1908] THIESSEN—DIOON EDULE 371 
a larger development of metaxylem; also secondary wood may have 
developed in the lower extremities, so that the transition from endarch 
to exarch has become more prominent and can be made out with 
greater clearness. The secondary wood accompanies the bundle as 
yet only for a short distance, and ceases long before the transition 
from endarch to exarch is complete. As the secondary xylem and 
centrifugal metaxylem diminish, the centripetal xylem (which of 
course is all primary) increases in bulk (fig. 9, a). Ina plant three 
years old no other secondary wood was present (figs. 22, 23). Even 
in the oldest leaves of quite old plants the secondary wood, which at 
the origin of the strand is quite massive, decreases very rapidly, and 
in the petiole just above the leaf base has thinned out to a few elements 
(ig. 24), remaining quite uniform to the rachis, where it disappears 
still more; while in the pinna no secondary wood whatever is present, 
all the xylem being primary and centripetal. Although in the transi- 
tion region the secondary wood diminishes in the same ratio in which 
the Primary wood increases, it must be noticed that the centrifugal 
Wood is not restricted to the secondary wood alone, as was shown in 
the younger bundles of the embryo, where the transition is clearly 
Carried out in the protoxylem and metaxylem alone. Thus in the 
seedlings the transition from centrifugal to centripetal wood is carried 
a after the appearance of secondary wood, and is completed in the 
Primary wood. 
In the older strands where secondary wood has been developed, a 
Considerable amount of the centrifugal wood therefore is metaxylem. 
This is shown by the amount of procambium that has been developed 
into centrifugal xylem; as may be seen by comparing the younger 
Strands in figs. ar, 4, a, for example, where there is a certain amount 
of centrifugal procambium, the amount depending upon the distance 
om the point of egress from the central cylinder, with the older 
Strands in fig, 24. Sometimes all of the centrifugal procambium 
aS become xylem; more often, however, patches of procambium 
T isolated cells of it are never lignified and retain their 
nuclei, and are then referred to as the thin-walled cells. These 
m-walled cells do not necessarily lie against the secondary 
Wood, though they most often do, and become most evident 
the upper extremities of the transition. A series of cross- 
