of small thick-walled elongated cells destitute of markings. a * Magazine of Natural History, vol. iv. p. 500, 1833. + On the Parasitism of Lot.un.th and Viscutn, bg w. Griffith, Esq. (Trans. Linn. SOC.vol. xviii. p. 78, 1841). $ Beitrgge zur Kenntniss der prrasitischen Pflanzen, Annalen desWiener Museums der Naturgeschichte. Wien, 1840. 5 Lehrbuch der Anatomie und Physiologie der Gesiichse, v01. ii. p. 465, 1860. ]I Botanische Zeitung, von Hugo von Wohl, 1861. Leipzig, 4to, p. 61. DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 17'7 Arranged in the same radiate manner as the plates of prosenchyina, and in the larger roots associated with it, but in the younger occurring alone, are narrow bundles of vessels, formed of one, two, or three rows of very delicate reticulated ducts composed of elongated cells, &Sth of an inch long and Athof an inch wide, joined to each other by their oblique ends. The woody fibres and ducts take the same direction as the root. The extremities of the young roots are altogether destitute of prosenchyma, but here the ducts are very numerous. The parenchymatous cells wliich form the surface of the root, and connect it with the tissues of the nourishing plant, are narrower than those lying more internally, and measure only the i%&th of an inch wide. A similar con- traction is observed to occur in the reticulated ducts as they approach the surface of the root, and before they come into connesion with the surrounding wood they become reduced to half their original width (Pl. XXX. figs. 15, 17). In order to understand the structure of the woody portions of the roots of Piscum, it will be necessary to describe briefly that of the stem. Structure of the #tern and zooody Base of Piscum uZbum.-The medullary rays of the stem of the Mistletoe are large and numerous ; they are, however, very irregular, and each varies in size several times in its passage from within outwards. They average about the &th of an inch in depth and the 8iTithof an inch in width, and are composed of large tubular cells, which also vary in size, and averzge the &th of an inch wide : the majority of these cells have thick walls, marked by a few scattered transversely elliptical dots. The rays are often confluent longitudinally, and so form wide plates of parenchyma : they are separated laterally by intervals of about the u+&h of an inch. One-third of this interval is occupied by the prosenchyma, the remaining two-thirds by slit-marked vessels. The prosenchyma, or wood-fibres proper, is composed of long plain fibres, the &&h of an inch wide, and so much thickened that their original cavities are reduced to mere canaliculi : this tissue immediately surrounds the medullary rays, forming a thin layer two cells wide. The slitted vessels form wide bundles composed of three or four rows of cells, lying between the prosenchymatous fibres which on either hand bound the medullary rays : the constituent cells have thick walls, and the reticulated deposits are broad and close, converting the intervals between them into short and very narrow slits ; they measure the &$h of an inch wide, and are joined together by almost straight extremities. The porous wood of the Mistletoe is, therefore, chiefly composed of coarse parenchyma and thick-walled ducts, the prosenchyma being very scanty, and forming a thin, wide- meshed network surrounding the medullary rays.