48 Scott and Brebnem — On the Secondary 
bundles in their whole length are differentiated later, and 
have no protoxylem-elements 1 . 
The protoxylem has the usual spiral structure ; we did 
not determine whether its elements are vessels or tracheides. 
The later-formed xylem (which is alone present in most 
of the bundles) contains tracheides only, with recticulated 
or pitted walls. A certain amount of xylem-parenchyma 
is present among the tracheides. The phloem calls for no 
special description ; the elements bordering on the xylem 
are parenchymatous ; the central group consists of sieve- 
tubes and companion-cells. 
The ground-tissue of the cylinder consists of rather 
elongated parenchymatous cells. Those surrounding the 
bundles are thicker walled and often prosenchymatous. 
Towards the outside of the cylinder the whole ground-tissue 
assumes the latter character. 
The cortex is thin-walled throughout ; many of its cells 
contain tannin. 
2. Secondary Tissues. —If we next examine a transverse 
section through an older part of the stem, which has 
already assumed a cylindrical form, we find a very dif- 
ferent structure (see Fig. 10). In the middle part of the 
section we recognize the lenticular outline of the primary 
cylinder, which is unaltered. But superadded on this we 
find an entirely new zone of tissue. Its maximum radius 
is at right angles to the major axis of the primary 
cylinder. Hence the effect of the addition of the secondary 
zone has been to give a circular section to the entire 
vascular system. At the same time the transverse section 
of the stem as a whole also becomes circular ; this is assisted 
by the formation of periderm, which has been produced near 
the surface of the flat sides of the stem, but in a more in- 
ternal position opposite its prominent edges. At the stage 
figured in Fig. 10 the cortex outside the periderm still remains; 
later on it is thrown off altogether. 
1 Cf. Strasbnrger, Hist. Beitrage, III, p. 398; also Roseler, 1 . c. p. 295. 
