DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 177 
Arranged in the same radiate manner as the plates of prosenchyma, and in the larger 
roots associated with it, but in the younger occurring alone, are narrow bundles of 
vessels, formed of one, two, or three rows of very delicate reticulated ducts composed of 
elongated cells, gğpth of an inch long and 535th of an inch wide, joined to each other by 
their oblique ends. The woody fibres and ducts take the same direction as the root. 
. The extremities of the young roots are altogether destitute of prosenchyma, but here 
the ducts are very numerous. The parenchymatous cells which form the surface of the 
root, and connect it with the tissues of the nourishing plant, are narrower than those 
lying more internally, and measure only the 44l;5th of an inch wide. A similar con- 
traction is observed to occur in the reticulated ducts as they approach the surface of the 
root, and before they come into connexion with the surrounding wood they become 
reduced to half their original width (Pl. XXX. figs. 15, 17). 
In order to understand the structure of the woody portions of the roots of Viscum, it 
will be necessary to describe briefly that of the stem. 
Structure of the Stem and woody Base of Viscum album.—The medullary rays of the 
stem of the Mistletoe are large and numerous; they are, however, very irregular, and 
each varies in size several times in its passage from within outwards. They average 
about the -th of an inch in depth and the 335th of an inch in width, and are composed 
of large tubular cells, which also vary in size, and average the gj oth of an inch wide: the 
majority of these cells have thick walls, marked by a few scattered transversely elliptical 
dots. The rays are often confluent longitudinally, and so form wide plates of parenchyma : 
they are separated laterally by intervals of about the g41;5th of an inch. One-third of 
this interval is‘occupied by the prosenchyma, the remaining two-thirds by slit-marked 
vessels. The prosenchyma, or wood-fibres proper, is composed of long plain fibres, the 
izsoth of an inch wide, and so much thickened that their original cavities are reduced 
to mere canaliculi: this tissue immediately surrounds the medullary rays, forming a thin 
layer two cells wide. The slitted vessels form wide bundles composed of three or four 
rows of cells, lying between the prosenchymatous fibres which on either hand bound the 
medullary rays: the constituent cells have thick walls, and the reticulated deposits are 
broad and close, converting the intervals between them into short and very narrow slits; 
they measure the 44l55th of an inch wide, and are joined together by almost straight 
extremities. 
The porous wood of the Mistletoe is, therefore, chiefly composed of coarse parenchyma 
and thick-walled ducts, the prosenchyma being very scanty, and forming a thin, wide- 
meshed network surrounding the medullary rays. us 
The woody base of the Mistletoe, contained within the nourishing plant, has a still 
looser texture: the prosenchyma forms a wider network of narrower strands, while the 
medullary rays become proportionately enlarged; by far the greater number of the cells 
composing the latter have walls as thick as the slitted vessels, and present elliptical dots 
which hardly distinguish them from the vessels ; the deposits, however, are very soft and 
are readily coloured. The rest of the parenchymatous cells preserve their original delicate 
Condition. The prosenchyma’ gradually thins away, and ultimately disappears at à little 
distance from the line of junction between the two plants. And here is noticed a 
