1908] THIESSEN—DIOON EDULE 373 
way vertically through the parenchymatous cells from one bundle to 
another. They are in all senses true reticulated vessels, without 
cross-walls, establishing a connection between the bundles of the vascu- 
lar system. 
Discussion 
In the embryo of Dioon edule the vascular cylinder is a protostele, 
which in.some specimens contains a solid xylem mass. From this 
solid cylinder all gradations are found to the siphonostele. The cells 
of the pith are actively meristematic, as shown by the mitotic figures, 
and often in older specimens xylem elements are found in the central 
Part at the level of the vascular plate. The xylem cylinder also con- 
tinually increases in size, new elements being added to it constantly 
between bundles already existing, as well as by the cambium. 
It should be emphasized that the vascular cylinder in the embryo 
and seedling does not consist of the short xylem cylinder only, but is 
continuous in a tissue, very different from the cortex outside and the 
pith within, which gives rise to strands of procambial tissue running 
into the leaf primordia. The pith is also a well-defined tissue from 
the vascular plate to the stem tip, and nowhere suggests that it arises 
6M an intrusion of the cortex through the leaf gaps. An inspection 
of fig. 8 shows that it has its beginning at the very tip of the growing 
Point, where it is seen to consist of a single row of cells or a tier of a 
few cells, gradually expanding as the stem grows into the large pith 
found in the older part. 
The transition of the protoxylem from the endarch to the exarch 
Position was first described by Metrentus (4), who also suggested the 
descriptive terms centripetal and centrifugal xylem. The situation 
"as interpreted by BERTRAND and RENAULT (§), who also established 
that the centripetal wood is in the same relative position throughout 
Its whole length; that it increases in bulk toward its upper end; and 
that the Centrifugal elements are reduced more and more. They say 
that the centripetal wood is intercalated between the pole and the 
Suter face. It is regarded as the primary wood (developed from the 
Precambium), while the centrifugal wood is regarded as secondary 
(developed from the cambium). 
7 present investigation shows that this statement needs modi- 
‘on. While all centripetal wood is primary, all primary wood is 
