186 DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 
in the wood, but always separated from it by a layer of dead compressed bark (Pl. XXIX. 
fig. 11 نه‎ a). 
The young roots of the Mistletoe and the bark of the nourishing plant live contigu- 
ously in organic union with each other, the delicate green larger cells of the former 
being intimately connected with the brownish smaller cells which constitute the meso- 
phloeum of the latter, in which the roots are immediately imbedded. "The boundary-line 
between the tissues of the two plants is very distinct, but often uneven. 
With regard to the relation between the cortical systems of the two plants, I find 
myself at variance with the statements of DeCandolle. In his ‘ Physiologie Végétale,’ 
p. 790, he says, “The bark of the Apple-tree, although juxtaposed against that of the 
Mistletoe, does not appear to be joined to it. In fact, every part of the bark which 
touches the Mistletoe is dead, and especially below the Mistletoe, where the necrosis is 
seen to extend for nearly an inch.” Again, at page 1412, “The Mistletoe, therefore, 
communicates with the tree only by the soldering together of its base with the wood, 
and not by the bark.” Since this eminent botanist has drawn some important physio- 
logical deductions, involving the relative growth of the two plants, from these statements, 
I have thought it necessary to examine this matter. The growth of the Mistletoe causes 
great thickening of the bark in its vicinity, and its older layers are pushed outwards and 
cracked by the distending base of the parasite, and thus a superficial appearance of 
nécrosis is produced ; but, on closer examination, it is found that the younger layers of 
the bark are in intimate living contact with the corresponding layers of the bark of 
Viscum. The contiguous margins of the barks are mutually bevelled, that of the 
Mistletoe at the expense of its outer surface, that of the nourishing plant at the expense 
ofits inner surface; and the bark of the parasite is received within, and thus shortly 
invaginated by the bark of the nourishing plant (Pl XXIX. figs. 8,10, 12). At the 
surface, and for some little distance inwards, the barks are often separated by a little 
chink ; there is, in fact, no union between the dead outer layers of the barks; but more 
internally they form a continuous living stratum covering over the smooth line of junc- 
tion between the woods of the two plants, and thus the uninterrupted circulation of the 
sap through both is secured. 
The foregoing observations were made upon the Maple, Hawthorn, Apple, Willow, and 
Oak ; and from more superficial examination of the attachment of the parasite to other trees 
upon whieh it grows, I conclude that they are strictly applicable to all. The conclusion to 
which they lead is, that a confluence of the cellular system of the Mistletoe with that of 
the plants which support itis the essential condition of its parasitism. Whether the roots 
of the parasite are implanted in the medullary system of the plant upon which it grows 
by any invasive action of their own is not the object of this paper to determine, but 1 
cannot pass by the question without specifying the principal anatomical facts which 
support this view. "They are, first, the evidence of repeated absorption of the fibro-vas- 
cular bundles, allowing the extension of the roots into the exposed medullary rays; and 
second, the confluence of the extremity of the roots with the central pith, which could 
only result from invasive growth of the roots; for, although a branch may be very early 
affected with the parasite, we can hardly suppose that this would occur simultaneously 
