Shaw. —The Seedling Structure of Araucaria Bidwillii . 327 
to fuse with that of an adjacent bundle. The secondary xylem and the 
phloem of the root is linearly continuous with the secondary xylem and 
phloem of the bundles in the hypocotyl. 
The reduction of the root to the diarch type ultimately takes place 
whether the original structure was pentarch, hexarch, or heptarch. In some 
specimens, however, the youngest portions of the root still showed triarch 
structure, the diarch stage not having yet been reached. Had the plants 
been older the diarch appearance would doubtless have been shown. 
Secondary roots are always diarch, and derive their primary vascular 
tissue from a single primary xylem group lying between two masses of 
secondary xylem. This was particularly clear in one specimen examined, 
in which the main tap-root had decayed away ; the base of the hypocotyl, 
therefore, terminated in a blunt projection, on one side of which a thin 
adventitious root arose. This root was diarch ; a transverse section of the 
hypocotyl, however, showed pentarch structure. On tracing the adventitious 
root into the hypocotyl, it was found that it arose from a single primary 
xylem group lying between two bundles ; had each primary xylem group 
given rise to an adventitious root, there would have been five roots in a ring 
at the base of the withered tap-root. 
An Anomalous Seedling. 
One specimen in the material first attracted attention from its abnormal 
external appearance, a variation which was found to be accompanied by even 
stranger diversities in the vascular tissue. The seedling in question 
(Diagram IV, Fig. 9) had a longer, more tuberous hypocotyl than usual ; 
at the base of the swollen portion of the hypocotyl the seedling took a sharp 
bend, running at right angles to its former course for some 3 cm. ; it then 
became sharply constricted and continued as a very thin root. 
The vascular structure of the seedling was normal down to a point (p.) 
about half way along the horizontal portion of the root, here hexarch 
in structure, derived from six pairs of bundles in the hypocotyl. A section 
taken at the constriction (d.) in the root, about 1-5 cm. lower down, showed 
diarch structure ; the transition from hexarch to diarch in the intervening 
portion (t.) was marked by some curious abnormalities. 
The hexarch stele passes quickly into pentarch in the normal way ; 
during this process a series of cell-divisions takes place in the pith. The 
divisions occur tangentially in a zone forming a closed circle ; they give rise 
to a circular cambium (c.) in the pith, and this cambium produces xylem 
externally and phloem internally. Thus a ring of vascular tissue with 
inverted orientation is formed inside the root (Diagram V, Fig. 1). While 
this is proceeding, changes take place in the five primary xylem groups ; 
each splits up into several smaller groups which return to the positions 
which they formerly occupied in the hypocotyl. That is to say, they 
