PLANT TISSUES 131 
in deeper regions of the bark and form cork or periderm. (See 
Fig. 131.) 
BorkE is a term which has been applied by German botanists 
to designate the dead tissues which sluff off when cork cambium 
arises and forms layers of cork in deep regions of the cortex, 
pericycle and phloem. ‘This occurs in the oak, birch, grape 
vine, elm, viburnum, hickory, cherries, etc. 
PERICYCLE 
The PericycLe or PERICAMBIUM, as it was formerly called, 
is a derivative of the ground meristem. It is a narrow cylinder of: 
tissue which. lies between the endodermis and the outer margin 
of the primary phloem of the fibrovascular bundles of stems or 
between the endodermis and the radial fibrovascular bundle of 
roots. In many stems it is composed entirely of thin-walled 
parenchyma cells but in others, whether woody or herbaceous 
and notably climbing or twining stems, it is composed of two 
kinds of tissues, thin-walled parenchyma and sclerenchyma. 
Frequently stone cells as well as sclerenchyma fibers occur in 
this region, as in stems of Cinnamon, etc. The sclerenchyma 
fibers occurring in the pericycle are called “‘pericyclic fibers.” 
Normally these fibers are lignified but may consist of cellulose, 
as in Flax and Mezereon stems. The parenchyma cells 
sometimes contain chloroplasts and function in the slow con- 
duction of sap and the storage of starch. The sclerenchyma 
fibers give strength. In roots, the pericycle consists of one or 
two rows of thin-walled cells which undergo division in places 
to form meristems which give rise to lateral roots. In roots of 
primary growth of Exogens the pericycle divides into two layers, 
the outer layer becoming a phellogen or cork cambium. The 
pericycle also may give rise to meristems forming adventitious 
roots and stems. 
LATICIFEROUS ‘TISSUE 
This form of tissue comprises either /atex cells, latictferous 
vessels, or coenocytic latex cells, differing from each other in 
origin and method of development. (1) Latex Cexts (latex 
sacs) are uninucleate cells which take their origin from minute 
