52 Scott and Brebner . — On the Secondary 
which have already assumed the character of permanent 
tissue. 
With this object in view we traced the development of 
the stem as shown in a number of transverse sections, made 
at measured distances from the apex. Several such series 
were examined, and the results checked by comparison with 
longitudinal sections. We will base our description in the 
first instance on a vigorous stem, in which we traced the 
differentiation of tissues from the apex downwards for a 
distance of 5’ 2 cm. Of course it will be understood that 
the absolute distances from the apex have no general value, 
and would come out very much smaller in less vigorous 
branches. 
At about 1 mm. from the apex nearly all the bundles 
of the cylinder are very oblique, the internodes not having 
lengthened much as yet. The leaf-trace bundles entering 
through the cortex are the most developed ; the larger of 
them have their protophloem and protoxylem already dif- 
ferentiated. The smaller leaf-trace bundles, however, even 
in the cortex, are still quite in the procambial 1 2 condition. 
In the cylinder too only the largest bundles (obviously 
belonging to the upper portions of the principal leaf-traces) 
show a differentiation of the first xylem- and phloem-elements; 
all the rest are procambial. There is no regular centrifugal 
order in the development of the different bundles in the 
cylinder. As however the smaller leaf-traces, and the lower 
ends of the principal ones, are limited to the outer part of 
the cylinder, it is here that we find the largest proportion 
of procambial strands, some of the outermost of which are 
in the very earliest stages of formation. In the outer zone 
of the cylinder active cell-division is in progress, especially 
towards the edges of the flat stem, and new bundles are being 
originated. 
1 We did not concern ourselves with the actual growing-point, the investigation 
of which has no bearing on our main question. 
2 We use procambium and primary desmogen as synonymous terms. Our use 
of the term secondary desmogen has been already explained (p. 22). 
