DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 193 
thereby an observable swelling of the part ensues;" and in tab. iii. fig. 18, he has repre- 
sented the germination of Viscum upon a small branch of Tilia Europea, and shows that 
in the act of germination, and before the young plant has put forth a leaf! it causes the 
young branch to swell out pretty uniformly to about twice its diameter. 
DeCandolle (Phys. Végét. p. 1411), speaking of the effect of the Mistletoe upon the 
bark of the supporting branch, says, * It results from this mortification of the bark, that 
the Mistletoe produces upon the supporting branch an effect analogous to that produced 
by a section or ligature of the bark, and a swelling is formed above the Mistletoe which 
is greater in proportion as the trunk of the Mistletoe approaches the thickness of the 
branch which supports it.” 
Griffiths also supports the theory of the downward circulation of the sap, and the effect. 
of the Mistletoe in arresting it, in stating (Trans. Linn. Soc. vol. xviii.) that *the upper 
bourrelet surrounding the suckers is always the larger.” I find that this swelling is due 
to the regular hypertrophy of the woody layers of the nourishing plant around the base of 
the parasite, and, discordantly with these statements, that as long as it does not interfere 
with the vitality of the branch beyond it, the hypertrophy takes place equally around it. 
Dr. Hooker (Flor. Ant. vol. ii.) states that “the Fagus-branch dilates around Myzoden- 
dron into the form of a cup, and is formed of as many layers of wood as years have 
elapsed since the germination of the parasite ;” and he represents the cup as being 
equally developed in all directions. Now the difference between the swelling around 
the base of Myzodendron and that of Viscum is only one of degree and regularity. The 
hypertrophy is in both cases produced by the uniform and uninterrupted growth of the 
parasite and its supporting branch. In the one case, owing to the simple condition of 
the base of Myzodendron, a regular cup-shaped expansion is formed ; but in the other, on 
account of the ramified condition of the base of Viscum, and a more or less complete 
confluence of its roots, the swelling around the base is generally irregular, though not 
more developed actually in any partieular direction, if the whole branch bearing it be 
alive. If, however, the base of Viscum be simple, then the woody layers, being regularly 
raised up and everted, often form a cup-shaped swelling around it, which is sometimes 
very well developed. : 
When, however, the parasite has killed or very much atrophied the branch beyond it, 
then, for the obvious reason that the lower part of the base of the parasite is situated in 
the midcurrent of the nutrient fluid, while its upper part is but just within its limits, the 
hypertrophy takes place to a greater extent around the lower part of the base than the 
upper (PL XXIX. fig. 12). I have never, in any case, seen the swelling greater above 
the base of the parasite than below it. : 
If we examine more closely into the mode in which the hypertrophy is produced, we 
observe that the thickening is effected by the gradual widening out of the woody layers 
from the side of the branch opposite to the Mistletoe to their junction with it; and when 
the parasite is seated on one side only of the branch, that side in the neighbourhood of 
the Mistletoe is inereased to twice, thrice, or even four times its natural thickness, the 
medullary canal becoming excentrical (Pl. XXIX. figs. 9, 10, 11, 12). 
In some specimens the concentric layers of the wood appear to be more numerous in 
