Anatomy of Teratological Seedlings. /. 
5i9 
have ultimately shown a diarch structure. Finally, there are those forms in 
which triarchy is established and persistent throughout; these, which include 
more than half the total number of tricotyls examined, forming the culmina¬ 
tion of the series (Fig. 11). 
One further point of interest must be noted. In one instance reduction 
from the triarch to the diarch condition was brought about not by the 
fusion of two xylem plates and the disappearance of the intervening phloem, 
but by the disappearance of a xylem plate, and the subsequent fusion of 
two phloem groups. The protoxylem was the first to disappear, followed 
later by the metaxylem. The possible significance of this interesting 
difference in the mode of reduction will be dealt with later. 
C 
Fig. 11. a-c. Tricotyl showing persistent triarchy. 
Fig. 12. a-d. Tricotyl, Type a. 
In the tricotylous as in the hemitricotylous seedlings morphological 
characters gave no clue to the behaviour of the vascular strands. In some 
cases thet hree cotyledons were of equal size, whilst in others two cotyle¬ 
dons were considerably smaller than the third ; both conditions, however, 
being found to be associated with persistent triarchy. A feature which gave 
a much more reliable indication was found in the slightly earlier fusion of 
two of the cotyledons, as where this precocious union occurred it was 
usually followed by the ultimate union of the associated xylem poles. 
The double bundles themselves showed a considerable variation in the 
degree of separation of the metaxylem elements of the triad, this being in 
some cases very pronounced, whilst in other cases the phloem only was 
double and the bundle appeared to have undergone lateral compression. 
No relationship could be traced, however, between the degree of compres¬ 
sion and the ultimate fate of the vascular bundles. 
Usually the young epicotyledonary leaves formed a trimerous whorl 
