Haustorium of Striga lutea . 1071 
dinal section, with several papillae touching its outer wall. This has 
become so swollen and dissolved by the ferment they have secreted that 
only its inner line is distinguishable ; and the inner wall has also begun to 
swell. The walls of the endodermal cells in the maize are thicker than 
those of the cortical cells, the inner considerably so, and the haustorium 
may enlarge to almost its mature dimensions while the papillae are trying 
to penetrate them. 
Meanwhile, the strand of elongated cells forming the axis of the young 
haustorium has become converted into the line of tracheides seen in Fig. 5, 
running from the vessels of the parasite down through the centre of the 
haustorium to the stele of the host. Sometimes this line of tracheides begins 
to form almost immediately after the haustorium has begun to penetrate 
the tissues of the host ; sometimes it is only laid down after some amount 
of cell-division has taken place in the haustorium. The latter seems to be 
the case when parasite and host lie close together, so that a comparatively 
short haustorium is formed ; the former when the haustorium has had to 
grow out to some length, and thus is further from the water supply of its 
mother root. In any case, this line of tracheides is formed in the same way, 
and apparently always before the haustorium has penetrated through the 
endodermis, which it takes some time to do. Spiral markings appear in 
the cells of the two or three layers which lie between the vessels of the 
mother root and the base of the line of elongated cells forming the axis 
of the haustorium. The spirals run in the same direction as those of 
the vessels, and the tracheides thus formed make a little hump on the 
vascular strand, which links it with these axial cells. (These connecting 
tracheides can be seen at the junction of the vascular system of the mother 
root with the axial strand of tracheides in Fig. 5. The two lines of tracheides 
which appear in the figure to end blindly in the nucellus close to the 
vascular strand are seen at a slightly higher focus to link on to it by similar 
connecting tracheides.) In the lowest of these axial cells, i.e. the one con- 
tiguous with the connecting tracheides, spiral thickening now appears, and 
each successive cell in the same longitudinal row becomes converted into 
a spirally thickened tracheide. Later on, other tracheides are added to 
this strand by the appearance of spiral markings in the papillae that are 
striving to pierce the endodermis of the host, and in the cells running up 
behind them (cf. Fig. 8). A few tracheides have been added in this way to 
the axial strand of Fig. 5. Comparing it with that of Fig. 6, it will be seen 
that as these additional tracheides follow the line of the maize stele, the 
central strand is lenticular in shape, although the haustorium when mature 
is almost globular. Occasionally in the case of a large haustorium a branch 
of the axial strand may be formed later on in the * nucleus ’ (cf. the left-hand 
haustorium in Fig. 1). 
While this axial strand has been forming, active division has been 
