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RAFFLESIA MANILLANA. 217 
the xylem and phloem. The growth of the embryonic flower 
shoot does not, at first, cause any distortion of the tissues of the 
host, which is probably due in part to the absorption of the lat- 
ter by the parasite. Soon, however, the embryonic shoot begins 
to grow more actively in the region toward the phloem, so that 
the rounded mass of cells becomes top-shaped (fig. 18). This 
is accompanied by a tendency of the xylem rays to be spread 
apart around the developing shoot, which process may take place 
to some extent even before the parasite begins to be top-shaped. 
The change in the shape of the parasite may take place simply 
by a more rapid enlargement of the portion in the phloem of 
the host. The top-shaped appearance is, however, frequently 
preceded by an irregular outgrowth of the parasite into the bark. 
This is usually the case when the first proliferation takes place 
almost entirely in the woody cylinder. At the same time that 
the xylem rays are becoming spread apart, the bark is pushed 
up in a dome-shaped projection (figs. 5, 8-11-13). 
The presence of the developing parasite in the tissues of the 
host causes an increased activity of the cambium of the host in 
the region around the parasite. This, together with the tend- 
ency of the parasite to cause the spreading out of the xylem rays, 
results in the production of a vase-shaped mass of xylem which 
projects from the side of the root, and in which the base of the 
parasite is embedded (figs. 21, 22). This vase-shaped mass be- 
comes more prominent as development proceeds. The increased 
production of xylem is accompanied by a similar increase in the 
phloem. 
The presence of the developing shoot not only causes a greater 
activity in the cambium, but there is also frequently a tendency 
for the host to develop a phellogen-like layer around the parasite. 
When this layer is produced it is usually formed first in the bark 
and then later may extend completely around the embryonic 
shoot. The phellogen-like layer, when once formed, gives rise to 
cells which soon become empty and have an appearance entirely 
similar to cork. Whenever these cells completely surround 
the parasite the latter is cut off from its food supply and con- 
sequently dies. The time at which the phellogen-like layer is 
produced, as well as the extent to which it is developed, varies 
very greatly in different specimens. It would seem that in some 
cases it does not make its appearance at all, but that in the major- 
ity of cases the embryonic shoots are killed by its growth. Usu- 
ally the shoots are surrounded and killed before they are large 
enough to cause any appreciable distortion in the tissues of the 
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