4 
28 STUDIES IN ROOT-PARASITISM 
into a single vertical plate with a delicate layer of cambium on 
each side. In figure 1 on Plate VII a median section is shown 
through an older haustorium. Secondary thickening has com- 
menced in the vertical strand, and it is interesting to note that 
this usually commences some way below the vascular loop, in 
fact, at a point about opposite to the lacunz inside the middle 
parts of the collapsed layers. These two and subsequent thicken- 
ings are illustrated by a series of diagrams 6—10 on Plate IX, 
each one of which is taken from an individual haustorium which 
has been chosen as the type of aclass. Figures 6 and 7 show 
the early stages already described ; figure 8 corresponds with the 
haustorium shown in figure 1 on this Plate. The thickest part of 
the strand gradually passes upwards in succeeding stages until it 
embraces the base of the vascular loop (figs. 9 and 10). At this 
the final stage, the whole vascular mass assumes a wavy outline due, 
to all appearance, to its being penetrated by broad medullary rays. 
The elements of this vascular mass are mainly narrow 
tracheide-like vessels with simple circular perforations at their 
ends, and of remarkably uniform character (Plate X, fig. 3). The 
following differences have been noted in various parts of the axis. 
In the region of greatest thickening, at the base of the vascular 
loop, the elements are short and arranged in rows derived from 
the same cambial mother-cell, with medullary rays here and there 
between them. There is little difference in the inner and outer 
vessels unless it be a slightly greater width in the outer ones. 
Passing down the axis, the medullary rays are less conspicuous, 
the vessels become longer and, if anything, narrower, until they 
separate near the host’s root to isolated strands of elongated 
elements. The vessels in the region of the loop show greater 
differences. Those in the lower half are very long and narrow, 
but remain uniform among themselves. Above the middle, as 
might be expected, this regularity is disturbed and broader 
elements make their first appearance. The vessels increase in 
diameter upwards until, in the mother root, ones are found with 
many rows of pits. The number of pits met with across the 
walls of the various vessels forms a convenient mode of comparing 
