178 DR. HARLEY ON THE PARASITISM OF THE MISTLETOE. 
modification of the vascular tissue which is not observed in any other part of the wood 
of Viscum, viz. reticulated or **scalariform"' ducts, identical with those described as 
constituents of the cellular roots. The reticulated ducts are modifications of the thick- 
walled slitted vessels, and may be traced into direct continuity with them. Thus at the 
distance of the 1th of an inch from the line of junction between the woods of the two 
plants the vessels composing the vascular bundles become less numerous, while the con- 
stituent cells of the remainder gradually enlarge, their walls become thinner, the deposits 
forming more delicate reticulations, and the slits consequently elongating into linear 
ehinks, which extend uninterruptedly over a considerable portion of the cell-wall. At 
the distance of about the ith of an inch from the wood of the nourishing plant the con- 
version is complete, and the enlarged bundles subdivide into single or double rows of 
delicate, reticulated, branching cells, which come into contact with the wood of the 
nourishing plant at intervals varying from the 3}>5th to the 4.35th of an inch, and form 
the smooth undulating lines which grain the wood of the parasite in its vicinity. Just 
. where the ducts appear to terminate, however, they change their original direction, and 
instead of continuing at right angles to the medullary rays, they first become oblique 
and then run parallel to them. It is further to be observed, that the cells com- 
posing them contract at the line of contaet to about the width of the slitted vessels or 
dotted parenchymatous cells. 
This description of the wood of Viscum differs considerably from that given by Decaisne. 
He states “ that the ligneous bundles are composed of short fibres having thickened and 
punctured walls, described by Kieser under the name of porous cells” (which, I presume, 
correspond to what I have called slitted vessels) ; * between these there are other tubes, 
much longer and more or less regularly attenuated, with very thick walls destitute of 
dots or reticulations” (these are the prosenchymatous cells proper). ‘The medullary 
rays of the Mistletoe are extremely numerous and small, and divide the vascular bundles 
into very thin plates, so as to render it extremely difficult to distinguish each of the 
elements of a wood bundle.” * 
The great discrepancy between our descriptions appears to be due to M. Decaisne’s 
misapprehension of the homology of the “porous cells” with those composing the 
medullary rays, and of that of the slitted vessels with vessels proper. 
In oblique sections of the wood of the Mistletoe there is considerable difficulty in 
distinguishing between the thickened parenchymatous cells and the slitted vessels, and 
this may be another source of disagreement. That the above homologies are correct is 
clearly demonstrated by examination of the woody base and roots of the Viscum at their 
junction with the wood of the nourishing plant; they may be more readily understood 
by a reference to Pl. XXX. figs. 18 & 19. 
The peculiarity of the wood of Viscwm is due, first, to the irregularity of the medullary 
rays producing a corresponding irregularity of the prosenchyma immediately surrounding 
them, so that im transverse sections it is not very easy to trace a given ray from centre 
to circumference ; and, secondly, to the thickening of the walls of great numbers of the 
parenchymatous cells and of the whole of the vessels. | 
® Op. cit. 
