DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 183 
root, and connect it with the tissues of the nourishing plant, belong to the medullary 
system of the latter or to that of the former. 
The young cellular root of Viscum may be regarded generally as a prolongation of the 
céntral pith of the parasite, and the contiguous medullary rays of the nourishing plant 
are successively confluent with its surface, which sometimes presents deep crenatures 
resulting from its lateral extension into the positions occupied by the medullary rays. 
This assimilation and confluence of the medullary systems of the two plants is perhaps 
best seen'at the angles formed by the divergence of the woody fibres; but, in many roots, 
it may be traced continuously around them for the greater part of their circumference, 
the growing roots evidently producing absorption of the woody fibres interposed between 
them and the adjacent medullary rays, which thus become, one after another, confluent 
with them. At the same time great distortion and distention of all the tissues of the 
wood is effected. "These points are well seen in such sections as is represented in 
Pl. XXX. fig. 13. Five young roots are here seen in transverse section, bounded 
by sharp undulations of the woody tissue, each of which is terminated by a rugged, 
sometimes bifurcated, spicular process projecting into the root. 
Pl. XXX. fig. 15 represents an oblique section, and the line of demarcation between 
the tissues of the two plants is irregular and ill-defined. 
The appearances here described are seen in sections made at some distance within the 
heart-wood. In the younger layers the divergence of the woody fibres around the roots 
is less complete, and the cells of the latter occasionally lie against the jagged free 
extremities of the fibro-vascular bundles, indicating clearly enough that absorption of the 
woody tissues of the nourishing plant has been taking place. This effect is also best 
observed where the fibres first begin to diverge. 
The few small reticulated ducts of the parasite which come to the surface of the root 
appear in these cross sections to be attached indifferently to the parenchymatous cells, 
fibres, and ducts of the nourishing plant. 
The woody fibres of the two plants never, as far as my observations go, come into 
actual contact: the plain, thick-walled prosenchyma of Viscum gradually thins jaway, 
and disappears towards the attached surface of the parasite, where it is entirely absent, 
With regard to the relation of the vascular tissues of the parasite and its nourishing 
plants, Unger (Beiträge zur Kenntniss der parasitischen Pflanzen *) considers their 
inosculation to be an essential condition of all cases of parasitism. In fig. 17, tab. iii., he 
has represented “ the scalariform ducts of the extremity of the root-substance of Viscum, 
regularly apposed to,” and directly inosculating with, “the dotted vessels of Orategus,” 
and opposed to none other of its tissues. Adolphe Pitra (° Botanische Zeitung,' 
Hugo von Mohl, 1861, Leipzig, 4to) appears to have adopted the same view. He says, 
“In a transverse section of the root (sucker or senker) of Viscum, which also euts the 
mother branch tangentially (fig. 5), we obserye not only that the wood-tissue runs round 
the sucker, but also that the vessels of the wood attach themselves to the short vessels of 
the sucker which radiate from the centre of it in all directions fo the contiguous wood. 
From this position of the elements of the wood, it may be eoncluded that here occurs a 
* Annalen des Wiener Museums der Naturgeschichte. Wien, 1840. iM" 
