182: DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 
of the surface containing ninety such roots, varying in diameter from the jth to the goth 
of an inch. 
The size of the woody roots varies according to the age of the plant. Owing to the 
tendency of the roots to increase more in the longitudinal than in the transverse direc- 
tion, and also on account of their more frequent confluence in this direction, the greater 
number of the woody bases of the Mistletoe have an elliptical form in transverse section, 
but few of them preserving a regular conical shape. | 
The ultimate anatomical relation existing between the Mistletoe and its nourishing 
plants may be very well studied in sections which cut the roots of the former and 
the medullary rays of the latter at right angles to their length. The nourishing plant 
best suited for examination is the Maple (Acer campestre). Before proceeding there- 
fore to describe the appearances presented in such sections, I will, for the sake of 
greater exactness, briefly describe the wood of the Maple. The tissues are arranged 
as is seen in Pl. XXX. fig. 14, which represents a vertical section of the wood across 
the medullary rays. The woody fibres diverge at intervals of about the jj5th of an 
inch, and enclose the bundles of cellular tissue composing the medullary rays, which 
measure, on the average, the 45th of an inch deep and the gl;th of an inch wide, 
the long diameter of the ellipse taking, of course, the same direction as the long axis 
of the branch. In this view the parenchymatous cells appear small and round; they 
are tubular, measuring the z;/g5th of an inch wide and the g4isth of an inch long, 
and are joined end to end and collected into bundles. The medullary rays thus consti- 
tuted pass in straight lines from centre to circumference. Pitted ducts, the 43,th of an 
inch wide, run amongst the wood-fibres at regular intervals. If now we examine the 
tissues in relation to the young cellular roots of Viscwm, we shall find that, where the 
woody fibres and ducts of the Maple meet with the root of the parasite, they suddenly 
diverge, and, sweeping round it, as suddenly converge above it, and having thus enclosed 
the root, resume their original direction. In fact, the woody fibres and ducts are 
arranged about the root just in the same way asthey are about the medullary rays of the 
plant to which they belong; and the smaller roots are often separated by only single 
fibro-vascular bundles of the nourishing plant, and thus resemble contiguous but greatly 
hypertrophied medullary rays, save that the cells composing them are larger and some 
of them delicately reticulated (Pl. XXX. figs. 14, 16). Occasionally we see the section 
of a root which has scarcely attained twice the size of an average medullary ray, and 
which, like the latter, possesses a narrow elliptical form. 
Agreeable with their regular distribution, the medullary rays of the nourishing plant 
lie against the surface of the root, somewhere separated from it by a few woody fibres, 
otherwhere by the intervention of a pitted duct, while, at several parts of its circum- 
ference, the woody fibres bounding the rays disappear in places, and the constituent 
cells of the latter thus become confluent with those composing the root (Pl. XXX. 
figs. 14 & 15); and as the latter are here of intermediate size, and in other respects 
similar to the parenchymatous cells of the medullary rays of the nourishing plant, which, 
on the other hand, are invariably increased in size in the immediate vicinity of the 
root, one cannot say positively whether the cells which form the surface of the Viscum- 
