DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 179 
With regard to the direction and arrangement of the roots of Viscum which lie within 
the wood, this is determined by the arrangement of the medullary system of the nourish- 
ing plant, the roots always lying strictly parallel to the medullary rays. The examination 
of the specimens figured in the plates led me, more than two years ago, to this conclusion ; 
more recently, however, Professor Oliver kindly referred me to the writings of the German 
botanists who have worked at the subject, and I then discovered that Dr. Hermann 
Schacht, in his * Lehrbuch der Anatomie und Physiologie der Gewüchse' (vol. ii. pp. 156 & 
465), had come to the same conclusion. He states that “ the root of the Mistletoe developes, 
on the side of the wood, branches which occupy the position intended for the medullary 
rays of the wood." Adolphe Pitra (Botanische Zeitung, von Hugo von Mohl, 1861, 
p. 61), however, refutes this statement. He says, “ Dr. Schacht's statement may be true 
with regard to certain definite stocks, such as firs and pines, but I cannot establish this 
fact, as I have not had opportunity of investigating such stocks ; but it certainly does not 
hold for all. With limes, willows, and other trees I have found the rule uniformly not 
established. The place where the sucker (senker) meets the wood is quite accidental, and is 
usually not that of a medullary ray; besides, the sucker is from the first much too strong 
and too broad (in the section of a yearling shoot) for it to correspond to a medullary ray." 
Since Dr. Schacht has not offered any arguments or observations in favour of the 
above-mentioned assertion, I will proceed to detail the facts which led me independently 
to the same conclusion. 
1. The base of the Mistletoe is always attached at right angles to that particular part 
of the branch upon which it grows, and this relation is maintained with regard to the 
roots of the parasite contained within the nourishing plant; the two plants, in fact, grow 
at right angles to each other. 
2. On making a transverse section of a branch infested with young roots of Mistletoe 
(Pl. XXVIII. figs. 1, 2, 3; Pl. XXIX. figs. 8, 10, 11), we find that the latter are divided 
longitudinally, and that they are arranged like radii from the circumference of the stem 
towards its centre, if this be occupied by the medullary canal; if not, then the points of 
the roots are directed towards the excentrieal pith (Pl. XXIX. figs. 8, 10, 11). Each root 
lies in the same horizontal plane, and terminates, by a very fine point, at a variable dis- 
tance from the central pith; occasionally it terminates in it (Pl XXIX. fig. 8). The 
roots vary in length; some attain to 2 or even 3 inches. 
On examining such sections more closely with the help of a pocket lens, the medullary 
rays of the wood are observed to lie strictly parallel with the sides of the roots through- 
out their entire course. The intrusion of roots of'various size and length throws the 
rays into simple curves or undulations, and no matter how tortuous soever these may be, 
the contiguous roots are correspondingly bent in the horizontal plane. The wide conical 
roots produce a great divergence of the rays and consequent curving of the younger roots 
in their neighbourhood (Pl. XXVIII. figs. 1, 2; PL XXIX. fig. 8a). 
When thin sections are examined under the microscope, the wood-cells and ducts of 
the nourishing plant are seen transversely divided and arranged in linear series, parallel 
` With the sides of the young roots (Pl. XXX. fig. 17), which simply interrupt the con- 
centric layers of the wood without disturbing them. 
