312 Peirce— Oil the Structure of the Haustoria 
mother-plant, is applied at its distal end against the ring of 
wood made by the confluent xylem of the host. The contact 
of haustorial xylem-cells with the xylem-cells of the host is 
therefore by their tangential walls, and is direct. When the 
haustorium has penetrated an oldish branch or stem this is 
the permanent condition ; but if a younger part is attacked, 
one in which the xylem has attained to no great thickness, 
the cells at the apex of the haustorium remain thin-walled at 
their tips, continue their solvent action, and presently make 
their way through the zone of wood into the pith. No very 
wide opening is made through the wood, and comparatively 
few cells emerge into the pith. Such as do make their way 
into the pith are noticeably larger in diameter than those in 
the main body of the haustorium, are very much longer, and 
have thinner walls. They grow through the pith in all 
directions, some attacking the ring of wood in various places 
along its inner face, but never making much impression upon 
it ; most cease to grow before they have reached the wood. 
Some of the smaller cells of the sucker which make their way 
into the pith grow only for a short distance, and their tips 
become enlarged as previously described. When the ring of 
wood has been thus penetrated, the central xylem-strand of the 
haustorium bifurcates at its tip and its tracheids apply them- 
selves by their radial walls against the radial walls of the 
xylem-cells of the host, their thick and thin places corre- 
sponding with the thick and thin places of the ducts with 
which they come into contact. 
So much has already been shown by Koch. Does the 
haustorium of C. epilinum consist merely of this central 
strand of xylem enclosed by elongated parenchyma-cells the 
conducting power of which is slight, or are there sieve-tubes 
as in C. americana and C. glo 7 nerata ? Koch did not find 
any. In a cross-section of the host through a well-developed 
haustorium one finds on each side of the central xylem-strand 
one or two rows of cambiform cells, long and narrow (see 
Fig. 17, c). Beyond these are one or two rows of larger 
cells of equal or slightly greater length, resembling sieve-tubes 
