16 STUDIES IN ROOT-PARASITISM. 
appearance, already become permanent. A series of parallel 
walls are formed and a layer of cells resembling cork makes 
its appearance, surrounding the decayed part. Thus, in the 
case of the yellowed collapsed layers figured on Plate VI, the 
cells on both sides have developed in a meristem-like manner. 
Similar secondary tissue has been noted around the lacune 
in young haustoria and the vellow band of decayed cells across 
the upper part of the nucleus. 
It is natural to assume that this cell-formation has for its 
object the provision of a separating layer of cork by which the 
dead tissues may be cut off from the living. It is to be noted 
in passing, that the cells thus formed are sufficiently turgid 
to project into the yellow mass, much as the external cells of 
the sucker project as papille into the mass of yellow secretion. 
It is probable that in both cases the form assumed is due to 
purely mechanical conditions. 
The cortical cells on the outer sides of the collapsed layers 
soon lose their contents and become apparently moribund. 
There is none of the radial arrangement of the cells which forms 
so marked a character of the old haustoria of Olax scandens, 
and the cells of this part are similar to those of Santalum 
album. And this similarity is emphasised by the occasional 
appearance in this place of meristematic cells resembling those 
found in haustoria of Santalum album attacking the roots of 
Pithecolobium dulce (Sant. II, Plate V, fig. 1, me). 
A much more striking feature is the occurrence of secon- 
dary thickening in the ends of the cortical folds, mentioned 
in para. 7 (Plate V, fig. 1), This appears to be independent of 
the lignification of a small patch of cells in the end of the cor- 
tical fold noted in para. 6 (Plate IV, 7). There is a tendency 
in certain plants for this lignification to extend upwards 
as a narrow band, three or four cells broad, immediately below 
the hypoderm (Plates VI, fig. 7, and VII, fig. 5). Itis thus not 
infrequent for the outer layers of the haustorium to assume 
some complexity, and this has been noted especially in the 
case of certain hosts, For instance, while no trace of the 
