DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 187 
with its original development: the branch must be preformed, which implies the 
existence of at least one ring of wood between the point of the root of the parasite and 
the medullary centre of the nourishing plant. 
Mutual growth of the Parasite and its nourishing Plant.—When the roots of Viscum 
album have become fairly infixed into the medullary system of the nourishing plant, 
their outer portions become gradually thickened by the formation of woody layers upon 
their surfaces. This increase in the lateral dimensions of the root takes place pari passu 
with that of the branch upon which it grows; for every layer of wood deposited upon 
the branch, a corresponding one is deposited upon the Mistletoe; and the growth of the 
two plants proceeding thus uniformly, the concentric rings of the stock pass uninter- 
ruptedly into those of the Mistletoe, and the woody layers become coincident. While 
the roots thus undergo increase and lignification about their outer portions, their inner 
extremities, which now lie deeply in the hard wood of the nourishing plant, constantly 
retain their original soft cellular condition. And, as far as my own observations go, the 
life of the parasite depends upon these delicate cellular processes ; they are, in fact, to 
Viscum, what the cellular rootlets of terrestrial plants are to them. When through 
accident or old age they die, secondary ones are thrown out. from the soft outer layer of 
the woody base, which, after traversing the bark for a shorter or longer distance, come 
into relation with the medullary rays of the nourishing plant. 
Guided by the above conclusion respecting the nature of the parasitic connexion of the 
Mistletoe, I was led to suppose that a difference in the size, number, and arrangement of 
the medullary rays might explain the reason why, apart from any natural affinity, which 
certainly does not exist, the Mistletoe is so frequently attached to certain trees, but 
rarely to some, and never to others. With a view of testing the truth of this supposition, 
I have examined the wood of those of our indigenous trees and shrubs which, from their 
size, distribution, and aggregation, are liable to the attacks of the parasite, and also that 
of some exotic plants to which it is naturally attached, or upon which it may be made to 
grow. The sections were made at right angles to the medullary rays, at the surface of 
the wood where the roots of the parasite would first come in contact with the rays; 
generally speaking, however, the rays have the same dimensions in the heart-wood as 
they have at the surface. The following are the results :— 
PINUS SYLVESTRIS and ABIES LARIX. 
Ray composed of a single row of about six wide cells, forming a narrow ellipse, the 
Tooth of an inch deep and the ryysth wide. Rays discrete, separated by intervals equal, 
in one direction, to twice their width, in the other to half their depth. 
ALNUS GLUTINOSA. 
Ray ;isth of an inch deep and 4455th wide, composed of a single row of about sixteen 
cells. Rays distinct, distant about thrice their width. Some are plates the q'sth of an 
