C. A. BARBER. 29 
their width. Those of the protoxylem, those in the end of the 
sucker and those in the lower half of the vascular loop usually 
have 1—3 rows of pits. The somewhat wider, outer vessels 
formed by the ecambium of the axis show 3—4 rows, while the 
vessels of the transitional region (above the middle of the vas- 
cular loop) and the mother root may have as many as 6 to 10. 
The sections thus far dealt with in the present paper have 
all been transverse, that is, cut across the root attacked. Sections 
in this direction, as we have seen in Santalwm, include more of 
the tissues of both host and haustorium than those cut in any 
other plane. To understand the vascular system of the axis, 
however, horizontal sections will be necessary (7.e., cutting the 
haustorium across its axis and the root attacked, if in the section, 
tangentially). Such sections are shown on Plate XII (figs. 1 and 
la), and the arrangement of the parts can be readily made out. 
Note in these figures px, the protoxylem, with scattered parenchy- 
matous elements, « the secondary xylem with its numerous 
medullary rays mr, and the cambiumc. The uniformity and 
small size of the vessels is clearly shown in transverse and 
horizontal sections, and on a comparison with a section of the 
mother root, it is seen that the haustorial vessels correspond best 
with those found in young roots. The larger vessels found in 
older roots are not met with in the axis of Olax haustoria attack- 
ing other plants. Those on Olax itself will be referred to in 
paragraph 18. 
The section drawn on Plate IX (fig. 1) being cut in the plane 
of the medullary rays, the latter are not seen, The vascular 
strand bifureates above and passes with its various elements 
upwards round the vascular loop into the transitional region. In 
Santalum attention was drawn to the fact that, in the upper part 
of the vascular loop, the vessels became indistinct, hyaline and 
filled with gum, and this region, occurring in all older haustoria, 
was called the “interrupted zone.” There is no trace of this 
feature in the vascular loop of Olax scandens, although it is 
markedly present in sections of Thesiwm, Osyris, Opilia and 
especially Cansjera. 
