195 
Seedling Structure of certain Centrospermae . 
placed xylem may be so extensive as to make them almost concentric 
(Diagram 7, Figs. 6 and 7), a tendency shown also by the bundles of 
the stem. 1 
While these changes have been taking place the plumular strands have 
not been idle ; p. 1 divides into two, p. 2 and p. 3, but only to rejoin (p. 10, 
Diagram 7, Fig. 6). Also, the strands p. 4 and p . 6 (Diagram 7, Fig. 5) 
branch into two ; the two branches of the former recombine (Diagram 7, 
Figs. 6 and 7), but not so the latter (p. 8 and p. 9). The changes at lower 
levels are much less complex ; the large strand c. 18 gives off a slender 
lateral branch (Diagram 7, Fig. 6) which fuses on to p. 7 and p. 9, these 
three forming the larger bundle,/. 11 (Diagram 7, Fig. 7). Further, there 
is a general clearing up of the smaller plumular traces ; p. 10 joins on to 
c. 17, similarly p. 8 unites with c. 18, the small strand on the right side of 
c. 10 joins with this bundle, and the xylem of the similar strand just below 
the right-hand protoxylem joins on to the protoxylem, whilst its phloem 
comes to an end (Fig. 7). Finally, the abnormally placed xylem elements 
in c. 17 and c. 18 creep inwards and join on to the normal wood (Diagram 7, 
Fig. 8). Thus there is formed a ring of collateral bundles, some of which 
consist mostly of cotyledonary vascular tissue and two, p. 4 and p. 11, 
mostly of plumular vascular tissue, from which the root-structure is 
organized. Of the root-structure, two xylem poles, those in the plane of 
the cotyledons, are already in position ; the other two are formed in this 
wise: the two large bundles, c. 17 and c. 18 (Diagram 7, Fig. 8), divide, 
each into two ; the halves do not show any rotation comparable to what 
obtains in so many of the foregoing plants, but move apart, leaving the 
protoxylem isolated (Diagram 7, Fig. 9). There is then a general centri- 
petal displacement, so that the bundles intervening between the four groups 
of protoxylem-elements fuse to form four large strands, and thus a tetrarch 
root-structure is organized (Diagram 7, Fig. 11). 
Leaving out of consideration the extraordinary series of anastomoses, 
it will be seen that the type of transition is the same as in Allionia. In 
brief, two protoxylems are isolated in the seed-leaves themselves, the coty- 
ledonary traces enter the axis and form four large bundles (c. i-c. 4), the 
corresponding pairs of which give origin to the two protoxylem poles 
situated in the intercotyledonary plane. 
The main differences between the two plants are these : whereas in 
Allionia the tetrarch arrangement gives place to a diarch root-structure, in 
Mirabilis the tetrarch structure, so far as our observations show, persists ; 
but this is not constant for the genus, for in M. divaricata reduction . to 
diarchy takes place, and the same also applies to M. multiflora. Further, 
in Allionia the anastomoses between the bundles in the hypocotyl, although 
1 Solereder: Systematic Anatomy. Oxford, vol. ii, 1908, p. 647. 
