1386 ELEMENTS OF PLANT ANATOMY. 
cells originate by certain living parenchymatic cells of this 
phloem tissue taking on the power to divide or form new walls. 
These living cells do not consist of a connected mass of tissue 
as is the case with the rind cells where the primary phellogen 
originates, but are separated by clusters of bast cells, stone cells, 
sieve-tubes, etc. For this reason the phellogen cells of the inner 
periderm do not appear to form a continuous ring, but vary in 
their course according to the variations in the order of arrange- 
ment of these clusters. Notwithstanding this apparent irregu- 
larity the result accomplished is similar; thatis, the inner periderm 
separates from the inner portions of the stem a zone or hollow 
cylinder composed of various elements, instead of the simple 
eround tissue cut off by the superficial periderm. This zone or 
hollow cylinder comprises what is commonly called bark, namely 
all that portion of the stem outside the inner periderm. 
This shows the difficulty of making a sharp distinction 
between the two classes of periderm, superficial and deep-seated. 
In fact the third kind of superficial periderm, where the phel- 
logen originates in the phloem of the original bundles, some- 
times serves the purposes of both, and in such cases no other is 
formed. A good example of this is the beech (Fagus). Here 
the first periderm is the only one, and by its constant develop- 
ment it keeps pace with the cylinder growing within. 
In those plants where successive inner periderms are formed, 
if the first originates somewhat deep in the rind, the secondary 
periderms follow the course more or less closely. On the other 
hand, if the first originates near the circumference the secondary 
periderms follow successively in such a manner that taking a 
radially oblique course, they attach themselves to the older peri- 
derm, cutting off scale-shaped sections of bark between. 
These periderms have been referred to as cutting off or 
separating outer portions of bark with no reference to an actual - 
separation of the part cut off, except in the first instance where 
the primary epidermis is said to crack and become rubbed off. 
