672 
Benson.—Root Parasitism in Exocarpus. 
The important question as to the presence of protoplasm, a nucleus, 
and plastids in these cells is left an open question by Heinricher (see 
later, p. 673). 
A thin matrix lining the walls is generally demonstrable. 
The task of securing evidence as to its nature is a difficult one, but in 
one series through a Thesium haustorium I have satisfied myself that 
delicate strands of protoplasm can be seen traversing the lumen, and that 
these are continuous with the matrix in which the granules are embedded 
(Fig. 3, PI. LV). 
It is much easier to obtain evidence as to the absence of nuclei, for 
those of the surrounding cells take up a staining reagent which is not taken 
up by the cytoplasm. Thus it can be shown again and again that the thin- 
walled, non-lignified cells interspersed among the phloeotracheides contain 
nuclei, but that nuclei do not occur in the lignified phloeotracheides. 
No evidence of the presence of plastids has been obtained. As none 
are visible at a high magnification, even when the amylodextrine granules 
are very minute (i. e. when the plastid might be expected to be clearly 
visible), it is probable that plastids are not present. 
The walls of the phloeotracheides much resemble those of the root 
tracheides, but show no circular bordered pits (Text-figs. 3 a and 3^). The 
areolae are non-lignified, and in Thesium appear to consist of nothing but the 
middle lamella (Fig. 3, PI. LV). In Thesium the lignification of the reticu¬ 
late bands is continuous from one cell to its neighbour, i. e. the middle 
lamella can only be demonstrated over the thin areolae, but in Exocarpus 
a middle lamella appears as a bright blue plate between the thickening 
bands of contiguous phloeotracheides. 
The lignified bands are of such thickness that under a high magnification 
the granules appear to be disposed only over the thin areolae, but with trans¬ 
verse sections it is easy to show that the granules are uniformly distributed 
over the inner surface of the cell. 
By means of a series of longitudinal sections of the haustorium of 
Exocarpus one can demonstrate that the sheath of these cells is much 
thicker on the proximal surface of the ‘ nucleus ’, i.e. there is a continuous mass 
of lignified cells lying between the transparent central cells of the nucleus and 
the neck of the haustorium. A series of sections also shows that there 
is continuity between this pad and the tracheides of the neck, but I have not 
been able to demonstrate the presence of the blue staining granules in any 
cells but those of the sheath. 
Cells apparently of the same character are found reaching down to the 
surface of absorption from the host. There is no reason to regard them as 
secretory. They are found to be surrounded, at the surface of contact with 
the host, by tubular, thin-walled nucleated cells, and hence appear to be 
primarily of importance as conducting cells. None the less, the apices, as 
