CELLS AND TISSUES X5 



a narrow canal which may be occupied in part by products resulting 

 from the disintegration of the protoplast. 



There are intermediate types, representing intergrades between 

 parenchyma and sclerenchyma, in which the elements have rela- 

 tively thick walls, simple pits, and retain their protoplasts. Such 

 forms are designated as substitute fibers. Another intergrading form 

 is the fiber tracheid, which exhibits some of the characteristics of 

 the tracheid but functions primarily as a mechanical element. 



Stone Cells. — A second type of sclerenchyma is the stone cell, 

 which is frequently short and approximately isodiametric but may 

 vary greatly in shape. The stone cells occur in compact or loosely 

 organized groups as in the fleshy parenchyma of the pear, Pyrus 

 communis. They are also common in the hard outer coats of seeds 

 and fruits, as well as in the bark and pericyclic regions of woody 

 stems. There is no sharp distinction between the fiber and the 

 stone cell, and intermediate types of all sizes occur, so that it would 

 be possible to arrange a complete series of sclerenchymatous ele- 

 ments ranging from the isodiametric to the elongated form. The 

 wall of the stone cell is thick, highly lignified, and may be much 

 stratified with clearly distinguishable lamellae. Canals, which 

 are usually branched, occur in the walls, and the continued addition 

 of materials thickens them until, finally, the lumen becomes very 

 small and may be almost completely occluded. 



CONDUCTIVE TISSUES 



The conductive elements of the vascular plant can be divided 

 into two principal categories, xylem and phloem, each of which is a 

 complex tissue consisting of an aggregate of four or more cell 

 types. There is also much general conduction through all paren- 

 chymatous tissues, especially that of the ray. 



The Xylem. — The xylem may include tracheids, vessels (tracheae^, 

 fibers, and parenchyma; and frequently contains ducts or glands of 

 various types. Developmentally, it is classified as primary xylem, 

 derived directly from the primary meristem; and secondary xylem, 

 differentiated from the cambium. The primary xylem is further 

 divided into the protoxylem, which is the first to differentiate and 

 mature at a given locus, and the metaxylem, which usually matures 

 later. (Figs. 8, 9.) 



The spatial relationship of the proto- and metaxylem is impor- 

 tant in anatomical diagnosis; and, in the development of the pri- 



