80 THE ELEMENTS OF VEGETABLE HISTOLOGY 



together with the associated ducts, is termed a xylem 

 bundle, and these bundles may be arranged in a definite 

 order in the plerom region. Lignification of cell walls 

 also occurs in groups of plerom cells other than those 

 of the xylem bundles; but the conducting elements 

 developed in these other lignified groups differ from 

 the ducts in that they are not continuous. They are 

 termed sieve tubes, because communication between 

 the various cells is through perforated walls. Each 

 group of sieve cells, with its associated lignified tissue, 

 is termed a phloem bundle, and these, like the xylem 

 bundles, may be arranged in a definite order. The 

 lignified cells of a xylem bundle are termed wood 

 fibers, while those in the phloem bundles are termed 

 bast fibers. As each xylem or phloem bundle consists 

 of a fibrous element and a vascular or conducting 

 element, the term fibro-vascular bundle is applied to 

 the combination. The xylem and phloem bundles 

 are placed in a circle near the outer boundary' of the 

 plerom region, and in the first stages of development 

 they usually alternate with each other. Narrow 

 strips of unchanged plerom parenchyma extending 

 between the fibro-vascular bundles form the primary 

 medullary rays. The unmodified parenchyma in the 

 center of the plerom is the pith. The arrangement 

 of the various primary root structures is diagram- 

 matically represented in Plate 30. 



Secondary Root Tissues. — In certain orders of 

 plants the primary tissues persist with but minor 

 changes throughout life. In the higher orders many 

 changes occur in these primary tissues, resulting in 

 the formation of secondary or permanent tissues, 

 such as those of the bark, and an extension of the 



