32 STUDIES IN ROOT-PARASITISM. 
A small inward projection sometimes, as in this case, marks the 
junction of upper and middle portions, and it is easy by this 
means to distinguish the three parts of the collapsed layer. 
There is no marked differentiation here of the cells which are 
about to become crushed and added to the layer, but this is not 
the case always, as is seen from a study of Plate IX, fig. 1, 
where the clearing of the cells for the formation of the 
uppermost part of the collapsed layers is well shown (uw. c. /.). 
This portion of the collapsed layers may be regarded as directly 
due to the increase in thickness of the axis of the haustorium by 
secondary thickening. As the thickest part moves upwards, it is 
followed by an upward extension of the collapsed layers on each 
side. We see then that the collapsed layers in Olax consists of 
three parts formed at different times and in different ways, all, 
however, arising entirely from the collapse of cortical cells which 
have lost their contents. 
The zone of radiating cells outside the collapsed layers, so 
closely connected in former stages with the deposition of large 
starch grains, has entered on its last phase. The radial arrange- 
ment is still visible, but the starch has all gone and, in its place, 
masses of calcium oxalate crystals have been deposited. Thus, 
throughout the life of the haustorium the curves of growth have 
been retained in this part of the cortex, an additional proof that 
it takes little or no part in the penetration of the host’s root. 
Such was not the case in Santalum ; but in certain cases, notably 
in attacks on Pithecolobium Saman and P. dulce, a narrow band 
of similarly arranged cells was found on the outer sides of the 
collapsed layers, having all the appearance of cells derived from 
a secondary meristem (Santalum, Part II, para. 7). From a 
consideration of the sections of Olax scandens haustoria examined 
it appears possible that this group of cells in Santalum is rather 
a remnant of the former curves of growth than a new formation. 
We saw in our study of the earliest stage of Olax haustoria 
met with (Plate III) that the rows of cells forming the body of 
the haustorium were separated from the older, irregularly 
arranged cells outside them by a lacunar band of crushed cells. 
