ROOT STRUCTURES 157 



type of bundle, found exclusively in roots of primary 

 structure. It must be noted that the concentric and 

 closed collateral types of fibro-vascular bundles are 

 present in the primary roots of monocotyledons, so 

 that while radial bundles indicate primary structure, 

 not all primary roots possess radial bundles. 



In powdered materials the ducts, wood fibers and 

 bast fibers are always seen in longitudinal view. 

 Sieve cells, being very delicate structures, are usually 

 disintegrated during powdering and therefore are 

 rarely apparent. The differentiation of wood fibers 

 from bast fibers in a powdered sample is usually dif- 

 ficult and sometimes impossible. As bast fibers are 

 generally longer and stronger than wood fibers, they 

 are less likely to be broken during grinding. The 

 lumen of a bast fiber is usually smaller than that of 

 a wood fiber. The pores in fibers may appear as pits, 

 or as slits extending diagonally in the wall of the 

 fiber. 



Pith Parenchyma. — This tissue represents the orig- 

 inal tissue of the plerom zone. The individual cells 

 are rather similar to the parenchyma cells of the cor- 

 tex but are apt to be separated by larger intercellular 

 spaces and to possess porous walls. The pith is in 

 the center of the plerom region and is gradually 

 replaced by xylem as the latter tissue increases. In 

 monocotyledonous roots possessing bundles of the 

 concentric and closed collateral types, there is no 

 regular arrangement of fibro-vascular tissues, and the 

 bundles are scattered within a central cylinder or 

 stele. The stele is bounded by the endodermis and 

 consists of the original parenchyma cells of the plerom 

 zone. 



