20 STUDIES IN ROOT-PARASITISM, 
haustorium, even in cases of self-attack: in Olax, it will be 
remembered, the presence of masses of sieve-tubes and other 
bast elements was a surprising but well-marked feature in cases 
of self-attachment. In older haustoria with much secondary 
thickening, a mass of cambiform tissue filled with contents is 
found outside the cambium, forming, as in Santalum. a sort of 
secondary cortex. Secondary thickening is practically con- 
fined to the upper part of the vascular loop, and this fact gives 
a peculiar and characteristic appearance to sections of old 
haustoria. Lastly, there is a marked difference in the size 
and pitting of the vessels of the sucker and those of the mother 
root. These points of interest will be dealt with in order. 
14. The Endodermis is extremely clearly shown in the 
upper part of the haustorium of Cansjera Rheedw. It is, how- 
ever, confined to the mother root and transitional region and 
stops short at the interrupted zone (Plates V, fig. 2, and VII, 
fig. 1). In ordinary haustoria it is seen to consist of a layer 
of crushed cells whose walls are much darker in transmitted 
light than those of the cells around them. Here and there, 
in sections of young parts, there are shadows on the walls, and 
in rare cases this is seen to be due to undulations similar to 
those in the endodermis of many roots. An endodermal 
layer appears to surround each bundle where they are 
separated in the transitional region, but good figures of this 
are not easy to obtain. 
The Interrwpted Zone is very well developed in these haus- 
toria. The change undergone by the vessels of this region 
is exactly that described for Santalum album (Sant. II, para. 
9). The walls lose their characteristic markings and assume 
a yellow colour and, in later stages, the whole vessel becomes 
filled with yellow substance, the lumen being entirely oblit- 
erated. The interrupted zone occurs at the point of junction 
of the transitional region and the haustorium proper, just above 
the place of greatest width of the vascular loop and in the place 
(mentioned above) where the bundles of the transitional 
