302 Peirce. — On the Str tic hire of the Haustoria 
a considerable area, careful examination shows that the wall 
is stained on both sides. Thin sections, careful staining, 
decolorizing, and clearing, and high magnification are neces- 
sary to demonstrate this ; yet with these precautions the 
results are positive. The companion-cells of the haustorial 
sieve-tubes unite directly with the companion-cells of the 
sieve-tubes of the foster-plant, as is to be expected from the 
direct union of the sieve-cells. 
We see then that certain haustorial cells abut directly upon 
sieve-tubes in the host, and that callus-plates are formed 
between these. These haustorial cells are therefore sieve- 
cells. In a transverse section stained as above described, one 
sees that these sieve-cells of the haustorium which unite with 
the sieve-tubes of the host are the terminal elements of sieve- 
tubes which run through the haustorium to a stem-bundle of 
the parasite (see Fig. 7, Plate XIII). Thus, as the haustorial 
xylem connects a xylem of the host with two xylems in the 
stem of the parasite, so the haustorial phloem connects 
a phloem of the host with a phloem in the stem of the parasite. 
As has already been shown on pages 298-299, the central 
cylinder of the haustorium is made up of bi-collateral fibro- 
vascular bundles, at first separate, later confluent, consisting of 
one xylem and two phloem-groups. When the proximity of 
the bundles of the host makes it possible for the haustorium 
to apply itself to two, as is generally the case in one of the 
Mimoseae, both phloem-groups of the haustorium become 
active. In such cases the single strand of xylem divides into 
two near the tip, each of which, accompanied by phloem, 
unites with one of the host bundles, xylem with xylem, 
phloem with phloem (see Fig. 6, PL XIII). Then the shaft 
of the haustorium consists for most of its length of bi-collateral 
bundles, but near the tip of twice as many collateral 
bundles. In the Apocynaceous host, as previously described 
(see Fig. 8, PI. XIII), the haustorium usually unites with 
three bundles, thus demanding a division of the phloem and 
a second division of the xylem. Since the third bundle with 
which the haustorium unites is small, and considerably 
