DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 185 
lary rays by the reticulated ducts, which are here the only representatives of the woody 
bundles. These large medullary rays become confluent with two or more of the 
medullary rays of the nourishing plant, the intervening fibro-vascular bundles of the 
latter also abutting against the parenchyma of the parasite. And, since the medullary 
systems of the two plants thus compass each other, it necessarily follows that the 
reticulated ducts of the parasite are opposed to the remaining fibro-vascular bundles of 
the nourishing plant, and hence, at first sight, the appearance of direct inosculation 
between the vessels of the two plants; but, in reality, the ducts of the parasite as often 
abut against the woody fibres of the fibro-vascular bundle as against its ducts. 
The relation of the fibro-vascular bundles of the two plants may be best made out in 
Populus nigra, the large ducts of which correspond in size with those of Viscum. 
The horizontal ramifications (side roots) of the base of the Mistletoe have precisely the 
same structure as the young perpendicular roots. I have been unable to distinguish 
in them a root-sheath (Wurzel-haube) and a bark, mentioned by Schacht (op. cit.). They 
pervade the bark of the nourishing plant freely, which they can of course only do by 
effecting its absorption simultaneously with their advance; some pass transversely and 
partially encircle the branch (Pl. XXIX. fig. 11), but by far the greater number, 
running parallel to each other and to the branch, traverse it lengthwise (Pl. XXVIII. 
fig. 5, & Pl. XXIX. fig. 6). Whichever direction they take, they produce at frequent 
and pretty regular intervals other tapering cellular roots, which, guided doubtless by the 
medullary rays of the bark, pass towards the surface of the wood, and are thus brought 
into contact with the ends of its medullary rays. "They are subsequently found imbedded 
at various depths in the hard wood of the nourishing plant, like the primary roots. These 
lateral roots also give origin to bud-like processes, which, deepening in colour, grow up 
obliquely through the bark, and appear as little shoots in its chinks, or, if the bark be 
young and smooth, they first greatly distend it,—the tense, shining cuticle becoming so 
thin that the already green buds of the parasite may be seen beneath it,—and subse- 
quently rupturing it, develope leaves and stems. 
The horizontal roots are cylindrical, and are wholly contained in the bark; and such 
being the case, they are each year removed further outwards from the surface of the 
Wood of the nourishing plant by the endogenous growth of its bark; and when the more 
external layers in which they lie have become cracked and dead and fall away, they are 
discovered (Pl. XXVIII. fig. 1 a, & Pl. XXIX. fig. 6 a). Meantime these roots increase 
in size and harden in texture by the formation of woody layers. ig 
Such being the relation of the horizontal roots to the bark of the nourishing plant, 
I cannot agree with Dr. Unger that they become imbedded in the wood by the growth 
of the branch; and I have never seen them lying indifferently in the bark hairs the 
Surface of the wood, as he (tab. iii. fig. 15, op. cif.) and Schacht (fig. 9, vol. ii. op. cit.) 
have represented. When, however, they are tied down at short intervals by the perpen- 
dicular roots, that portion of the bark which is included between them and the surface 
of the wood is often killed by the pressure resulting from the growth of the latter. It 
follows, of course, that the corresponding portion of the wood is arrested in its growth, 
and the horizontal roots are, in after-years, found lodged in vertical or horizontal grooves 
