founded on the Structure of their Seedlings. 55 
for about two-thirds of its length downwards from the inser- 
tion of the blades. In the lower part there is a small central 
cavity, not very clearly defined in transverse section, which 
gradually opens out as it descends, and so forms the conical 
chamber enclosing the stem-bud. 
Two opposite bundles, one from each cotyledon, run the 
whole length of the cylinder, and each remains single until 
it reaches the level of the plumular growing-point. Here 
both begin to open out into a double structure. The activity 
of the cambium within them is already marked : at this level 
in a seedling so young that the first leaf is a mere rudiment in 
which the midrib is just indicated, two rows of radial unligni- 
fied xylem elements are found in each trace inside the cambial 
zone. 
As in following the series of sections from this seedling 
we approach the first node, two unlignifled plumular traces 
appear opposite each other between the two traces from 
the cotyledon. The stele is elliptical as in B, Diagram IX. 
A well-marked cambium is present in all the traces, and also 
between them. It forms a complete ellipse enclosing all the 
xylem. The formation of secondary tissue has already begun 
within the traces, where we find two or more rows of unligni- 
fied xylem elements outside the primary xylem which is also 
as yet unlignified. 
The formation of secondary tissue reaches its maximum 
a little lower down, where in this young seedling the primary 
elements belonging to the plumular traces have almost dis- 
appeared. The four cotyledonary phloem groups are 
approaching each other in pairs. They are separated from 
the well-lignified primary xylem of the cotyledon by a con- 
siderable bulk of secondary tissue. The secondary formations 
are best developed in those segments of the ellipse which were 
occupied by plumular traces : they are thinnest outside the 
two groups of cotyledonary xylem, in which the protoxylem 
is already external. 
The diarch root-stele is formed as in Diagram IX. The 
activity of the cambium decreases in the lower region of 
