THE STRUCTURE OF THE ROOT 



37 



by a number of holes, so that their protoplasm is con- 

 tinuous (Fig. 13). With the sieve tubes are a certain 

 number of slightly elongated cells of the ordinary type. 

 The bast and wood strands are thus seen to occupy, 

 with a little supporting tissue, almost the whole central 

 cylinder of the root (Fig. 14). There is always an outer 



FIG. 14. Section of central part of root, b, bast strands; w, wood 

 bundles. Xioo. (After Kny.) 



continuous sheath over the whole, one cell thick as a 

 rule, which is called the pericycle. Outside the peri- 

 cycle comes the endodermis. 



The endodermis forms a sheath, one cell thick, round 

 the central cylinder. Its walls in some cases become 

 uniformly thickened and lignified. In others the 

 outer and inner walls remain thin, while the side walls 

 become changed in a different way. The cellulose is 

 replaced by another material which resists r the passage 



