ROOTS 



17 



tip. This protective structure is called the root cap. Just 

 back of the root cap is the region of greatest cell multiplica- 

 tion (Fig. 5), composed of cells that are actively growing. 

 The very tip of the cap is continually sloughing off, while 

 new cells are being added to it 

 just in front of the growing point. 



In addition to the root cap we 

 note that there are three distinct 

 parts to the root, namely, (i) 

 dermatogen, an outer layer or 

 layers; (2) plerome (axis); and (3) 

 periblem, between the dermatogen 

 and plerome. The dermatogen 

 becomes the epidermis, the plerome 

 the stele or central cylinder, and 

 the periblem the cortex. It is 

 often possible to strip the cortex 

 and epidermis from the central 

 cyhnder, which is composed of 

 tough, fibrous tissue. 



The cortex (Fig. 6) is composed 

 of large, thin-walled cells, which 

 do not fit closely together, but 

 leave air spaces (inter-cellular 

 spaces) between. The innermost 

 cortex layer is called the endo- 

 dermis. The outer cortex cells 

 may become prolonged to the side 

 to form root hairs. The central 

 cylinder or stele (Fig. 6) is bounded 

 by a single layer of cells, the pericycle, which lies adjacent 

 to the endodermis. Within the stele are found alternat- 

 ing bundles or strands. The woody, water-conducting bun- 



FlG. 6. — Cross-section of a 

 young root of Phaseolus mul- 

 tiilorus. A, pr, cortex; m, 

 pith; X, stele or central cylin- 

 der — all tissue within the peri- 

 cycle, inclusive; g, primary 

 xylem bundles; 6, primary 

 phloem bundles. B, cross- 

 section of older portion of 

 root; lettered as in A; b', 

 secondary phloem. (After 

 Vines.) 



