ANATOMY OF TISSUES. 1338 
the formation of new cells. This extension is entirely insuf- 
ficient to keep pace with the increasing growth of ordinary 
woody stems. For all such another plan is followed which 
results in what is called periderm formation. 
The word periderm, is used to denote all that growth which 
is generated by a secondary meristem arising somewhere in the 
rind tissues. According to the position of this meristem, there 
are two kinds of periderm, superficial and deep-seated. On 
many stems of limited growth the superficial periderm is suffi- 
cient to supply the necessary extension, and also furnish a better 
means of protection than that given by the primary epidermis. 
In such stems no deep-seated periderm is formed. Where the 
woody growth is protected many years, the superficial periderm 
is deficient in the same manner as was the original epidermis, 
and therefore its place is supplied by the deep-seated. 
There are three kinds of superficial periderm, the distinction 
here being based upon the place of the meristematic cells. 
These consist always of a single row of cells, as seen on the 
cross-section. In the superficial periderm they extend entirely 
around the stem thus forming a mantle or single-celled layer, 
unbroken save for the breathing spaces which will be described 
later. To this layer is given the name phellogen, and it arises 
either in the epidermal layer, in the second or third layer under 
this, or still deeper in the phloem of the vascular bundles. There 
are very few instances of the first class! the second being the 
most common. As there is but a single layer of phellogen cells, 
their manner of division is more easily ascertained than in case 
of the cambium cells. In general, it may be either centripetal, 
centrifugal, or reciprocal ; that is, the lasting cells may be cut 
off toward the centre, toward the circumference, or in both direc- 
tions alternately. The cells cut off lie in regular radial rows; 
those external to the phellogen layer are somewhat plate-formed, 
their radial diameter remaining short. Their walls soon become 
1 Nerium Oleander may be used to illustrate. 
