48 ELEMENTS OF PLANT ANATOMY. 
of meristem cells. Such a change may take place in the tissue 
lying between the bundles, and it may occur in one of two ways. 
Either new cambium groups form between the old, and develop 
into what are known as interfascicular bundles, or a process takes 
place by which a ring of meristematic tissue is made to extend 
around the axis. 
This ring, generally referred to as the cambium ring, is 
characteristic of all dicotyledonous stems, also of the woody 
stems of gymnosperms, and in such cases as we have described 
above it originates in the following manner. We have already 
a circle of bundles whose central cells are composed of cambium 
forming three or four tangential rows. The ends of these rows 
lie in contact with certain parenchymatic cells of lasting tissue. 
These lasting tissue cells now grow in length or in the direction 
of the long axis of the stem, until they become as long as the 
cambium cells; they then assume the nature of meristem and 
divide by tangential walls in the same manner as do the cells 
of the original cambium. This change of parenchymatic lasting 
tissue into prosenchymatic meristem or cambium begins, as has 
been said, at the ends of the original cambium rows. Now sup- 
pose it to continue from group to group ; there would then arise 
a continuous ring of cambium around the axis of the stem. 
This actually takes place, except that midway between the 
bundles are left a few cells which do not change their shape into, 
long or cambium cells, but assume the function of meristem, 
retaining their parenchymatic form. The meristematic ring is 
then unbroken, but it cannot be called a continuous cambium 
ring. It is now easy to see how a continuous zone of new cells 
may be intercalated between the central pith and the outer rind, 
by the cambium cells giving rise to long cells lke those com- 
posing the bundles, and the few isodiametric meristem cells, to 
the so-called medullary rays. This description of the origin of 
the cambium ring is based upon Naegeli’s investigations, and 
there are known to be certain well authenticated instances 
