CELLS AND TISSUES 



15 



require separate consideration. In root axes, a very common devel- 

 opment is for the pericyclic cells to continue active division in 

 both radial and tangential planes so that the pericyclic zone 

 increases in width and maintains its continuity circumferentially as 

 secondary thickening occurs in the stele. The individual cells also 

 tend to enlarge tangentially and frequently serve for storage. In 

 stem axes, the pericyclic zone may also be broad; but, in the 

 absence of a clearly marked endodermis, its outer limits are often 

 difficult to define since the pericyclic cells may resemble struc- 

 turally those of the cortex or phloem. 



Other tissues that frequently develop from the pericycle are 

 the protective periderm and fibers which are described in succeed- 

 ing sections under their appropriate functional classifications. 

 Another important pericyclic function is its capacity to produce 

 lateral and adventitious roots and shoots. Other less common 

 potentialities include the production of secondary cambiums 

 (Beta), the development of oil ducts (Apium and Pastinaca), and 

 the formation of glands, lactiferous cells, and canals. 



PROTECTIVE AND ABSORPTIVE TISSUES 



The superficial protective or absorptive tissues of the plant 

 may be differentiated early in ontogeny, as is the epidermis; or 

 they may arise later in development, as in the formation of cork 

 tissue or phellem. 



The Epidermis. — The epidermis serves for protection, or, in 

 some regions, for absorption and secretion; and the entire outer 

 surface, with the exception of certain protected growing points, is 

 covered by a single layer of epidermal cells which is continuous 

 except where it is interrupted by stomatal or lenticular openings. 

 Although it is usually one layer in thickness, the cells may divide 

 periclinally to form a double or multiple layer as in Solanum tu- 

 berosum, where an ephemeral periderm develops from the epidermis 

 early in the ontogeny of the tuber. The epidermal cells may vary 

 greatly in size and shape; but they are typically flattened parallel 

 to the surface, and square or rectangular in transection with the 

 free surface exhibiting more or less convexity. In surface view, 

 they may be tabular or platelike; but frequently the walls are 

 undulating or sinuous, with lobes or projecting angles. 



The cells contain cytoplasm and a nucleus, the former usually 

 occupying the periphery of the cell surrounding a large central 



