of some Phanerogamic Parasites . 301 
radial walls. If one can demonstrate the presence of a callus- 
plate between two cells, no doubt can be entertained as to the 
nature of these cells ; they are sieve-cells. If a tangential 
section of the host, passing through the sieve-tube area of 
a fibro-vascular bundle to which a haustorium had applied 
itself, be laid for a half-hour in a moderately strong aqueous 
solution of aniline-blue, then washed in distilled water to 
decolorize cellulose, lignified, and cutinized walls, and sub- 
sequently examined, in a clearing mixture of glycerine and 
water, under a high magnifying power, one finds the callus- 
plates of the sieve-tubes clearly defined by their blue colour. 
As is shown by Fig. 13, the haustorial tissues are easily 
distinguished from those of the host by the long axes of their 
cells being at right angles to those of the host. A careful 
examination will show that certain haustorial cells abut 
directly upon the sieve-tubes of the host, that the common 
wall between these haustorial cells and the sieve-cells is 
coloured along its whole face a faint blue, or that at certain 
places it is decidedly thickened and that these thicker parts 
are intensely blue (Fig. 13, c ). When the wall is faint blue it 
is due to a thin deposit of callus over a considerable area ; 
when the wall is deep blue in spots it is due to a thick deposit 
of callus in a limited area. One finds upon the radial walls 
between adjacent sieve-tubes of the host similar deposits of 
callus, extended and faintly coloured, or limited and deeply 
coloured. I have not seen such deposits on either side of the 
radial walls between sieve-cells and companion-cells. We 
know that callus is formed during the activity of sieve-cells as 
such, and not during their formation. Furthermore, by no 
means at present known can callus-plates be discovered in 
the youngest active sieve-cells. The deposit of callus is 
gradual and can be observed only after the cells have been 
active for some length of time. Between the sieve-cells of 
the host, and the adjacent haustorial cells, I have observed 
thick deposits of callus only on the side of the wall which 
limits the cavity of the sieve-cells of the host (Fig. 13, c)> 
When, however, a thin deposit of callus is formed over 
