ye! STUDIES IN ROOT-PARASITISM. 
IX, figs. 6-10), we see that the increase in thickness of the vas- 
cular strands assumes its maximum in the lower part of the 
vascular loop. In Cansjera this tendency is increased to 
such an extent that comparatively little thickening takes place 
in the sucker or axis, and the addition of secondary wood is 
chiefly in the middle of the vascular loop, immediately below 
the interrupted zone. This feature gives the old haustoria a 
very characteristic appearance. In them there is a curious 
bell-shaped expansion of the wood on each side of the loop, 
terminating upwards in the isolated strands of the interrupted 
zone. The haustorium thus becomes broader at the top with 
increasing age and the collapsed layers assume a vertical posi- 
tion (Plate IX, fig. 1). In very old haustocia the effect is still 
more striking. The whole haustorium assumes a turbinate 
form, the upper part being very broad and the point of attach- 
ment frequently minute (Plate I, figs. 5 and 6), like the peg of 
atop. The collapsed layers in such haustoria are inconspicuous 
and ultimately become almost horizontal, the cambiform tissue 
is broad and the cortex is reduced to a narrow strip charged 
with calcium carbonate crystals (Plate IX, fig. &). 
Yellowing of the old wood is not a very marked character 
in Cansjera haustoria, but it is met with here and there. It 
is interesting to note that, in this matter, as in so many 
others, Cansjera resembles Santalum rather than Olax. In the 
latter the yellowing commences in the primary xylem and 
gradually extends outwards through the secondary wood. In 
Santalum and Cansjera, while the inner layers of secondary 
wood become yellowed with age, the primary xylem remains 
white in colour like the younger part of the secondary wood, 
THE SUCKER. 
17. Penetration.—lt is assumed that the chief reason 
for the penetration, by green hemi-parasites, of the roots of 
other plants is to form a connection through the haustorium 
between the vascular systems of the host and parasite. By 
