20 STUDIES IN ROOT-PARASITISM. 
Plate V a fully mature gland is figured. Its lower part and the 
whole of the nucleus around it are completely disorganized, but 
some considerable growth has commenced in the nucleus above 
the gland, pushing the cap down into the lumen of the gland. 
The cells of this meristem are seen to be dividing so as to form 
rows of cells radiating from the lumen and they are also separated 
into two distinct regions, a median and a lateral (fig. c). The 
median strand consists of much smaller cells, divisions here taking 
place both in the radial and the tangential direction. This strand 
appears to be the first differentiation of what will be the axial 
plate of vessels. In Plate VI this nuclear meristem has grown 
considerably and the stage there illustrated may be taken to 
represent the last in a haustorium as yet wholly outside the 
host’s root. 
The downward thrust of the glandular mass exerts consider- 
able pressure in the interior of the haustorium. This is seen 
in the flattened shape of the peripheral cells of the nucleus and 
those of the adjoining cortex. It is also seen in the crushing 
out of recognition of the lower part of the gland. But of more 
interest is its effect upon the zone of clear cells and the adjoining 
starch-filled parenchyma of the cortex. We have here in fact 
the first step in the formation of the collapsed layers clearly 
depicted. These are at first formed entirely from the outer 
cells of the clear layers (Plate VI, fig. 3) but, later, the outer 
starch-filled cells also take part, those about to be added to the 
collapsed layers losing their contents with fragmentation of the 
starch grains and becoming clear (Plate VI, fig. 2, and Plate VIII, 
tigs. 1—3). A reference to what happens in Santalwm will show 
that this ‘formation of the collapsed layers is entirely at variance 
with what happens there. But this is not all. The collapsed 
layers of the mature haustorium in Olax are made up of three 
distinct parts, each formed at a different time and in a 
different manner, whereas in Santalum the collapsed layer on 
each side is uniform in its origin. The part of the collapsed 
layers formed at this stage of development in Olaa is the lowest 
one. The middle portion is not formed until after penetration, 
