ANATOMY OF TISSUES. OT 
This ring is formed in different ways, according to the pecu- 
liarity of the different plants where it occurs. In general it 
may be said to be formed either by the intercalation of new 
bundles, or by the formation of interfascicular cambium. The 
new bundles are either leaf-traces or they belong to the stem 
alone. New leaf-trace bundles are introduced when the arrange- 
ment of leaves on the stem changes so that the upper part bears 
more leaves than the lower. Both leaf-trace and stem bundles 
are so introduced that their respective parts exactly correspond 
with those of the older ones. The cambium of-the old and new 
bundles does not, however, form an unbroken circle as there 
are, in all cases, a few of the ground cells left between the 
bundles, which cells become parenchymatic meristem, thus com- 
pleting the circle of meristem cells; though the cambium is 
interrupted by as many of the parenchymatic meristem groups 
as there are bundles in the circle. 
These groups give rise to the 
primary medullary rays as they 
cut off cells in both directions, 
outward and inward, keeping pace 
with the growth of the cambium 
cells. On the cross-section they 
appear very similar to those of the 
cambium, and it is only on the 
long section that their real char- 
Fia. 54. 
acter is made evident. The pri- 
Diagrammatic cross-section through a 
mary medullary rays thus formed — young dicotyledonous stem. 0 epi- 
: : Tis dermis. p primary rind. m primary 
extend through the bundle cylinder J egunary ray. be h one of the five 
from rind to pith, and serve as vascular bundles. b& phloem. c¢ cam- 
A bium. hk xylem. P pith. — (Wiesner.) 
conductors of cell contents. Their 
walls remain thin and are often furnished with simple pores. 
A good example of this type is furnished by different species 
of Salix. (Fig. 54.) 
Euonymus and Berberis stems are examples of the other 
