Cuscuta and its Host . 667 
the tip of a hypha making its way straight through the wall of a host sieve 
tube ; the inner and outer layers of its wall both stain blue. 
In two of the elements shown in Fig. 57, and probably also In the left- 
hand element in Fig. 65, the blue stain is taken by the entire thickness of 
the much swollen wall at the tip of the hypha. 
When the hyphae have penetrated well into the functional phloem 
these swollen blue-staining tips are very rarely met with, though occasion¬ 
ally irregular masses of blue mucilaginous-looking substance are found, 
apparently in the cavity of the hyphae (see Figs. 62 and 67). 
Formation of junctions between the tips of the hyphae and the sieve 
areas of the host. It is by no means an easy matter to ascertain exactly 
what does occur when the tip of a hypha forms a junction with a sieve area 
of the host. The accumulated mass of evidence seems, however, to demon¬ 
strate conclusively that the formation of a junction is effected by the applica¬ 
tion of the naked protoplasm of the hyphal tip to a functional sieve plate or 
sieve field of the host . The swollen and mucilaginous parasite wall is 
entirely dissolved away in the region immediately over the sieve area. 
Before proceeding to give the evidence on which this conclusion Is 
based it may be well to state that it is opposed to that reached by Peirce 
and Strasburger. Peirce 1 demonstrated the fact that the haustorium forms 
junctions with the phloem as well as the xylem of the host; having described 
the sieve plates between host and parasite, and having seen callus on either 
side of their plates, he stated his view that half of the 6 compound ’ sieve 
plate is derived from the host wall and half from that of the parasite. 
Strasburger 2 came to a similar conclusion. He could not demonstrate any 
simple protoplasmic connexions between other cells of the host and parasite, 
a fact which he attributes to absence of near relationship between the two; 
he states, however, that open communication is established In the pores of 
the compound sieve plates, and he enumerates the facts among other 
evidence possibly in favour of his view that protoplasmic communication 
may be established between non-genetically connected tissues. 
When the junction sieve areas are carefully examined by suitable 
methods, the most obvious fact concerning them is that they resemble exactly 
in appearance and dimensions , and in every detail , the lateral sieve plates or 
sieve fields already described in the respective hosts, Salvia and Vitis. The 
junction sieve areas are certainly no thicker than those in the undisturbed 
host phloem, as they would almost necessarily be if they owed their origin 
to a fusion between the host sieve area and the thick swollen wall of the tip 
of the parasite hypha. The presumption is therefore that these are not 
1 Peirce, 1893, p. 301 and p. 314, Fig. 13, PI. XIV. The account given by Peirce of his 
observations and the figures given by him are by no means conclusive. This is partly due to his 
researches having been conducted on alcoholic material. His figures by no means preclude the 
possibility that in the formation of the junction sieve plate, one or other of the host and parasite 
walls is absorbed. 2 Strasburger, 1901, pp. 601-2. 
