Pedicularis vulgaris , Tournef . 503 
is formed in the developing haustoriuni a mass of small-celled tissue, the 
‘ nucleus *, using the terminology of former writers ; 1 this is bounded by the 
elongated palisade-like cells of the piliferous layer on the side adjacent to 
the host root, and by a cortex a few cells in thickness on its flanks. The 
haustorium penetrates the host root by means of the palisade-like piliferous 
cells, which grow straight forward into its tissue, dissolving the cortical cells 
through which they pass, and living on their contents, and eventually 
reaching the xylem elements of the root. After applying themselves by 
their tips to the elements of the host xylem, some of them become thickened 
in an annular or reticulate manner, still retaining their contents. Meanwhile 
a change has been taking place in the parenchymatous cells adjacent to the 
xylem elements of the mother root opposite the position of the haustorium. 
These become thickened in a reticulate or annular fashion and form a mass 
of tracheidal cells, which, however, still retain dense protoplasmic contents. 
Then a single row of the small cells of the central mass or £ nucleus 
connecting the tracheidal penetrating cells with the mass of the tracheides 
adjacent to the xylem of the mother root, become lignified and thus tracheidal 
connexion is set up between the parasitic and host xylems. So much has 
been described by du Sablon, but little has been said by him about the 
internal anatomical structure of the mature haustorium, or the orientation 
of its internal conducting elements beyond mention of the fact that the 
penetrating cells are lignified in one plane only. 
The orientation of the conducting tissue of the haustorium bears a 
definite relation to the orientation of the host root. It has already been said 
that the haustorium always attaches itself to the host root in such a manner 
that the latter is always perpendicular to the portion of the parasitic root 
bearing the haustorium (see Fig. 1). 'Hence if a median longitudinal 
section of the haustorium be made, in such a plane that a longitudinal section 
of the parasitic mother root is obtained, a transverse section of the attached 
host root will be made. Conversely, if a median longitudinal section of the 
haustorium be made in such a plane that a transverse section of the mother 
root is obtained, a longitudinal section of the host root will result. Always 
bearing in mind the orientation of the host root with regard to the parasitic 
root, and with the help of a few diagrams it is a simple matter to understand 
the internal structure of the haustorium. Fig. 3, A, represents a median 
longitudinal section of the haustorium taken in such a plane that a 
longitudinal section of the mother root and transverse section of the host 
root are obtained. The haustorium is seen to consist of a central mass of 
small -celled tissue 71. c. Adjacent to the vascular strand v.s. of the mother 
root is the plate of tracheidal cells and running directly from this to 
1 Barber, C. A., 1 . c. ; Fraysse, A.: Contribution a la biologie des plantes phanerogames 
parasites. Montpellier : Societe Anonyme de Tlmprimerie Generate du Midi, serie A, No. 515, 1906 ; 
Benson, Margaret, 1 . c. ; Stephens, E. L., 1 . c. 
