128 ELEMENTS OF PLANT ANATOMY. 
class, where growth in thickness arises from the increase in size 
of the original bundles, no new bundles being intercalated. This 
kind of growth we have referred to as ring formation by inter- 
fascicular cambium. To describe this, we may assume a cross- 
section so near the stem apex that the original bundles are yet 
distinct and at some distance from each other. The continuous 
ring of meristem cells is formed here by the cells lying on either 
side of each bundle and in contact with the cambium, changing 
or growing into cambium cells. Their number, reckoned radially, 
corresponds to that of the cambium cells. This change in form 
continues until only a few parenchymatic cells are left between 
the bundles. These in turn become meristematic but retain 
their parenchymatic form, and develop the primary medullary 
rays, in this case the same in number as the original bundles. 
A cross-section of a stem belonging to either of the above 
types may be described as consisting of three parts, a central 
circle of pith cells, followed by a zone of vascular tissue inter- 
rupted by the primary medullary rays, and succeeded by another 
of ground tissue cells. All that growth which takes place pre- 
vious to the completion of the cambium ring is called primary, 
and the outer zone or that lying outside the cambium is 
called the primary rind. When, on the completion and sub- 
sequent development of the ring, another entirely new zone is 
formed, it is called secondary or additional growth. This 
secondary zone always consists of two parts, an outer phloem, 
and an inner wood part. The name secondary rind is some- 
times applied to the phloem part of this secondary growth. In 
pharmaceutical as well as in common language, the term rind 
alone denotes all that part of the stem outside the cambium 
ring. | 
The separate elements of the secondary growth, except the 
medullary ray cells, have the same general character as those 
corresponding in the primary growth. The cambium cells 
themselves are long or prosenchymatic, with the long axis 
