DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 181 
Bearing in mind the appearances presented by the sections above described, the 
modified appearances of the larger roots when seen in vertical sections of the nourishing 
branch, made parallel to its medullary rays, will be readily understood. In such sections 
the base of the Mistletoe often appears to terminate in a * sucker-like expansion ” 
furnished next the wood with little nipple-shaped processes, or coarse fibres dipping into 
its substance (Pl. XXIX. fig. 12). 
The young taper roots also appear wider and shorter (Pl. XXVIII. fig. 5; Pl. XXIX. 
fig. 6) than in sections made in the other directions. These are the appearances which 
doubtless led Mr. Griffith to the conclusion that the Mistletoe is attached to the stock by 
sucker-like processes. If we could make a longitudinal section of the entire root parallel 
to the medullary rays, it would have an appearance similar to that seen in cross sections of 
the branch, but modified in most cases by the greater dimensions of the root in the vertical 
direction. It is difficult, however, to make a complete section of the root in this direc- 
tion, first, because many are narrow-elliptical wedges terminated by two or three tail-like 
cellular processes, of which altogether it is much more difficult to make a longitudinal 
section in this direction than in cross sections of the branch; and secondly, owing to the 
undulations or simple curvings of the taper roots in the horizontal plane, we are unable 
to make complete longitudinal sections of them in the vertical. And thus it is that in 
ordinary longitudinal sections of the parasite and the branch supporting it we make such 
oblique, or partial, sections of that part of the former which is contained within the 
latter, as resemble ** sucker-like expansions " or gibbous processes imbedded in the wood 
of the stock (Pl. XXIX. fig. 12). When the modifying conditions are absent, or nearly 
so, we may make a more complete section of the root (Pl. XXVIII. fig. 5c; Pl. XXIX. 
fig. 6). 
When longitudinal sections of the Mistletoe-bearing branch are made at right angles to 
the medullary rays, the roots will be seen in transverse section as represented in Pl. XXIX. 
fig. 7, and also in Pl. XXX. figs. 13, 14, 16, which are magnified sections. They have 
à cireular or elliptical form, the long diameter of the ellipse corresponding to the long 
axis of the branch; and they manifest a greater tendency to increase in this direction 
than in any other, the section occasionally presenting a depth many times greater than 
its width. Confluence of two or more roots is observed to have often occurred, and this 
is most common in the longitudinal direction. The roots being taper, their sca oiim 
according to their distance from the surface of the wood; and since the conical roots of 
the larger and older plants taper so suddenly as to be, at the depth of an inch within the 
wood, but little thicker than their cellular prolongations, sections made at this distance 
from the bark rarely expose roots of a greater diameter than the ¿th of an inch, while 
Some measure only the lth of an inch. Their greatest dimensions are of course at the 
Surface of the wood, where they commence, and where, as may be seen in 8 decorticated 
branch, the cellular roots have a mean diameter of an eighth of an inch, but even here 
there are numbers which measure no more than the j;th of an inch across. 
These young cellular roots are in some cases exceedingly numerous. On the decorti- 
cated surface of a branch of Maple, a foot long and 15 inches mean circumference, I have 
estimated several thousands, a single square inch of by no means the most crowded part 
VOL. XXIV. 2B 
