Seedling Structure of certain Centrospermae . 
i93 
may give off branches which join on to the adjacent bundles. Eventually 
the plumular strands d. 1 and d . 1 join on to one of the bundles situated on 
their flanks (Diagram 6, Fig. 9), and a definite rotation of the protoxylems 
of the bundles b takes place (Diagram 6, Fig. 10) ; thus there is organized 
a tetrarch root-structure (Diagram 6 , Fig. 11), which, however, in a very 
short vertical distance downwards gives place to a diarch arrangement. 
The phloem and metaxylem of b . 1 and b. 3, and b . 2 and b. 4, effect 
a junction and enclose the proto- 
xylems which were derived from 
these same bundles (Diagram 6, 
Figs. 11 and 13). These proto- 
xylems die out and thus there 
supervenes a diarch root-structure, 
the groups of tracheae c. 1 and c. 2, 
which have gradually been in- 
creasing in number, forming the 
permanent protoxylems. 
Mirabilis longijlora , L. The 
seedlings of this plant (Fig. 7) are 
large and have prominent epigeal 
cotyledons, often unequal in size, 
with long petioles inserted on a 
rather massive hypocotyl, at the 
base of which there is a conspicuous 
Peg (/•). 
The essential features of the 
transition phenomena resemble 
those of Allionia albida ; but the 
structure of the hypocotyl is ex- 
ceedingly complicated and is illus- 
trated in Diagram 7. 
The vascular changes in the 
cotyledons are so like those of 
Allionia that no further description 
is necessary (see Diagram 6, Figs. 
i-6). At the top of the hypocotyl, as in Allionia , there obtain two isolated 
groups of protoxylem-elements — four large vascular bundles ( c . 1, c. 2, c. 3, 
and c. 4) derived from the cotyledons, and two masses of plumular vascular 
tissue (p). Tracing these bundles downwards, the plumular vascular tissue 
passes outwards between the strands c. 1 and c. 3, and c. % and c. 4, deploys on 
either side, and breaks up into isolated vascular strands (/), which sometimes 
consist only of phloem (Diagram 7, Figs. 2, 3, and 4). Concurrently the 
Fig. 7. Mirabilis longijlora. Natural size. 
