300 Peirce.— On the Structure of the Haustoria 
where, owing to the plane of the section, only one row is 
shown) whose walls are being unequally thickened, leaving 
pits or reticulations. The differentiation extends through the 
pericycle to the base of the haustorium, the two rows gradually 
converging and uniting with its xylem. Thus the xylem of 
the haustorium becomes connected with the xylems of two 
fibro-vascular bundles in the mother-plant. In similar fashion 
two rows of cells, which, if large, divide by radial walls, run 
from the two phloems of the same two axial bundles of the 
parasite to the two phloems of the haustorium, and become 
differentiated into sieve-tubes and companion-cells (see 
Fig. i 2, s, s). In sections old enough, callus-plates can be 
demonstrated in the sieve-tubes by the use of aniline-blue. 
Thus the phloems of the haustorium are connected with the 
sieve-tubes of two fibro-vascular bundles in the mother-plant. 
The question now presents itself. — Do the vascular tissues 
of the haustorium unite with corresponding tissues in the 
host ? For convenience, a transverse section is made of the 
host where a tolerably old haustorium penetrates it. As is 
shown by Fig. 7, t, the xylem-elements of the haustorium 
apply themselves directly to the xylem-elements of a fibro- 
vascular bundle in the host ; that is, those cells of the 
haustorium (consisting so near its apex of central cylinder 
mainly, with little or no cortex) which come by the solution of 
the intervening cells of the host into contact with the ducts, 
quickly differentiate into tracheids, the thick and thin places 
in their walls corresponding with the thick and thin places in 
the walls of the ducts to which they apply themselves. Thus 
the haustorial xylem connects a xylem-group of the host 
with two xylem-groups in the stem of the parasite. 
By a similar process of solution of the intervening cells of 
the host the phloem-cells of the haustorium are united with 
the phloem-cells of the same bundle. That the union of 
haustorial sieve-elements with sieve-elements in the host is 
direct may more clearly be seen in a tangential section, such 
as is shown in Fig. 13, Plate XIV, for the contact of 
haustorial cells with the cells of the host is usually by their 
