218 
BROWN. 
host, but in other cases death does not occur until the swelling 
on the side of the root is a centimeter or more in diameter. To 
the host the advantage of the process just described seems to be 
evident, for it must cause a great reduction in the number of 
flowers produced by the parasite. It may also be of advantage, 
indirectly, to the parasite for it probably saves the host from be- 
coming exhausted and allows the flowers of the parasite to be 
produced over an extended period. Why the phellogen-like layer 
should be produced in some cases and not in others is not evident, 
but this is probably connected with the relative vigor of host 
and parasite. 
As the shoot of Rafflesia develops it would soon burst through 
the bark of the host if considerable growth did not take place 
in the latter. The shoot, however, does not break through until 
a swelling, about 3 cm in diameter, has been formed on the side 
of the root (flg. 6). If the shoot is formed in a small root, the 
diameter of the swelling may be much greater than that of the 
root (flgs. 5, 6). The growth of the bark is accomplished by 
the division of cells throughout that part pushed up by the 
parasite. These cell divisions sometimes take the form of cam- 
bium-like layers. The result is that the cells of the bark are 
arranged in rows running toward the apex of the swelling 
(figs. 19, 21, 22). There is, however, considerable irregularity 
in this arrangement, as the divisions take place in a number of 
places, and the direction in which the phloem is cut off from 
the true cambium is constantly changing due to the continual 
spreading of the xylem. When the parasite is small, the phloem 
cells, formed from the cambium around it, are cut off in a plane 
parallel to the longitudinal axis of the root (fig. 19) ; but as the 
excessive growth and spreading apart of the xylem rays con- 
tinues, the phloem cells come to be cut off in a plane which 
approaches more and more toward being perpendicular to this 
(fig. 3). 
As mentioned before, the most rapidly developing part of the 
embryonic shoot is the rounded portion in the bark. The en- 
largement of this soon carries the region of most active growth 
of the parasite beyond the cambium of the host (figs. 18, 19). 
As the parasite continues to grow it becomes oval in longitudi- 
nal sections, with a pointed base (fig. 19). At this stage the 
vascular bundles are differentiated in a circular arrangement 
in the outer part of the parasite. These are shown as dark 
lines in figure 19. A cambium layer is formed on the outer 
side of each bundle. During the early stages there is also a 
