24 STUDIES IN ROOT-PARASITISM. 
remain white while those of the unaltered starch-filled parenchyma 
outside become slightly discoloured and browned (figs. 1—8). 
The formation of the middle part of the collapsed layer is 
well seen in the section. It is formed entirely from cortical cells 
and follows the line of junction of cortex and nucleus. Its mode 
of origin from the innermost cortical cells is seen to agree with 
that described above for the increase in thickness of the lower 
part (fig. 3), so much so that the middle and lower portions of the 
collapsed layer appear as an undivided whole and form an 
unbroken curve, the inward projection half way up alone indicating 
the place of junction (figs. land 2). The upper limit of the middle 
part of the collapsed layer is indicated by the ending of the clear 
cells outside it (fig. 2). The starchy cells beyond retain their 
character during the life of the haustorium, losing their contents 
very gradually. The third or uppermost part of the collapsed 
layer is unimportant, appears only in old haustoria with secondary 
thickening and follows a different course (see paragraph 16). 
The axial tissues at this stage require a word of explanation. 
When the nuclear meristem undergoes its great extension and 
thrusts itself downwards into the host’s root, its cells are stretched 
to their utmost limit. In one place this extension is too much 
for the cohesion of the tissue and it is ruptured, forming a 
characteristic and well-marked lacuna (fig. 2, lac, but better in 
Plate VII, fig. 1). The cells above and below this lacuna, being 
subjected to very different degrees of tension, are not affected 
in the same way. While those above become greatly elongated 
(Plate VIII, fig. 2. «), they remain connected with one another 
and retain their radial arrangement (they correspond with the 
cells marked «@ in Plate VI, fig. 2). They have delicate proto- 
plasmic sacs and are readily distinguishable from the starchy cells 
which were early pushed in between the nucleus and the vascular 
loop (Plate IV, fig. 1, ¢). 
The cells below the lacuna are more difficult to explain, and 
it is not altogether certain to what part of the nucleus they 
belong. They form a sort of “nest” of cells, oval in shape, 
rounded towards the collapsed layer and drawn out downwards 
