194 DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 
the hypertrophied portion of the branch than below, where it is of normal size; but it is 
doubtful whether they really be so, for it is very difficult to count the obscure and 
crowded rings of a young branch. 
It appears from the facts just mentioned that the roots of the Mistletoe stand to the 
nourishing plant in the relation of an hypertrophied medullary system, and one which 
induces an excessive flow of the sap of the branch, resulting at first in the local hyper- 
trophy of its tissues, but subsequently, the supply of sap, or the power of transmitting it, 
failing, the central portion of the wood becomes exhausted and dies, involving in its 
death that of its destroyer also; for the dark-coloured sapless wood extends from the 
centre in irregular wavy processes, and enshrouds the dead roots of the parasite. The 
branch, however, still maintains its vigour, and slowly buries the inveterate intruder,— 
the roots and woody bases of the Mistletoe, some full three inches in circumference, 
becoming imbedded in the centre of the stem by its subsequent growth (Pl. XXVIII. 
figs. la, 2a, 3a). But other roots are meanwhile penetrating the newly formed layers 
of the wood, and its whole cireumference is in time more thickly beset with parasites 
than ever; the branch is heaved out into a spindle-shaped swelling, and the outer layers 
of the bark are rent into wide breaches favourable to the continued encroachments of 
the invading parasite, while its inner layers become immensely thickened and form a 
suitable nidus for its increase. After a while the second crop of roots spread destruction 
still further outwards, and, like the former crop, implicate themselves in it; the branch 
still struggles vigorously with its enemy, but as fast as one generation of roots are dying; — 
a later and more numerous progeny attack it. Many of the medullary rays are destroyed, 
and, the development of the wood being interrupted in the direction of the dead roots, 
deep fissures filled with dead bark, and resembling involutions of the stem, are formed 
and extend sometimes to the very centre of the branch, which, in this condition, resembles 
in section that of a Malpighia (Pl. XXIX. fig. 9). The infected branch, moreover, 
assumes various contortions, being twisted sometimes in one direction and sometimes in 
another: it is frequently found bent at right angles to itself. But it wrestles in vain 
with a veritable Hydra, which, having killed its centre, spoiled and occupied its bark, 
and invaded anew the little living wood that remains, now gradually completes the work 
of destruction. 
While the Mistletoe is thus affecting the branch in its immediate neighbourhood, it is 
producing, by interception of its juices, atrophy of that portion beyond it. ‘With the 
increase of the parasite this interception becomes complete, and the atrophied portion of 
the branch dies. 
The death of the supporting branch thus precedes that of the Mistletoe; but decay 
first affects the softer wood of the parasite, and its roots quickly crumble down and leave 
corresponding cavities in the wood, by which the branch is laid open, and thus fully 
exposed to the inroads of decay ; and if the atrophied portion have not already dropped 
away, it now rots off at the infested part. 
Such are the effects produced by the Mistletoe on large branches of the Maple; and I 
would here record my obligations to my friend, Mr. Frederick Nash, of Ludlow, for having 
kindly furnished me with most of the material upon which the foregoing observations 
