PLANT TISSUES 129 
nucleus; starch grains are frequently but not always present 
and sometimes mucilage, tannin and crystals occur. Its 
functions seem to be to give protection to the stele (tissues within 
it), to reduce permeability between primary cortex and stele 
Py. 
Fic. 70.—Transverse section of the outer regions of the stem of the elder (Sam- 
bucus canadensis), showing cork, cork cambium, etc. (From Strasburger, Macmillan 
Co.) 
and to serve as an air-dam, preventing the clogging with air of 
the water-conducting elements. 
Cork 
Cork or suberous tissue is a protective tissue composed of 
cells of tabular shape, whose walls possess a layer of water-proof 
substance called suberin. It is formed by the division of the 
cells of the phellogen or cork cambium which cuts off cells out- 
wardly. Its cells, when first formed, contain cytoplasm and 
nucleus but, as the cork cambium forms new layers of cork on 
its exterior, the older cork cells lose their protoplasmic contents, 
die, become filled with air or a yellowish or brownish substance, 
and gradually sluff off. Under the microscope, dead cork cells 
usually appear black due to air filling their cavities. Thin 
sections of cork, when mounted in suitable reagents, will show 
