56 Scott and Brehner . — On the Secondary 
elements of the secondary c wood ’ do not necessarily fit on, 
in any way, to the cambial cells bordering on them externally. 
From the irregular character of this inner secondary zone, 
and the marked absence of any definite initial layer, one 
might be tempted to doubt whether this tissue can properly 
be called secondary, and whether it may not simply form the 
completion of the primary cylinder. The long interval, in 
normal cases, before the formation of the zone in question 
begins, an interval during which the primary cylinder has 
become fully differentiated, negatives any such idea, which 
is also inconsistent with the fact that the bundles of this zone 
are cauline, and only indirectly connected with the leaf- 
traces. 
The development of the inner secondary zone appears to 
go on slowly. In a piece of stem io cm. long it was just 
commencing at the top, and just completed at the bottom. 
Hence the completion of this zone must have taken place at 
a distance of about 1 5 cm. from the apex. (Cf. p. 55.) It is 
possible that this may correspond to one year’s growth, but of 
this we could obtain no evidence. 
Not till the inner zone of thickening is nearly completed 
does the cambium extend round the ends of the cylinder (in 
transverse section); consequently there is never more than 
a thin layer of tissue belonging to this zone at these points. 
Its maximum thickness is at the middle of the broader sides 
of the stem (see Fig. 10). 
When the transition to the outer secondary zone takes place 
the cambial divisions become more regular, and we find 
longer continuous radial series of cells. Henceforward the 
development goes on quite normally, and so far as we could 
tell the same cambium is active throughout. The normal 
process of secondary thickening is now established, and con- 
tinues year after year. It is not, however, until after several 
series of the distinct secondary bundles have been formed, 
that any appreciable amount of secondary cortex begins to 
be developed. 
As regards the details of development of the secondary 
